Practical questions

Mark scheme

Examiner's comment

This question discriminated well. It requires the use of the graph to determine that 60 cm3 was the volume of gas evolved and hence calculate moles of H2. Many low scoring candidates did not read the volume of gas from the graph, using 24.5 cm3 instead. ‘Error carried forward’ marks could be given to candidates who subsequently multiplied an incorrectly calculated value by two to reflect stoichiometry and hence calculate moles of HCl reacted. The final mark needed use of moles = concentration x volume to calculate HCl concentration (as well converting to standard form and giving to 2 significant figures). Candidates who scored low on the whole paper tended to struggle to make an attempt on this question.

Notes from the mark scheme and report

• 2 sig figs AND standard form required
• final volume 60 cm3 read from graph
• common errors: using 24.5 cm3 as H2 volume, 60/24 (units), 0.204 not in standard form (2 marks each)
• 2:1 HCl:H2 ratio

Skills: M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question required candidates to appreciate that initially concentration of HCl is at its highest, leading to more frequent collisions. As the reaction proceeds, concentration and collision frequency decrease until HCl is used up and the reaction stops (shown by the graph plateauing). The first marking point ignored candidates incorrectly discussing concentration of solid magnesium, but candidates need to be clear that only the (limiting) reactant, HCl, was used up when the reaction stopped for the third marking point. This last point was often missed, and this would be a good question to use with high-attaining candidates. Apart from the last point, there was an even spread of marks given; the second point was often missed through lack of ‘frequency’ or other link to collision rate. Analysing data in graphs Lower scoring candidates often stated that ‘the line increases because rate/concentration increases’. Students should be encouraged to carefully read axes to make sure they are identifying the correct trend from a graph.

Notes from the mark scheme and report

• less frequent collisions/fewer per second (IGNORE 'fewer collisions' or 'fewer successful collisions' with no link to time)
• acid used up (DO NOT ALLOW magnesium/reactants used up, Mg is in excess)
• IGNORE 'magnesium concentration decreases'
• DO NOT ALLOW rate increases

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question was a very good discriminator. Most candidates knew that silver nitrate should be added to the solutions and knew the colour of precipitates produced. Some candidates did not score the second mark by only giving colours and not stating they were solids. Many candidates were also able to give a correct ionic equation, with only the highest scoring candidates explicitly stating it was the AgX precipitates which had different solubilities in ammonia. A small number of candidates mistook this for a question on halogen displacement, scoring no marks. naming the solution and its result. Ionic equations are given (although only one is needed) and relative solubilities of the precipitates in ammonia are stated. This response was given the full 4 marks.

Notes from the mark scheme and report

• AgNO3
• any two precipitate colours
• ionic equation
• AgCl dissolves in dilute NH3, AgBr in concentrated NH3, AgI insoluble (must be clear the precipitate dissolves)
• HCl/H2SO4 or BaCl2 is a CON (IGNORE HNO3)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Many well-structured calculations were seen, even if all the marks weren’t achieved. Almost all candi- dates calculated the number of moles involved. For those candidates’ hat didn’t get full marks, it was often caused by using an incorrect mass in Q = mcΔT. Some tried to calculate the mass of NaOH and HCl (from moles and Mr values) while others didn’t add the total volumes together i.e. used 40 g rather than 80 g. Some used moles × 2 i.e. 0.112 rather than per mole of water formed. One of the most common errors was the omission of the negative sign for the final enthalpy change. Some put a positive charge instead. Some lost a mark for the incorrect significant figures.

Notes from the mark scheme and report

• common errors (3 marks): –28.1 from using 40 g instead of 80 g or 2 × 0.056 mol, –1690 from adding 273 to the temperature change, +56.1 (no – sign)
• 3 SF AND – sign required
• ALLOW ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Many candidates found this question challenging. Some managed to recognise the idea that the moles remained the same. Fewer recognised that the en- thalpy change stayed the same while the mass/volume decreased. A minority combined both ideas to- gether. Many gave responses in terms of increasing concentration increases rate, attributing a higher tempera- ture to more collisions.

Notes from the mark scheme and report

• same number of moles so same energy released AND smaller total volume/mass heated
• IGNORE responses about rate of reaction
• Candidates struggled to understand the difference between the rate of reaction and the en- thalpy change, often indicating that more collisions would result in more energy being re- leased. Make sure that candidates know that overall energy change depends on the net dif- ference in bond energies between reactants and products per mole. While an increase in con- centration can lead to a faster rate of reaction, it will not change the enthalpy change of the reaction.
• We have some useful resources to support this: Topic Exploration Pack: Kinetics and Catalysis https://teachcambridge.org/item/9fbefaed-4ade-4f22-95ec-3defcfef5a46 Delivery guide (H032) Enthalpy changes https://teachcambridge.org/item/6f16de69-f654-407c-b513-3ed7902ef9ce

Skills: M0.2 Use ratios, fractions and percentages; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most were able to correctly calculate the number of moles with the molar mass given in the question. Many were then able to use the molar gas volume at RTP, given on the data sheet, to find the volume of gas. The most common errors seen here were an incorrect number of significant figures, not converting to cm3 but leaving as dm3, and some multiplied by 2. A few candidates attempted to use the ideal gas equation here and if this was done correctly marks were credited.

Notes from the mark scheme and report

• answer must be 3 SF and in cm3 (184)
• ALLOW ECF from incorrect n(BaCO3)
• ALLOW ideal gas equation with appropriate T and p

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme

Examiner's comment

A very challenging question with most candidates failing to score any marks. Only a few correctly sug- gested the gas dissolving in water or the use of non-standard conditions. Most correct responses were for gas lost while replacing the stopper. Most mentioned loss of gas but didn’t link to the procedure to say how it was lost. Lots of candidates suggested that the reaction was incomplete despite being told the acid is in excess in the question. Candidates should consider that if a reagent is in excess, the reaction is expected to go to completion. Many suggested faults with setting up apparatus – possibly from personal experience. We wanted them to consider the limitations should this be set up without ‘human’ errors. A lot of candidates discussed the various leaks in the apparatus as the sources of gas loss. Some made reference to gas present in the tubing already not understanding that it would push through and displace gas to fill the measuring cylin- der, with equivalent amount being left in tubing at the end.

Notes from the mark scheme and report

• gas escapes while bung is replaced
• some CO2 dissolves in the water
• conditions not at RTP
• IGNORE incomplete reaction and human errors such as unsealed apparatus
• We provide support to help students prepare for questions based on the practical work they have carried out. Each of our suggested practicals has extension questions which can be used to explore practical work in more detail and evaluate different procedures.
• This question links well with PAG 1.1: https://teachcambridge.org/item/b9be7815-8c49-4fd6- b0eb-8f476393c653

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Around half of the candidates gained a mark for this question. Many suggested the use of “gas syringe” but from their response in (i) it was clear that a significant number didn’t understand how this would im- prove the experiment. Lower-attaining candidates sometimes couldn’t give the correct name for a gas syringe suggesting “gas volume measurer” or “gas tube” instead. Some other incorrect responses included: • Improve resolution on measuring cylinder • Use a larger mass of BaCO3 • Leave reaction for longer • Heat to a constant mass • Make sure delivery tube is further in the measuring cylinder • Make sure connections are airtight

Notes from the mark scheme and report

• gas syringe in place of upturned measuring cylinder, or carbonate in a small tube released into the acid without removing the stopper
• IGNORE gas cylinder

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Very few lost this mark and when they did it was usually for the giving the first titre to only 1 decimal place. Occasionally arithmetic errors were seen.

Notes from the mark scheme and report

• 23.30, 23.05, 23.15
• DO NOT ALLOW 23.3 (must be 2 d.p.)

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most scored here. A minority used all three titres for the mean and even fewer used 23.30 in the calcula- tion with either of the other two values.

Notes from the mark scheme and report

• mean of concordant titres only (23.05 and 23.15) = 23.10
• ECF titres must be within 0.1 cm3

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many responses were well constructed and showed most of the stages. This made it much easier to award error carried forward (ECF) marks if a correct response was not obtained. Around a quarter of candidates were able to score all 5 marks. Almost all candidates scored the first 2 marks. However, it was sometimes hard to follow the steps a candidate had taken to obtaining a final response, due to: • Steps not being labelled • Steps merging into one another (all equalling each other despite different processes) • Competing alternative methods described without crossing through incorrect working. Many candidates did not gain marks for unit conversions i.e. not converting volumes from cm3 to dm3 and incorrectly converting g to mg. Some did not gain marks for missing the × 10 step. Lower-attaining candidates often didn’t use the titre value obtained in (ii) but used 25 cm3 instead, muddling which vol- ume value should be attributed to which reactant. Maths Skills for AS Chemistry It is important for candidates to have the opportunity to develop their maths skills as part of the course. Particular attention should be paid as to how to: • Setting out the working for calculations • Keeping the full value of any step in their calculator to use in the next step • Common unit conversions, e.g. dm3 to cm3 • Giving the final answer to an appropriate number of significant figures or that asked for in the question • Checking their working for any errors Some support can be found in our Maths Skills Handbook: https://www.ocr.org.uk/Images/295468-chemistry-mathematical-skills-handbook.pdf This article from the RSC contains some useful resources: https://edu.rsc.org/resources/maths-skills-for-chemistry/4013899.article

Notes from the mark scheme and report

• common errors (4 marks): 0.413/4.13/41.3 incorrect conversion to mg, 1240 not divided by 3 tablets
• 1:2 acid:NaOH
• ×10 for 250 cm3
• 3 SF AND mg

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Around half of the candidates scored both marks, understanding the impact on the titre of rinsing the bu- rette with water. It was essential for candidates to refer to the “titre” and not just the volume in their re- sponse. Some understood that the NaOH would be more dilute but then suggested it would give a smaller titre or didn’t give impact on the titre value at all. Some suggested that the water provided OH- ions thus de- creasing the amount of NaOH required to neutralise. A few gave non-committal responses such as “Water would alter the concentration of NaOH, so you wouldn’t get an accurate titre” so scored 0 marks.

Notes from the mark scheme and report

• titre would be greater
• NaOH in burette more dilute so larger volume needed

Skills: PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Examiner's comment

Many understood the necessary steps to purify a liquid and were confident with the calculation. Most knew at least one or two purification steps and could explain their purpose. Reasons marks were lost included: • Not remembering the correct name for equipment i.e. separating funnel • Getting the layers the wrong way round despite being given density information • Unclear which layer an anhydrous salt was added to for drying • Using an inappropriate salt for dry step, e.g. Na2CO3 or MgCO3 • Slips in calculations, e.g. incorrect Mr values, yield inverted or not accounted for • Thinking a solid would be formed • Describing reflux (including drawing apparatus) • Saying to distil at a specific boiling point but not stating the actual value (given in the question) As with all calculations, it is important to set out working clearly as it helps to avoid slips. Remind candi- dates of the importance of checking their Mr values – many made slips here. Candidates should also avoid rounding intermediate values and consider the number of significant figures used for the final re- sponse. Some rounded intermediate values to less than 3 significant figures but gave a final response to 4 or more. The response scored Level 3 and gained all 6 marks. All three purification steps are given in the correct order and explained. The calculation for mass is set out clearly and the final mass is given to 3 signifi- cant figures.

Notes from the mark scheme and report

• separating funnel - organic layer is the top (less dense) layer
• dry with anhydrous MgSO4/CaCl2
• distil collecting fraction at 70 °C
• 0.125 mol × 0.65 × 92.5 = 7.52 g
• IGNORE washing with carbonate/water
• ALLOW one small slip e.g. Mr 73

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

This question required the candidates to know the observations of the reactions of group 7 and to recognise a change in colour to show the evidence of a chemical reaction. This question proved difficult for candidates with few being given all three marks. Colours were not well-known with I2 in aqueous solution often described as purple. Other incorrect answers included no colours stated, effervescence and descriptions of the formation of the halide precipitates. Several candidates gave an equation instead of an observation. The PAGs – section 1.2.1 (d) mentions accurate use of terminology to describe change instead of just final colours. A good number of candidates were able to provide the correct ionic equation, although several incorrectly included Na in their equations or wrote half equations. Occasionally candidates wrote more than one incorrect equation, often forming Cl2.

Notes from the mark scheme and report

• NaI: orange to brown (DO NOT ALLOW purple)
• NaCl: stays orange/no colour change (IGNORE 'no reaction'
• DO NOT ALLOW 'turns orange')
• Br2 + 2I– → 2Br– + I2
• IGNORE half-equations
• We have produced a teacher and delivery guide to assist with learning about the reaction of group 7 elements and their compounds: Teach Cambridge (ocr.org.uk) Links to the legacy coursework tasks and PAG practice question sets can be found on Teach Cambridge.
• Exam hints for students can be found on the OCR website.

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Lots of candidates were given 5 marks. They were able to use the experimental evidence to deduce the orders, write a rate equation and calculate k. The best candidates listed their reasoning for the orders rather than writing in large paragraphs of prose which sometimes got confused and led to contradictions. A good proportion of candidates recognised that 2.25 was 1.52, although a few thought that the 2nd order with respect to NO was a result of ‘2.25 being approximately 2 times 1.5’. Less successful candidates assumed that if they couldn’t identify what the relationship was between the concentration and the rate, that meant it was zero order. Less successful candidates were also unsuccessful in working out the units. A few candidates omitted k in their rate equations or didn’t write a rate equation. Many candidates were given marks as ECF from incorrect orders.

Notes from the mark scheme and report

• rate = k[NO]2[O2]
• k = 13.3
• units dm6 mol–2 s–1
• ALLOW ECF from incorrect orders and rate equation

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Nearly all candidates were able to draw the line of best fit and linked the equation of a straight line to the equation. A few candidates drew a curve, and some included the anomalous result. Gradients were calculated with accuracy with a few candidates calculating the gradient using an inverted formula. A few didn’t notice the x-axis didn’t go to 0, so used Δx in their gradient at -7x10-4 and ended up with a gradient of about -12800 and Ea of 110 as ECF. Most candidates knew to multiply by R but a few incorrectly divided by R. Some candidates didn’t give to 2 SF and / or convert to kJ and gave a negative value. A few candidates tried to substitute values into lnk = - Ea/RT + lnA, rather than use the graph. Many realised that they didn’t have a y-intercept and then calculated the gradient. Those that were able to complete the maths calculated the correct answer within the range.

Notes from the mark scheme and report

• best-fit straight line excluding the anomalous point at ln k = 11.4
• gradient –28 300 to –31 300
• Ea = –gradient × 8.314
• 2 SF in kJ mol–1 (240–260)
• DO NOT ALLOW a negative Ea

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.2 Plot two variables from experimental or other data; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Examiner's comment

Candidates made good progress and produced good diagrams with many given 4 marks. More candidates appreciated how the vanadium half-cell was set up than in previous series. Common errors were to miss the labelling of the salt bridge or to quote an incorrect temperature, e.g. 273K or 293K. Occasionally a hydrogen electrode was drawn, or V was used instead of Pt. Some candidates put H+ in with their V2+ /V3+.

Notes from the mark scheme and report

• complete circuit with voltmeter AND labelled salt bridge
• Pt electrode in V2+ AND V3+
• Zn in Zn2+
• 1 mol dm–3 AND 298 K (DO NOT ALLOW 1 mol
• IGNORE pressure and 'equimolar')
• electrodes/salt bridge must dip into solutions

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Most candidates were able to identity the correct colour and followed this by describing a precipitate or solid. A few candidates missed the observation of the precipitate. Incorrect colours suggested were yellow, cream and brown. The latter was perhaps as a confusion with the Fe3+ ion.

Notes from the mark scheme and report

• green precipitate (Fe2+)
• (pale) blue precipitate (Cu2+)
• 1 mark for both colours without 'precipitate'
• DO NOT ALLOW dark blue

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Candidates appeared better prepared for this type of question than in previous sessions, with a significant proportion achieving 3 or 4 marks. It differentiated well between candidates at different grades. Most candidates recognised the organic layer would be at the bottom but couldn’t always name the apparatus used to separate these two layers with incorrect responses including burettes, pear-shaped flasks, separating flasks and separating tubes. Some were confused about the density information, thinking that as the organic layer is denser it would be on top, or if adding more water it would then switch around the layers. Many gave the first step but not step 2 and 3. Lots of detail was given about how to use the separating funnel, e.g. invert to mix, vent to release pressure build up etc., to fill up the space provided. Most that went on to add an anhydrous salt were able to explain that it was used to dry the organic layer and were able to give a suitable example. Some candidates often misunderstood how to use them suggesting the anhydrous salt should be added to the separating funnel or the aqueous layer. Some candidates also suggested washing the layer with water or aqueous sodium carbonate to remove the acid once the organic layer had been dried. Many candidates either omitted the temperature to distil at to obtain pure 1-bromopropane or confused it with the boiling point of propan-1-ol (reactant) also given in the question. Some said to collect the distillate at its boiling point but didn’t quote the value that they had been given in the question so did not score the final mark. Many candidates introduced additional, unnecessary steps. Lower attaining candidates often confused the purification with that of an organic solid i.e. recrystallisation and scored 0 marks. A clear and concise response which scored all 4 marks.

Notes from the mark scheme and report

• separating funnel
• keep bottom (denser organic) layer
• dry with anhydrous salt (MgSO4/CaCl2)
• redistil collecting at 71 °C (70–72)
• no further marks if purification of a solid described
• We have produced a range of practice exam questions linked to the purification of an organic liquid (PAG5) which can be found on the Teach Cambridge website.
• This video produced by MaChemGuy is helpful for reviewing PAG 5: PRACTICAL SKILLS - PAG 5 - SYNTHESIS OF AN ORGANIC LIQUID.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

The majority of candidates scored both marks here. A few lost the second mark for just saying ‘turned orange’ without ppt. Some gave tests in (c) here as well but were usually consistent showing all three reacting in the table.

Notes from the mark scheme and report

• 2,4-DNP(H)/Brady's reagent
• orange/yellow/red precipitate (marked independently)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Very few candidates did not score any marks for this observation question. Common errors included: • gas/CO2 formed for the reaction with carbonate. The observation was needed • no reaction for methylglyoxal with dichromate, as they had not realised aldehydes will react as well as primary and secondary alcohols • positive test for all three with dichromate − often linked with incorrect answer in Question 9 • white precipitate with the reaction with carbonate • bubbles with acetol with carbonate (presence of C=O and O-H leading to possible confusion with an acid?). Candidates gave a range of responses for the reactions with no change, e.g. ‘no silver mirror formed’, ‘stays orange or ‘nothing’, these were still accepted but for clarity candidates should respond as the question asks.

Notes from the mark scheme and report

• one mark per row
• Tollens': silver mirror only for methylglyoxal (ALLOW acetol positive)
• Na2CO3: effervescence only for pyruvic acid (IGNORE 'gas/CO2 formed', must be an observation)
• dichromate: orange to green for methylglyoxal and acetol

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

The majority were able to correctly calculate Rf values and gain a mark here. A few did not gain marks for calculator errors (they had corrected measurements but gave the wrong answer) or for incorrect measurements, e.g. from the bottom of the TLC rather than the start line.

Notes from the mark scheme and report

• Rf = 0.75 (0.71–0.79) AND 0.25 (0.22–0.28)
• ±1 mm on each measurement
• working not required

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

There were a variety of responses to this question, with many obtaining correct answers of 206 mg for ibuprofen and 494 mg for paracetamol. From the 2:1 stoichiometry in the provided equation, the amount of ibuprofen is 4.00  10–3 mol. Candidates then needed to multiply this value by the molar mass of ibuprofen (206 g mol–1). Most candidates obtained 2.00  10–3 mol for CO2. Common subsequent mistakes resulted from candidates ignoring the 2:1 stoichiometry, working out the molar mass incorrectly, not dividing by 4 for 1 tablet and not converting to mg. Some candidates used the ideal gas equation to work out the moles of CO2. The question clearly stated RTP and the molar volume at RTP is stated on the Data Sheet. When RTP is stated, candidates are wasting valuable time by using the ideal gas equation, which is harder and opens up calculation errors, particularly with powers of 10. Overall, the question was answered well. However, many calculations were unclearly displayed, with random numbers often sprayed across the page. This could make it very difficult to find answers, especially those containing errors.

Notes from the mark scheme and report

• 2 mol ibuprofen per mol CO2
• ÷ 4 for one tablet
• 206 mg and 494 mg
• common errors 103/597 (no × 2) and 824/1976 (no ÷ 4) score 3
• ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme

Examiner's comment

This question was answered confidently with most candidates identified the conditions for a maximum equilibrium yield from the equation: a high pressure from fewer gaseous moles in the product and a low temperature from the exothermic reaction. This was all that the question asked but candidates often wrote far too much, considering operational conditions, rate, safely and expense. Unfortunately, candidates could end up contradicting themselves and wasting valuable time. Candidates are advised to read what the question is asking and to limit their response to this. The calculation was answered well, with most identifying that 2 significant figures is appropriate as the lowest number of significant figures present in the data provided in the question. Thus 0.48 mol dm–3 was commonly seen, rather than 0.476. The Kc expression was usually correct and most candidates rearranged this correctly to make [H2] the subject. The square root calculation showed that some candidates did not appear to have the required calculator skills. A significant number of candidates showed the Kc expression upside down and the mark scheme allowed ECF to be applied following this error.

Notes from the mark scheme and report

• high pressure because fewer gaseous moles on right (DO NOT ALLOW gaseous atoms)
• low temperature because forward reaction exothermic
• square brackets required in Kc
• square root needed
• 0.48 to 2 SF (0.476 loses a mark)
• ECF only from inverted Kc or no square root

Skills: M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations

Mark scheme, with the levels and indicative points

Examiner's comment

Most candidates were able to gain some marks for this Level of Response (LoR) question, with very few obtaining no marks. To achieve Level 1, candidates needed to identify at least 2 of the 3 orders with no further correct analysis. For Level 2 candidates were also expected to show the correct rate equation to match their orders or to provide a reason for why the mechanism must proceed via more than one step. Most candidates reaching Level 2 wrote a correct rate equation and usually calculated the rate constant. At this level, comparatively few produced a valid analysis of the mechanism. For a Level 3 response, all three aspects of the analysis were required: correct orders, correct rate equation and rate constant, and a correct explanation of the mechanism. Many candidates were able to provide all three aspects. Candidates usually determined the 1st order with respect to propanone from the initial rates data provided. The 1st order with respect to H+ from the rate–concentration graph was often seen. The zero order with respect to bromine from the concentration–time graph caused more problems, with many candidates incorrectly choosing 1st or 2nd order. The communication strand for this LoR question was given if the candidate had linked their orders with the evidence provided. Most candidates gave the initial rates but the evidence from the graphs was often omitted and vague. Responses such as ‘because of the graph’ are inadequate. The candidate’s response also had to be structured in a comprehensive way. This strand was usually given. beyond the minimum requirements for Level 3 including the communication strand.

Notes from the mark scheme and report

• propanone first order ([ ] × 3, rate × 3)
• Br2 zero order (constant gradient on concentration–time graph)
• H+ first order (straight line through origin)
• rate = k[CH3COCH3][H+]
• k = 4.53 × 10^-4
• Br2 in overall equation but not in rate equation (and H+ in rate equation but not overall) so more than one step

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Candidates produced a range of answers, and the question discriminated extremely well. The higher attaining candidates often gave 6 or 7 correct formulae. The flowchart contained many clues which most candidates used. Most were able to provide at least 3-4 of the required formulae and there was no consistently wrong answer. Most candidates gave H+ and Cl– as two of the three ions, and AgCl for the white precipitate with Ag+(aq). Both S and S8 were allowed for sulfur but S2 was not allowed. As SO42– ions are present, Ag2SO4 was also allowed for the precipitate with Ag+(aq). Common errors included acid formulae such as H2SO4 in place of water, an incorrect formula for BaSO4, giving BaSO3, writing SO4– for the sulfate ion, and OH– as one of the three ‘acidic’ ions. Lower attaining candidates added formulae without using information in the question. Examples include full formulae such as HCl in the box labelled ‘ions’ and CuCl42– for the yellow solid, despite Cu being absent in the flowchart.

Notes from the mark scheme and report

• all responses must be formulae: AgCl (ALLOW Ag2SO4), BaSO4, three ions from H+, Cl–, SO42– (ALLOW ClO–, HSO4–), S (ALLOW S8
• DO NOT ALLOW S2 or 'sulfur'), H2O

Skills: PAG4 Qualitative analysis of ions

Mark scheme, with the levels and indicative points

Examiner's comment

As with the other Level of Response (LoR) question (Question 3(a)), most candidates were able to gain some marks for this question, with very few obtaining no marks. Overall fewer candidates were given Level 3 for Q6 than for Q3(a). To achieve Level 1, candidates needed to calculate a value for the energy change q from mc∆T using at least two of m, c and ∆T. Candidates needed to first process the raw experimental results provided to extract the required values for the calculation. In the mc∆T calculation, the most common incorrect values used were 1.97 (the mass of Na2CO3) for m, 8.314 (the gas constant) for c and 277 (Kelvin) for ∆T. Only the lowest attaining candidates were unable to carry out this task. For Level 2, candidates were expected to determine the energy change per mole using the correct mc∆T value. Most candidates reached Level 2. A Level 3 response proved to be more elusive, requiring candidates to use an energy cycle to determine an enthalpy change. This task was far more testing than the requirements for Level 2. Some leeway was allowed for values used within the response. For m, a value in the range of 30 – 33.5 was allowed to take into account either HNO3(aq) or the mass of solution. Responses calculated per mole of Na2CO3 or HNO3 were allowed. This allowed for the fact that, even though the question described an excess of HNO3 being used in the reaction, the volume and concentration specified (30 cm3 of 1.00 dm3 HNO3) is short of being an excess. The approach taken is fully outlined in the mark scheme for this question and ensured no students were disadvantaged. The communication strand for this LoR question was given for responses that were arranged in a cohesive way. Unfortunately, a significant number of candidates produced responses that were difficult to follow with unlabelled quantities and numbers spread randomly across the page.

Notes from the mark scheme and report

• m = 33.50 g (mass of final solution, not 30 cm3)
• q = 33.50 × 4.18 × 4.0 = 560 J
• n(Na2CO3) = 1.97/106 (or via n(HNO3) = 0.0300 if acid limiting)
• ∆H2 = –30.1 (temperature rise so exothermic)
• ∆H1 = 2∆H3 – ∆H2 = +69.7 kJ mol–1
• energy cycle and signs

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most candidates were able to work out these simple subtractions. Candidates were told that the titration readings were read to the nearest 0.05 cm3, requiring volumes to be recorded to two decimal places, which may include a ‘0’. The right-hand initial reading is therefore 24.10 cm3 and not 24.1 cm3, which continues to be the commonest error seen.

Notes from the mark scheme and report

• titres to 2 DP
• DO NOT ALLOW 24.4

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Candidates are expected to use only concordant titres when working out the mean titre and the left-hand titre of 24.40 cm3 should be rejected. Most candidates did this to produce 24.20 cm3 as their mean titre. Use of 24.2 was allowed because rounding of a ‘0’ as the second decimal place had already been penalised in Question 21 (b) (i). Predictably, the most common error was to use all three titres to produce the incorrect mean of 20.27 cm3.

Notes from the mark scheme and report

• use concordant titres only (24.15 and 24.25)
• DO NOT ALLOW mean of all three titres
• ALLOW ECF from (b)(i)

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many candidates followed a well drilled method to analyse their titration results: • Moles of Na2CO3 in the mean titre • Moles of CH3COOH in 25 cm3 • Scaling 10 for moles of CH3COOH in 250 cm3 Candidates then needed to process their titration results further to determine the percentage composition: • Mass of CH3COOH in 250 cm3 • Percentage composition of CH3COOH to 3 significant figures. Most candidates were able to make some progress through the analysis. Common errors included: • Not 2 to obtain the moles of CH3COOH • Omission of the scaling stage. Some candidates ignored the titration results entirely, instead calculating the number of moles of CH3COOH in 6.5 g of the descaler as 0.1083 mol by assuming that all of the descaler was CH3COOH. This approach was flawed and could not be given any marks. A final comment must be made about the presentation of many of the responses. Numbers had often been sprayed across the page and it could be difficult to see how these related to a cohesive solution. It was often impossible to give marks for such responses. The question discriminated extremely well with some candidates given all 5 marks. Less successful responses demonstrated problems with approaching this type of question and some were given no marks at all. CH3COOH from 25 to 250 cm3. The result is a percentage composition of 8.94 % instead of 89.4 %. The clear presentation allowed the examiner to follow how the incorrect response had been obtained. Error carried forward allowed marks can be given for a correct method, giving a total of 4 out of 5 marks.

Notes from the mark scheme and report

• 2:1 ratio CH3COOH:Na2CO3
• ×10 scaling to 250 cm3 (8.94% = 4 marks)
• 3 SF required
• common errors: omitting ÷1000, using 25.0 cm3 instead of mean titre
• DO NOT ALLOW n(CH3COOH) = 6.50/60
• ALLOW ECF from mean titre

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; PAG2 Acid–base titration

Mark scheme

Examiner's comment

The presentation of many candidates’ diagrams in questions such as this one requires improvement. The mark scheme was generous, requiring to first see that a candidate knew what distillation is. A first mark was given for a set up comprising a flask connected to a roughly horizontal condenser. There had to be no gap above the condenser and the overall set up could not be a closed system. A second mark was given for minimal labels: condenser and correct direction of water flow. Nearly half the candidates received no marks for their diagram with only just over a quarter being given both marks. Typical errors included: • a reflux set up • open above the flask • a closed system • water flowing the wrong way • the condenser labelled as condensation tube, cooling tube, cooling funnel, distiller, etc. Less successful responses showed a conical flask, beaker or test tube being heated. The best advice is to spend some time instructing students how to draw diagrams and to label the apparatus using the accepted scientific names. Students increasingly appear to be finding these questions challenging.

Notes from the mark scheme and report

• DO NOT ALLOW any marks for a reflux set up
• condenser horizontal or angled down
• not a sealed system
• water in at bottom and out at top
• IGNORE no heat or thermometer
• Practise the drawing of common organic apparatus as part of the practical work carried out for the practical endorsement.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Higher-attaining candidates often gave a very detailed responses with all stages, including warming the NaOH, use of damp litmus paper and some included an ionic equation. Quite a few lost a mark as they missed the addition of hydroxide, just warmed, but they still gained mark for testing with indicator paper. Some thought that the indicator paper would turn red or be bleached and a few gave incorrect ion test e.g. add silver nitrate, add acid. Over a third of candidates did not score on this question, with a significant proportion not even attempting it.

Notes from the mark scheme and report

• (warm with) NaOH/hydroxide
• damp litmus/indicator paper turns blue
• DO NOT ALLOW ammonium hydroxide or ammonia as reagent

Skills: PAG4 Qualitative analysis of ions

Mark scheme, with the levels and indicative points

Examiner's comment

The calculation of enthalpy change was generally well-answered and the majority of candidates were able to recall the equation q = mcT. Many candidates forgot the minus sign or gave a positive sign for final enthalpy change. Errors in calculation were most commonly for using an incorrect mass, usually by finding the mass of copper(II) nitrate (from moles and Mr). Some also used the wrong value for the heat capacity, selecting the value for R from the data sheet instead. Many gave the final answer to an inappropriate six significant figures. Candidates often found it challenging to give appropriate assumptions and improvements, limiting the level achieved to Level 2. However, many candidates did correctly give the assumptions that the specific heat capacity and density of the solution was the same as water. This is usually stated for the candidates with these types of questions. The most common improvements suggested were use of a polystyrene cup, adding a lid or using a thermometer with a higher resolution. Quite a few candidates suggested using a larger volume of solution which would indeed reduce the % uncertainty in the volume measurement. However, it would lead to a smaller temperature change, increasing the % uncertainty in the temperature measurement. Some confused the question with a calculation of enthalpy change of combustion and gave improvements accordingly, e.g. ‘use a copper or bomb calorimeter’, ‘draft shields’, ‘heat for longer’, ‘position of flame and supplies of oxygen’. This response achieved Level 3 - 6 marks. There is a correct calculation for H, the final value has a correct negative sign and is given to 4 significant figures. Lots of valid assumptions and improvements are given.

Notes from the mark scheme and report

• q = 100 × 4.18 × 18.6 = 7.77 kJ
• n(Cu(NO3)2) = 0.05
• ∆H = –155 kJ mol–1 with negative sign
• assumptions (density/c same as water, no heat loss, ignore zinc)
• improvements (polystyrene cup, lid, cooling curve, more precise thermometer)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most scored both marks here. Some didn’t circle the anomalous result and some lost a mark for a poorly drawn curve. Candidates must ensure they have a sharp pencil and draw a single line through all the points (except the anomalous point). Some didn’t start at the origin or didn’t level off at around 90 cm3.

Notes from the mark scheme and report

• smooth curve through all points except 100 s
• anomaly at 100 s circled
• curve levels off at about 90 cm3

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

More candidates were able to correctly draw a tangent than seen in previous years with similar questions. A generous range was given for both tangent and gradient so many scored both marks. The most common reasons for losing marks was for having a gap between the curve and the tangent or calculating the gradient incorrectly, e.g. misreading scales, dx/dy, or by using interpolation rather than a tangent.

Notes from the mark scheme and report

• tangent at 50 s (±10 s)
• rate = gradient about 0.67 cm3 s–1
• DO NOT ALLOW interpolation
• ALLOW ECF from tangent or straight line of best fit

Skills: M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

More than half of candidates achieved both marks and most scored at least 1. The most common reason for losing marks was for not being specific, by just saying ‘concentration’ or ‘volume’, or suggesting volume and concentration for metal oxide. Some suggested use of the same gas syringe without realising that the apparatus used should not affect volume of gas produced, just how easy and accurate it is to measure.

Notes from the mark scheme and report

• any two: amount/mass/surface area of catalyst, temperature, volume or concentration of H2O2
• DO NOT ALLOW concentration/volume of catalyst

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Nearly all candidates were able to draw the line of best fit and linked the sharp vertical section of the graph with the volume of Ba(OH)2 needed to neutralise the ethanoic acid. Candidates should aim to produce a smooth line of best fit and avoid ‘tram’ lines when the pencil is taken off the paper and the curve started again. The line should go through most points. Some candidates misinterpreted the graph and used values of 8, 12, 12.2, 12.25 and 18. However, the remainder of the calculation was accessible, and most candidates scored well with ECF marks from this point. There was occasional division of 2 for the moles of ethanoic acid and dividing by the original volume of Ba(OH)2 rather than the 10cm3 of ethanoic acid. This candidate scored all available marks. This is a very good example of a candidate displaying their working. The response was well communicated indicating the end point, links were made to what was being calculated and how the next number was obtained.

Notes from the mark scheme and report

• smooth S-shaped best-fit curve through the majority of points
• equivalence at 12.5 cm3
• 1:2 ratio Ba(OH)2:CH3COOH

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M3.2 Plot two variables from experimental or other data; PAG11 pH measurement; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Successful candidates drew a complete circuit and voltmeter with the labelled salt bridge dipped into the two solutions. Both cells had Pt as the electrodes. One cell contained 1 moldm-3 Fe2+ and Fe3+ where the other had a delivery mechanism for H2 (at 1 atm) and H+ (at 1 mol dm-3). Standard temperature of 298K (or 25oC) was stated. Common errors included: not have solutions in the beakers, used Fe or Fe2+ electrodes, the hydrogen cell was missing and/or the H2 with a device for adding it. A few candidates suggested two cells with only Fe2+ one side and only Fe3+ on the other.

Notes from the mark scheme and report

• Pt electrode in Fe2+/Fe3+ half-cell
• standard hydrogen electrode with H2 delivery and H+
• salt bridge and voltmeter
• 1 mol dm–3, 298 K, 1 atm/100 kPa

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

An unfamiliar question that proved challenging with only around a fifth of candidates obtaining the mark for correctly suggesting that reflux would prevent loss of volatile compounds. Many candidates suggested that reflux ensures the reaction goes to completion but here this was insufficient as esterification is an equilibrium reaction and additional information in (b)(i) indicates that there is unreacted compound G present. It was necessary to focus on the purpose for reflux rather than other ways of heating a reaction, such as the energy needed to break bonds or speed up the rate of reaction. Some less successful responses linked to oxidation reactions, presumably as they understand the importance of either reflux or distillation in this context. For example, ‘reflux is required for complete oxidation’ or ‘if distillation had been used an aldehyde would have been formed’.

Notes from the mark scheme and report

• prevents loss of volatile reactants/products by evaporation (mixture boiling dry)
• IGNORE rate or completeness of reaction

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

The best responses correctly identified that using sodium hydroxide was not an improvement and explained this either by stating that it would react with the phenol group or hydrolyse the ester group in compound H. However, most candidates appeared not to consider a reaction with H in their answer. Many focused on the neutralisation of sulfuric acid in a similar way to sodium carbonate and gave responses such as: • stronger base • no effervescence so harder to see when completely reacted • no CO2 produced so easier/safer/higher atom economy/less waste • requires double the moles compared to Na2SO4 to react

Notes from the mark scheme and report

• NaOH would react with the phenol –OH OR hydrolyse the ester (compound H)
• DO NOT ALLOW 'reacts with alcohol'

Mark scheme

Examiner's comment

Many candidates identified the need to describe the isolation and purification of a liquid in response to this question. Some struggled to remember the name of the apparatus required i.e. ‘separating funnel’ but could describe the separation of layers. The best responses used the density data provided to explain that the lower layer would contain compound H. A few gained a mark for a description of the addition of water to identify the correct layer to discard. In generally, candidates only have experience of the organic layer being less dense in their practical work which was reflected in their responses. The use of an anhydrous salt to dry the organic layer was well-known, but many candidates gave vague answers such as ‘add anhydrous salt’ failing to justify why the salt was needed. Many recognised the need to purify the resultant liquid by distillation, but not all linked to the boiling point of compound H to secure this final mark. Some who didn’t dry with an anhydrous salt said to heat to 100oC to remove water. Approximately a third of candidates gained no marks here. Candidates thought that compound H would form crystals so described recrystallisation rather than a method to purify an organic liquid. Other responses attempted to merge both methods together. This candidate scored 1 mark for this response for the use of a ‘separating funnel’. They have attempted to use the density information but have incorrectly identified that the density of H as being low which would make it the top layer. Candidates needed to know that the density of water is 1 g cm-3 to be able to make a correct comparison. There is no addition of an anhydrous salt to dry the organic layer. They have recognised the need to distil but have not given the correct temperature at which to collect the pure compound. However, if they had given the boiling point for distillation no mark would have been given as they would lose this final mark for describing the purification of a solid. It was very common to see responses which described the purification of a solid, often in addition to that of a liquid.

Notes from the mark scheme and report

• steps in order: separating funnel
• keep the bottom (organic, denser) layer
• dry with anhydrous MgSO4/CaCl2
• redistil at 222 °C
• no further credit if the method treats H as a solid (recrystallisation)
• The method to purify an organic liquid was poorly understood by many candidates. Many of the candidates answered this question using techniques to purify an organic solid which is covered in the second year of A Level, rather than an organic liquid which is covered in the first year. It is important to spend time comparing both methods and helping candidates identify when each method is required.
• OCR have produced a range of practice exam questions linked to the purification of an organic liquid (PAG5). These can be found on Teach Cambridge.

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

This question differentiated between candidates extremely well. See Exemplar 1 below. some of the issues encountered in the responses. Unfortunately, nearly a quarter of candidates could not be given any marks at all for their responses. To improve, it is worth studying Exemplar 1. The candidate has communicated the key steps required to prepare the standard solution: • Calculation of the mass of hydrated copper(II) nitrate required. • Dissolving the hydrated copper(II) nitrate in water in a suitable container (a beaker). • Transferring the solid to a 100 cm3 volumetric flask, washing the beaker with water and transferring the washings also to the volumetric flask. • Making the solution up to the 100 cm3 mark in the volumetric flask and inverting the flask to mix the contents thoroughly. Issues with responses which arose by not reading the question closely enough: • Omitting to calculate the mass of hydrated copper(II) nitrate required. • Calculating the mass of anhydrous copper(II) nitrate instead of the hydrated salt. • Dissolving in 100 cm3 of water and then adding more water for rinsing. • Not rinsing out the original container at all. • Making the solution up in the volumetric flask. • Using of a 250 cm3 volumetric flask for preparing 100 cm3 of solution. • Omitting the inversion stage. • Answering a different question, e.g. how to carry out a titration, how to determine an enthalpy change, how to work out the number of waters of crystallisation by heating in a crucible.

Notes from the mark scheme and report

• mass 4.83 g using M of the hydrated salt (241.5), 2 or more decimal places
• dissolve in less than 100 cm3 of distilled water in a beaker (DO NOT ALLOW 100 cm3 or more)
• transfer to volumetric flask AND rinse washings in
• make up to the mark AND invert

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates were given all 3 marks for their titration tables. One mark was allocated to the readings, the other 2 for the correct titres. Candidates are expected to record burette readings to 2 decimal places with the last figure being 0 or 5, reflecting the accuracy of the burette, which is stated in the question. Two issues were seen: • Some candidates ignored the terminal 0 in their readings, e.g. 1.9 for 1.90 cm3. • A small number of candidates recorded the initial and final readings the wrong way round despite these terms being used above each burette diagram. Titres were usually correct.

Notes from the mark scheme and report

• all readings to 2 decimal places with last figure 0 or 5 AND final/initial readings in the correct rows
• titres 21.65, 21.85, 21.55 (2 correct for 1 mark)

Skills: M1.1 Use an appropriate number of significant figures

Mark scheme

Examiner's comment

Candidates are much better at calculating percentage uncertainties than in previous series. Almost all candidates realised that the uncertainty in the titre resulted from two readings and that the overall uncertainty in the titre would be 2 × 0.05 = 0.1 cm3, resulting in a percentage uncertainty of 0.46%. Comparatively few candidates ignored the ‘2’ and gave 0.23% as their answer. Some candidates worked out the percentage uncertainty from the mean titre but this was still given marks.

Notes from the mark scheme and report

• 2 × 0.05 because two readings
• 0.46%
• 2 decimal places minimum
• DO NOT ALLOW 0.50%

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined

Mark scheme

Examiner's comment

Candidates are well versed with calculating an Rf value, with nearly all candidates obtaining a value in the acceptable range of 0.12–0.18.

Notes from the mark scheme and report

• working required
• measure from origin line to solvent front (about 9.1 cm), not from the bottom of the plate
• ALLOW 0.12–0.18

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

This novel question assessed whether candidates realised why chemists used TLC when carrying out organic reactions. A good response would identify the following key features after 20 minutes: • The C–Cl bond energy is greater than C–Br and so the reaction would be slower. • The haloalkane spot would be larger and the propan-1-ol spot smaller. • The propan-1-ol is spotted on the chromatogram to monitor the progress of the reaction. The question differentiated very well between candidates, but many did not seem to know where to start with many candidates not scoring any marks. This suggested that candidates recognised chromatography as a technique but did not appreciate its relevance in organic chemistry. Some candidates referred to pigments, recalling their early chromatography experiments in finding the colours in ink.

Notes from the mark scheme and report

• propan-1-ol spot smaller/1-chloropropane spot bigger
• C–Cl bond stronger than C–Br so reacts slower (C–Cl bond required
• DO NOT ALLOW chloride/bromide
• IGNORE halogen reactivity)
• propan-1-ol run to show product formation/monitor progress
• IGNORE 'as a control/comparison' alone

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Although high attaining candidates responded with the formulae of chromium-containing species, it was common to see organic compounds being suggested. Consequently, a large proportion of candidates did not score either of the 2 marks. Many candidates seem to expect to only give organic species in their responses on this paper and would benefit from understanding that inorganic species may also need to be provided.

Notes from the mark scheme and report

• green Cr3+ (or [Cr(H2O)6]3+)
• orange Cr2O7 2–
• formulae AND charges must be correct
• DO NOT ALLOW Cr6+
• 1 mark if the wrong way round

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

This Level of Response question was answered well with many candidates identifying compounds F–I correctly to reach Level 3. Structures were usually shown skeletally and this practice is to be recommended. Not only is it far quicker and clearer, it eliminates writing every atom in a displayed or structural formula. Some candidates were not given marks for missing hydrogen atoms or for ‘sticks’ being shown. In these structures, the chemical meaning of a stick is a terminal CH3 group. Candidates were also asked to show how the results of the chemical tests helped the identification of the unknown compounds and this formed the basis of the communication strand of the LOR mark. Candidates answered this part of the analysis extremely well and most were given marks for their good communication skills. This question differentiated very well between well-prepared and less confident candidates. The latter often did not know how the results of these organic tests can be used to identify the functional groups present. It was common for such candidates to identify only one of the four compounds, scoring within Level 1 only. This exemplar is concise and very clear. The candidate has clearly linked the result of each test to the functional groups that must be present. The candidate has drawn skeletal formulae and clearly has experimented with many possible structures before deciding on which must be correct. Notice that the candidate has crossed out the structures that they have rejected. This is an important exam technique – if two structures are drawn, with one correct and the other incorrect, the correct structure cannot be given marks. The response is clearly at Level 3 for the four correct structures and the good communication ensures that the communication strand can be given. This response received all 6 marks.

Notes from the mark scheme and report

• H and I carbonyl (2,4-DNP)
• I aldehyde (Tollens')
• F, G, I oxidised by dichromate
• F has C=C (bromine): F unbranched unsaturated primary/secondary alcohol, G alicyclic alcohol (DO NOT ALLOW tertiary OH), H pentan-2-one or pentan-3-one, I pentanal
• To better prepare candidates, we recommend using either the digital multiple choice quizzes on Teach Cambridge or creating targeted practise materials using ExamBuilder. If you are unsure of how to access these or ways to make the most of them, get in touch via science@ocr.org.uk.

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Most candidates recognised that a volumetric flask is used to accurately prepare volumes of solutions. A common error was a conical flask, perhaps by not reading the information clearly and giving the name of the flask used in the titration itself.

Notes from the mark scheme and report

• ALLOW graduated flask

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates were able to work out these simple subtractions. Candidates were told that the titration readings were read to the nearest 0.05 cm3, requiring titres to be shown to two decimal places, which includes a ‘0’. The middle titre is therefore 20.60 cm3 and not 20.6 cm3, which continues to be the commonest error seen.

Notes from the mark scheme and report

• all three titres to 2 DP
• DO NOT ALLOW 1 DP e.g. 20.6 for 20.60

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Candidates are expected to use only concordant titres when working out the mean titre and the middle titre of 20.60 cm3 should be rejected. Most candidates did this to produce 20.30 cm3 as their mean titre. Predictably, the commonest error was to use all three titres to produce the incorrect mean of 20.40 cm3.

Notes from the mark scheme and report

• mean of the concordant titres only (20.30)
• DO NOT ALLOW mean of all three titres (20.40)

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many candidates followed a well drilled method to identify the unknown metal M as potassium: • Moles of H2SO4 in the mean titre • Moles of KOH in 25 cm3 • Scaling 10 for moles of KOH in 250 cm3 • Molar mass of the metal as 39.11 and identified as K. A number of candidates omitted the scaling stage to obtain a molar mass of 391.1. By ECF, the ‘correct’ identity would be caesium or francium. Some candidates then ‘fiddled’ their response, dividing by 10 to ‘identify’ the metal as K. This incorrect approach was not credited. A common error, illustrated in Exemplar 2, was for candidates to calculate 39.11 but to think that this was the mass of MOH and not M. They then subtracted 17 (for OH) from 39.11 to obtain a response of 22.11 and identified M as sodium instead of potassium. This error probably stems from candidates either not reading the question closely enough or confusion about the mole concept.

Notes from the mark scheme and report

• ×2 for MOH:H2SO4
• ×10 scaling to 250.0 cm3 (if absent Ar = 389, Cs/Fr)
• ALLOW ECF from incorrect mean titre in (c)(iii)
• use of 25.0 for acid volume loses first mark only

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

There were some excellent responses to this question which discriminated extremely well. Unfortunately, there were a significant number of incorrect responses and some less successful candidates had clearly struggled to recall and apply this important material. The identification of the isomers was usually correct, as was the identification of the oxidation products from the primary and secondary alcohols, and the conditions required to produce the organic products. The equation proved to be the hardest requirement with the H2O by-product often being omitted or H2 shown instead. A general point applies to organic structures. Some candidates did not show the structures of the isomers and attempted this question using the molecular formula of C3H8O supplied in the question for both alcohol isomers and no structural formulae. It was then impossible to know which isomer was being reacted and this could cost the candidate a significant number of marks. It is essential in organic chemistry to use unambiguous formulae which can be any combination of skeletal, structural or displayed. Unless a question specifies that a molecular formula is required, candidates should assume that an unambiguous formula is required.

Notes from the mark scheme and report

• propan-1-ol AND propan-2-ol
• distillation for aldehyde AND reflux for carboxylic acid or ketone
• one correct [O] equation (2[O] for the acid)
• ALLOW named products if functional group not given

Skills: PAG5 Synthesis of an organic liquid; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Most candidates did not gain any credit here. However, the range of responses seen highlighted some misconceptions in their understanding of how different mixtures can be separated. Many assumed that sodium chloride was in solution/aqueous, not recognising that water was not present in this reaction. Responses such as “sodium chloride will evaporate” or “remove the water” were seen. Some gave a description of the purification method for an organic liquid - the use of a separating funnel and/or distillation were common. Some suggested the use of a magnet to remove Ti despite it being a non- magnetic metal.

Notes from the mark scheme and report

• add water AND filter
• NaCl dissolves / Ti insoluble (Ti is the residue)
• Understanding how to separate mixtures is covered in both KS3 and KS4 but it is important that these concepts can be applied during further study. Asking this type of problem solving question would make a good starter activity.
• Some useful activities for separating mixtures can be found in the GCSE Chemistry B (Twenty First Century Science) Chemical analysis transition guide

Mark scheme

Examiner's comment

This was a tricky calculation, made more challenging if candidates hadn’t been able to successfully complete (i). Many were able to calculate the moles of BaSO4 but often rounded their answer to only 2 significant figures at this stage i.e. 0.014. Many assumed a direct ratio between BaSO4 and NaCl so mass was found by multiplying moles by 58.5 (molar mass for NaCl) - if this was done then credit was given for ECF for the final marking point.

Notes from the mark scheme and report

• n(BaSO4) → mass Ba(NO3)2 (× 261.3) → mass NaCl by difference → 26.6% to 3 SF
• using n × 58.5 gives 16.4 (ECF only)
• ALLOW ECF from equation in (b)(i)
• The M1 section of the Mathematical Skills handbook contains useful information on handling data, including M1.1 use of significant figures.

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme

Examiner's comment

Another tricky question with less than half gaining credit. Many were able to recognise the addition of silver nitrate as the test for halide ions but did not realise that it could be used quantitatively. Many didn’t read the question carefully and assumed Na2SO4 was still present, giving a mixture of two precipitates. Some, despite recognising the formation of AgCl, could not then see how to calculate the mass of NaCl i.e. “you won’t have formation of BaSO4”. Some suggested that barium nitrate would also form a precipitate, perhaps confused by the (s) state symbol in the question.

Notes from the mark scheme and report

• AgCl precipitate would form
• mass of NaCl calculated from mass/moles of AgCl
• IGNORE incorrect formula of silver chloride

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

The majority of candidates scored both marks. Some scored 1 mark only mainly for correct colours of the precipitates but with incorrect formulae. Common errors included AgBr2 and AgI2, giving the formula of the haloalkane or formulae involving a halogen and a nitrate ion.

Notes from the mark scheme and report

• AgBr cream, AgI yellow: formula AND colour for each mark
• 1 mark if both formulae OR both colours correct

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Most responses did not gain credit here. Some recognised that AgI would form first but then gave an incorrect reason (such as ‘held by weaker London forces’) or their answers lacked detail (e.g., ‘weaker bonds’ without specifying which bonds). Many said AgBr because Br is more reactive or more electronegative.

Notes from the mark scheme and report

• AgI/yellow first AND C–I bond weaker / lower bond enthalpy
• IGNORE polarity and electronegativity
• ALLOW ECF from (d)(i)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

More than half of candidates gave the correct answer. Common mistakes included not multiplying the uncertainty by 2 for each measurement (0.81) or using the initial or final temperatures rather than the difference in their calculations.

Notes from the mark scheme and report

• two readings: 2 × 0.25/31 × 100 = 1.6%

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG3 Enthalpy determination

Mark scheme, with the levels and indicative points

Examiner's comment

Many candidates limited themselves to only L1 here by not calculating the enthalpy change from the student’s results. Remind candidates to read the question carefully and to use all the information provided (such as tables of results). Some missed out on L3 as their evaluation lacked detail, as they needed to include at least two differences and two suitable improvements. Not all improvements given were suitable, with some suggesting candidates confusing different practicals - for example, ‘use a polystyrene cup’ or ‘heat to a constant mass’. It was evident that some lacked understanding about complete combustion, with suggestions to ‘measure mass until it doesn’t change any further’ or ‘heat water for longer’ to make sure complete combustion had occurred. Some also suggested ‘using a larger mass’ but further information was needed for credit to be given. This exemplar shows a candidate who achieved L3 with all 6 marks given. Working for the calculation of H is clear with all steps shown, in a logical order, and the final answer has correct negative sign. More than 2 differences and 2 improvements are given. The communication is clear, linking potential differences with suggestion for improvements.

Notes from the mark scheme and report

• q = 200 × 4.18 × 31 (mass of water, not fuel)
• n = 1.31/74
• ∆cH = –1460 to –1464
• heat losses, incomplete combustion, evaporation, non-standard conditions
• improvements: draught shield, lid, copper can, cap on wick, bomb calorimeter
• Further information about practical skills assessed on written exams can be found in Section 3 of the practical skills handbook. If using the suggested practical activities developed by OCR, then encourage candidates to answer the extension opportunity questions to help develop a deeper understanding in preparation for written assessments.

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme, with the levels and indicative points

Examiner's comment

The first Level of Response question in the paper was answered well. Almost all candidates were able to conclude that the experimental results showed that they were consistent with first order with respect to Br-. Some candidates were able to use a sequential approach to determine the orders with respect to BrO3- and H+, reaching a Level 3, but others found this more problematic. Some did not notice that more than one concentration had been changed between experiments. This led to many determining the rate to be 0 order with respect to [BrO3-] and [H+]. Candidates should focus on the quality of their descriptions when linking data to their conclusions with some candidates creating their own data set to fit their explanations. Having determined orders, nearly all candidates were able to give a corresponding rate equation and could calculate a value for the rate constant, albeit with frequent omission of units. Some candidates confused the rate equation with a Kc expression.

Notes from the mark scheme and report

• first order Br– (expts 1 and 2)
• first order BrO3– (expts 1 and 3, both concentrations change)
• second order H+ (expts 1 and 4, ×5 gives ×25 after allowing for BrO3–)
• rate = k[Br–][BrO3–][H+]2
• k = 16.4 from any row of data

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Many candidates calculated the correct value of q and scored all three marks. Common errors saw some candidates using the incorrect mass, of either 0.133 or that added to or subtracted from the 150. The temperature change was given but a few candidates added the 10.5 to 273 in order to, incorrectly, convert to K. The final mark required the candidate to link the temperature increase to an exothermic value and include a minus sign.

Notes from the mark scheme and report

• q = 150 × 4.18 × 10.5
• n = 0.133/100
• – sign required
• common errors for 2 marks: +4950, –5077/–5064 (use of 2 SF)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Nearly all candidates were able to draw the line of best fit and linked the equation of a straight line to the equation for delta G. Some hadn’t used a ruler to draw a straight line and candidates should be reminded of this requirement. Candidates were able to recognise that the gradient corresponded to (-) delta S but some did not to use the scale correctly. Some candidates tried to use very small triangles to get the gradient and candidates would be advised to use the largest scale possible to minimise errors in the line of best fit. Most candidates recognised that the minimum temperature was the x-intercept (when delta G = 0) and that delta H was the y-intercept. Some did not use this approach and instead used the ∆G=∆H-T∆S equation, using previously obtained value(s), to calculate the minimum temperature, ∆S or ∆H. Some candidates used ∆H = -186 from a previous question or attempted to solve both ∆S and ∆H by simultaneous equations.

Notes from the mark scheme and report

• best-fit line extrapolating to y-axis 43–47 and x-axis 820–840
• ∆S = –gradient × 1000 (122–131)
• minimum T where ∆G = 0 (340–370 K)
• ∆H = y-intercept (43–47)
• ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M3.1 Translate information between graphical, numerical and algebraic forms; M3.2 Plot two variables from experimental or other data; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Examiner's comment

Only a few candidates were given this mark. Some had the colour change inverted and most stated a variety of different colours.

Notes from the mark scheme and report

• colourless to (pale) pink
• DO NOT ALLOW purple

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Almost all candidates calculated the titres correctly. A significant number used all three titre values to derive their mean value.

Notes from the mark scheme and report

• titres 12.65, 12.95, 12.75
• mean of concordant titres only = 12.70
• Links to the legacy coursework tasks and PAG practice question sets can be found on Teach Cambridge and can help students prepare for practical-based questions like this one. Exam hints for students is useful to share with candidates.

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

ocr.org.uk/science Oxford Cambridge and RSA CHEMISTRY A H432 For first teaching in 2015 A LEVEL Version 1 H432/01 Summer 2023 series Examiners’ report Contents Introduction .......................................................................................................................................... 4 Paper 1 series overview........................................................................................................................ 5 Section A overview ............................................................................................................................... 6 Question 1 .................................................................................................................................... 6 Question 2 ....................................................................................................................................... 6 Question 3 ....................................................................................................................................... 7 Question 4 ....................................................................................................................................... 7 Question 5 ....................................................................................................................................... 8 Question 6 ....................................................................................................................................... 8 Question 7 ....................................................................................................................................... 9 Question 8 ....................................................................................................................................... 9 Question 9 ..................................................................................................................................... 10 Question 10 ................................................................................................................................... 10 Question 11 ................................................................................................................................... 11 Question 12 ................................................................................................................................... 11 Question 13 ................................................................................................................................... 12 Question 14 ................................................................................................................................... 12 Question 15 ................................................................................................................................... 13 Section B overview ............................................................................................................................. 14 Question 16 (a) (i) ........................................................................................................................... 14 Question 16 (a) (ii) ......................................................................................................................... 15 Question 16 (b) .............................................................................................................................. 16 Question 17* .................................................................................................................................. 17 Question 18 (a) ............................................................................................................................... 18 Question 18 (b) .............................................................................................................................. 19 Question 18 (c) (i) .......................................................................................................................... 20 Question 18 (c) (ii) .......................................................................................................................... 21 Question 19 (a) (i) .......................................................................................................................... 23 Question 19 (a) (ii) ......................................................................................................................... 24 Question 19 (b) .............................................................................................................................. 24 Question 20 .................................................................................................................................... 25 Question 20 (a) (ii) ......................................................................................................................... 26 Question 20 (b) .............................................................................................................................. 26 Question 20 (c) (i) .......................................................................................................................... 27 Question 20 (c) (ii) .......................................................................................................................... 27 Question 20 (d) (i) .......................................................................................................................... 28 Question 20 (d) (ii) ......................................................................................................................... 28 Question 20 (e) (i) .......................................................................................................................... 29 Question 20 (e) (ii) ......................................................................................................................... 29 Question 20 (f) ............................................................................................................................... 30 Question 21 (a) (i) ........................................................................................................................... 31 Question 21 (a) (ii) ......................................................................................................................... 31 Question 21 (iii) .............................................................................................................................. 32 Question 21 (b) .............................................................................................................................. 33 Question 22 (a)* .............................................................................................................................. 34 Question 22 (b) (i) .......................................................................................................................... 36 Question 22 (b) (ii) ......................................................................................................................... 36 Question 22 (c) .............................................................................................................................. 37 Question 22 (d) .............................................................................................................................. 37 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 21 (a) (i) Only a few candidates were given this mark. Question 21 (a) (ii) Almost all candidates calculated the titres correctly.

Notes from the mark scheme and report

• × 5 for Fe2+
• ÷ 0.0250
• ECF from incorrect titre
• common errors for 2 marks: 1.27 × 10–3 (no × 5), 2.46 × 10–2 (volumes transposed)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates were able to successfully explain Step 1, scoring the first 2 marks. When comparing electrode potentials, candidates should avoid the use of higher/lower as these phrases are ambiguous due to the negative signs involved. Describing them as ‘more negative’ or ‘more positive’ is clearer. Candidates are advised to read the instructions contained within the question and to use or comment on all the data presented. Very few candidates linked Step 2 to the reducing effect of zinc to manganate(VII) ions hence the need for filtration. A few candidates explained Step 2 in terms of the warming/cooling and zinc crystalising rather than explaining the redox chemistry given in the table.

Notes from the mark scheme and report

• Step 1: E(Zn) more negative than E(Fe3+/Fe2+) so Zn reduces Fe3+ to Fe2+
• Step 2: E(Zn) more negative than E(MnO4–) so unfiltered Zn would react with MnO4–
• IGNORE larger/smaller

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

About a quarter of candidates gained all 3 marks. They were able to give clear, well-structured answers with all the steps as carried out in PAG 6 practical activities. The best answers were often in the form of bullet pointed steps and often included extra details such as a hot filtration step or promoting crystallisation by scratching the surface. Many candidates lost the first mark as their responses didn’t give sufficient detail, e.g. hot solvent, not just warm and minimum volume required. Some missed the cooling step essential for formation of crystals before filtration. Some missed that the crystals need to be dried following filtration but often if only given 1 mark this was the mark given. Lots of confusion with purification with organic liquid was seen, such as references to use of a separating funnel, drying agents and distillation. Some also went on to explained how to check purity using melting point determination.

Notes from the mark scheme and report

• dissolve in minimum quantity of hot solvent
• cool AND then filter
• dry
• DO NOT ALLOW a drying agent e.g. MgSO4
• IGNORE washing and hot filtration
• A useful resource for revising the steps for purification of an organic solid is the Topic exploration pack: Experiments on organic synthesis in Learner Activity 2.

Skills: PAG6 Synthesis of an organic solid

Mark scheme, with the levels and indicative points

Examiner's comment

Most candidates were able to deduce the empirical and/or molecular formula of the organic compound. Analysis of the IR spectrum was also well attempted, but some candidates assumed the unknown was a carboxylic acid, attributing the sharp peak just below 3000 cm–1 to an OH group. Others misidentified the C=O peak as a C=C group suggesting alkene or arene structure. They were often led to this conclusion as they believed no precipitate with 2,4-DNP suggested no C=O rather than no aldehyde or ketone. Good analysis of the NMR data was crucial for deducing the correct ester. Some candidates opted to annotate the proton NMR spectrum, some produced tables and others gave written details for each peak. It was vital that they were able to interpret all information for each peak i.e. number of proton environments, the type of environment from chemical shift, the number of protons in each environment from relative peak areas and use of splitting patterns to find information about adjacent protons. Many tried to make the data fit their proposed structure rather than the other way round. Some suggested structures that were only partially consistent with the data such as CH3CH2CH2COOC(CH3)3 and were awarded Level 2. Others did not take full note of all the information provided, for example omitting the 2,4-DNP observations, giving the ketone (CH3)3COCH2COCH2CH3 or not checking it matched the molecular formula CH3CH2COOC(CH3)3 so only achieved Level 1. Candidates need to be encouraged to draw a structure as without they can only achieve a maximum of 2 marks despite some excellent analysis of the data. Conversely, it is not sufficient to just give a structure, candidates must give analysis of the data provided. This is a good Level 3 6 mark response. As well as this clearly laid out analysis they also had details written on the question, e.g. no aldehyde or ketone due to no reaction with 2,4-DNP. This response has been selected due to the detailed analysis of NMR data that has been summarised in a table.

Notes from the mark scheme and report

• C4H8O then C8H16O2 from m/z 144
• IR C=O about 1740 but no reaction with 2,4-DNP so not aldehyde/ketone: ester
• singlet 9H C(CH3)3, triplet 3H + quartet 2H CH3CH2CO, singlet 2H OCH2 at 4.1
• CH3CH2COOCH2C(CH3)3
• IGNORE C–O peaks

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

This question was a good discriminator, producing marks across the whole 3 mark range. More successful candidates correctly calculated the energy change, moles of RbClO3 and enthalpy change of solution. However, there were pitfalls for many including the following: • calculating the energy change using the mass of water rather than the mass of the solution. This was despite the suppled information that the specific heat capacity of the solution is the same as for water. Candidates should understand that m in mc∆T is the mass of the substance that produces ∆T • calculating an incorrect value for the molar mass of RbClO3. Instead of 169, this was often seen as 120.5 (using the atomic number of 37 for Rb, rather than the mass number of 85.5) and 185 (for RbClO4) • using values of m at the wrong stages in the calculation. e.g. 2 g with the energy change and 102 g or 100 g with the moles calculation • calculating the correct numerical value for the enthalpy change of solution, but then placing a ‘–‘ sign in front of the value, despite ∆T being for a decrease in temperature. Finally, as with all multi-step calculations, candidates are advised to use calculator values throughout. Any intermediate rounding introduces rounding errors in the final value. The final value can be rounded either to the significant figures demanded by the question or to the lowest number of significant figures used in the provided data.

Notes from the mark scheme and report

• mass of solution 102 g (not 100 or 2.00 g)
• ∆T = 1.5
• n = 2.00/169
• +54.0 to 54.2
• IGNORE sign
• common errors: 100 instead of 102 (53.0) 2 marks, 102 and 2 swapped 1 mark, 8.314 used for c 2 marks
• ALLOW ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

The majority of candidates did not answer this question correctly. Candidates were expected to refer back to the formula of the Tutton’s salt, spot that there was water of crystallisation present, and that this would be lost if all the solvent was boiled off. Many responded vaguely in terms of decomposition or formation of larger crystals but a mark was only awarded if there was a definite link to the water contained within the crystals.

Notes from the mark scheme and report

• prevents water of crystallisation being removed / anhydrous salt forming
• IGNORE 'all the water would be removed', prevents decomposition, larger crystals
• In the specification, Section 2.1.2a states the following: (a) the writing of formulae of ionic compounds from ionic charges, including: (i) prediction of ionic charge from the position of an element in the periodic table (ii) recall of the names and formulae for the following ions: NO3 –, CO3 2–, SO4 2–, OH–, NH4 +, Zn2+ and Ag+u.
• This section will be studied at the start of the 2 year course. Candidates need to be confident with using these common formulae. For success in chemistry, these ions must be learnt.

Mark scheme

Examiner's comment

Addition of NaOH(aq) to the Tutton’s salt results in two reactions: precipitation of copper(II) hydroxide and a reaction of an ammonium ion, used to show its presence as a qualitative test. As with Question 4 (c) (i), this part discriminated very well with many candidates able to be rewarded with some of the marks. The formula of copper(II) hydroxide, as Cu(OH)2 or Cu(OH)2(H2O)2 were both acceptable. This was correct more often than the responses related to the ammonium ion. The formula of the gas formed in the reaction of NaOH(aq) with the ammonium ion caused problems, with NH3 and its subsequent test with moist indicator turning blue seen much less than the reaction of Cu2+(aq) ions. Hydrogen (the ‘squeaky pop test) and oxygen (relighting a glowing split) were common incorrect responses. This was another question in which referring back to the formula of the Tutton’s salt would have revealed important clues.

Notes from the mark scheme and report

• Cu(OH)2 and NH3 (formulae required
• IGNORE names)
• moist indicator/litmus turns blue
• no ECF for a test on the wrong gas
• DO NOT ALLOW unbalanced charges or 'bleaches indicator'

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Th final part of Question 4 required candidates to identify the anion in the Tutton’s salt as sulfate, and to recall that Ba2+ ions is used for the sulfate test to form a white precipitate. Any soluble barium compound was credited with barium chloride and nitrate being the commonest seen. As with earlier parts, this part discriminated very well. Most candidates who knew that barium ions were needed also collected the mark for the white precipitate observation. Over half the candidates did not score here, the most common errors being to repeat the test for the ammonium ion, or to use silver nitrate, clear confusion with the halide test.

Notes from the mark scheme and report

• BaCl2 or Ba(NO3)2 solution gives white precipitate
• observation only from a soluble Ba2+ reagent
• DO NOT ALLOW H2SO4 added
• DO NOT ALLOW AgNO3 before the sulfate test

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Compared with the application based Question 4, candidates answered this stock titration calculation well. Almost all candidates determined that the mean titre was 22.35 cm3 and went on to calculate the number of moles of HCl as 4.47  10–3 mol. Most scaled this value by 10 to determine the moles in 250 cm3. Most candidates then used the initial moles of HCl to determine the moles of aspirin in the 3 tablets as 5.30  10–3 moles. A significant number omitted this stage but they were able to be credited for the next stage of calculation using a correct method. Consequently over half the candidates were awarded 5 or 6 marks for this stock calculation.

Notes from the mark scheme and report

• mean of concordant titres 22.35 (not all three = 22.60, 5 marks max)
• n(NaOH) remaining × 10
• subtract from 0.0500 mol
• × 180 then ÷ 3 tablets = 318 mg
• common errors: no × 10 scaling, no subtraction from 0.05, omitting the titration calculation (3000 mg)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.2 Find arithmetic means; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates drew a diagram that looked like a vertical condenser above a flask. The quality of the diagrams was not very good. Candidates then needed to label their diagram. Errors included a bung or thermometer inserted at the top of the condenser and water flowing the wrong way in the condenser. For labelling, candidates were expected to use scientific terminology. Responses such as ‘condensation tube’ and vague terms such as ‘flask’ were not credited. These labels were often omitted. A significant number drew a set up for distillation instead of reflux.

Notes from the mark scheme and report

• flask AND upright condenser AND open at top
• labels pear-shaped/round-bottom flask, condenser, water in at bottom and out at top
• DO NOT ALLOW conical/volumetric flask, thermometer or sealed system
• heat not required

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Candidates generally answered the first part of this question well. Most candidates were able to identify silver nitrate (or a halogen displacement method), to describe the expected observations, supported with mainly correct ionic equations. Candidates found it much harder to identify B and C as NaBr and CaCl2. They could do this in various ways by matching possible formula with the provided molar mass ranges. The mark scheme did allow marks to be given when candidates described the identification process, although this was often very muddled, so, only the most able few candidates fully identified the unknown B and C.

Notes from the mark scheme and report

• AgNO3 then any two precipitate colours
• ionic equation for at least one halide
• B = NaBr and C = CaCl2 from molar mass range
• HCl CONs AgNO3 (IGNORE HNO3)
• IGNORE solubility in NH3
• ALLOW halogen displacement alternative

Skills: M0.2 Use ratios, fractions and percentages; PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

More successful candidates followed a well-rehearsed method for processing experimental enthalpy results to arrive at an enthalpy change. The result was usually the correct answer of –116 kJ mol–1. Most candidates combined the 2 volumes (25 cm3 and 50 cm3) to give 75 cm3 with m = 75 g and then calculated the correct energy change of 5799.75 J using mc∆T. Some candidates did not combine the volumes and used m = 25 g or 50 g instead. Most candidates worked out the amount in moles of Ba(OH)2 and HNO3. Unfortunately, these calculations were often scattered across the page with no indication of what the calculated values applied to. Some candidates incorrectly combined the moles (0.10 and 0.05) and divided this value (usually 0.15) into the energy change. The mark scheme accounted for these errors and allowed error carried forward marks to be allocated appropriately. Candidates are strongly advised to organise their calculation in a coherent way and to show what each calculated value applies to so that error carried forward can be applied for mistakes. misunderstanding the meaning of m in the equation q = mc∆T, used for finding the energy change q from experimental results. The information states that the density of the solutions is the same as for water, 1.00 g cm–3. Therefore, the mass m that changes temperature ∆T is 75.0 g and the energy change is 75.0  4.18  18.5 J. This candidate has correctly calculated the moles of one of the reactants, Ba(OH)2, as 0.0500 but has then calculated its mass as 75.0  4.18  18.5 J = 8.565 g and has used this in mc∆T. So the key message is that m in mc∆T is where the thermometer has been placed.

Notes from the mark scheme and report

• mass is 75.0 g (both solutions)
• divide by n(Ba(OH)2) = 0.0500 not n(HNO3)
• common errors –58.0, –38.7, –77.3 (3 marks)
• + sign loses 1 mark
• 3 sig figs needed
• In the equation q = mc∆T, mass m is the mass of the substance that changes temperature by ∆T. This substance has a specific heat capacity, c. Looking at the experimental results, the substance changing temperature is the mixture of the two solutions, which have volumes of 25 cm3 and 50 cm3. This is where the thermometer has been placed.

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Although most candidates gained some marks here, there were a significant number who did not gain all 4 marks. Some candidates need to practice reading burettes and recording their values – this can be checked during practical work. The most common errors included not recording values to 2 decimal places (especially if final number was 0), readings recorded the wrong way round in the table, and misreading 0.05 as 0.5. Most candidates were able to identify concordant results, but some still calculated the mean from all values. This led to an average that needed to be rounded to the nearest 0.05, as asked for in the question, resulting in them losing both the mean titre mark and mark for accuracy. Some had errors which lead to values which weren’t concordant – this should be a flag to students that they have made a mistake.

Notes from the mark scheme and report

• all six burette readings correct
• mean from concordant titres 27.30 and 27.40 only (27.35)
• all values to 2 d.p. ending in 0 or 5
• ALLOW ECF from incorrect readings but mean must use closest titres

Skills: M1.1 Use an appropriate number of significant figures; M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates managed to gain at least 1 mark for this question. The most common mark lost was for not multiplying by 10, having missed that only 25cm3 of 250 cm3 citric acid solution was used in the titration. Another mark that was often lost was for not dividing by 2 to find mass in 1 lime rather than the 2 used in experiment. Some candidates used 25cm3 to calculate their moles of NaOH rather than the titre value from (i). It is vital that candidates are given the opportunity to practice more complex multi-step calculations of this type, with modelling given for lower-attaining candidates. Identifying which information goes with each reactant is vital. All steps in the calculation should be separate and clearly labelled to help avoid confusion. Encourage candidates to keep full values in their calculators to avoid intermediate step rounding. When writing down intermediate values, ideally write down the full calculator value or where this is not possible the value must be given to at least 3 significant figures (correctly rounded). It is helpful to the examiner to know if calculator values are used and this could be indicated by using truncated answers followed by … , for example 0.00729… for n(A). The exemplar shows a response where each step of the calculation is shown clearly. The candidate has also used pictures to aid them, recognising that the NaOH is in the burette and lime juice in the conical flask. All values are identified and there is no intermediate rounding. All 5 marks are given here.

Notes from the mark scheme and report

• 1:3 ratio citric acid:NaOH
• ×10 scale-up from 25.0 to 250 cm3
• ÷2 because two limes used
• ALLOW ECF from titre in (b)(i) and through all steps

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

A very challenging question with very few candidates scoring both marks. The response needed a clear indication of how the method would be altered and a justification for why this would work. Lots of candidates recognised the need to dilute the citric acid to obtain the correct titre but were not able to give a method of how to do this or any indication of quantities needed. Some candidates said to use a larger volume of NaOH – not recognising that this would be the titre value, e.g. “in order to keep the same titre but lower concentration of NaOH the student should titre more NaOH”. Some gave the method of how to dilute the NaOH or even just said to add water. A few suggested using a higher concentration of lime juice. Candidates need to be given opportunities to plan practical work to fully appreciate the impact that any changes will have (specification 1.1.1).

Notes from the mark scheme and report

• use half a lime or make up juice in 1 dm3 volumetric flask
• justified by factor of 4 (NaOH 4 times more dilute)
• ALLOW any feasible method giving dilution factor of 4
• Further information about practical skills assessed on written exams can be found in section 3 of the practical skills handbook - https://www.ocr.org.uk/Images/208932-chemistry-practical- skills-handbook.pdf. If using our suggested practicals, then encourage candidates to answer the extension opportunity questions to help develop a deeper understanding in preparation for written assessments.

Skills: M0.2 Use ratios, fractions and percentages; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates were able to give the formula of the barium compound as BaSO4. However, they did not recognise that this would cause a white ppt to be seen, presumably as not in the context of qualitative ions testing. Many candidates said they would see bubbling/fizzing. Some gave a colour change as they were possibly considering what would be seen if an indicator is present. Others mentioned a precipitate but with no colour given. Some candidates gave the incorrect formula, such as Ba2SO4 or Ba(SO4)2, again showing the importance of practising writing formulae. In addition, some candidates wrote out the whole equation for the reaction.

Notes from the mark scheme and report

• white precipitate
• BaSO4
• We have produced a topic support pack to assist with learning about the reaction of group 2 elements and their compounds: http://www.ocr.org.uk/Images/364103-chemistry-of-group- 2.docx

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Most candidates recognised that they needed to draw a tangent. Candidates need to be instructed to draw their line clearly and make sure that it covers several large squares on the graph – preferably extending to the axes. The range given for the calculated rate from the gradient of the tangent was generous, however some did not realise that the y-axis started at 91.0 so incorrectly calculated their gradient. Some candidates used interpolation so gained no marks.

Notes from the mark scheme and report

• tangent drawn at t = 50 s (±10 s)
• DO NOT ALLOW interpolation/direct reading - must be a gradient
• range 5.7 × 10^-3 to 6.9 × 10^-3
• IGNORE units and sign
• ALLOW ECF from incorrect tangent
• M3 section of the Maths Skills handbook contains useful information on use of graphs in chemistry, including M3.5 on drawing and using the slope of the tangent to a curve as a measure of a rate of change: https://www.ocr.org.uk/Images/295468-chemistry-mathematical- skills-handbook.pdf

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

About a third of candidates didn’t score any marks here. Some did not follow the instructions to add their curve to the previous graph. The sketches produced in the blank space then usually lacked any scales on the axes so could not be given marks. Lines added were often inaccurately drawn with lines often extending below 91.55g before returning to this mass at the end. It was often difficult to interpret if a graph levelled off before the original with lots tracking the same curve with a very slightly steeper slope. Some gave a slower rate but did manage to gain marks for finishing at 91.55g.

Notes from the mark scheme and report

• steeper slope AND levels off earlier
• same loss in mass - levels off at about 91.55 g

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question was not answered well with over half the candidates failing to score any marks. While some candidates seemed familiar with the techniques required, describing the process to separate the layers, they often struggled to name the separating funnel. Common approaches were to attempt to ‘filter’ the layers or to use heat (via evaporation or distillation) to drive off the water. Some attempted to use Na2CO3 or NaOH to dry the organic layer – perhaps confusing neutralisation of any remaining acid. Although distillation appeared frequently many did not give the temperature so did not gain marks. The order of the procedure was also not always clear with distillation before using a drying agent. Some described attempts to crystallise the organic layer. The range of answers suggests students may need more practical experience with separating organic liquids.

Notes from the mark scheme and report

• separating funnel to separate layers
• dry organic layer with anhydrous salt (MgSO4, CaCl2)
• distil and collect fraction at 91 °C

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Many candidates were able to explain clearly the second order dependence on [NO] but many found it more challenging to identify and explain the first order relationship for [CO], where the candidate had to consider the second order dependence on [NO] to work out the effect on rate for the change in [CO]. Most gave a rate equation of the correct format based on the orders they had stated, and worked out a value for k. Although some did not determine the correct units, there was an encouraging number who managed this either from memory or by cancelling. The most common error was not including s-1

Notes from the mark scheme and report

• NO second order from expts 1 and 2
• CO first order needs both concentrations changing (expts 2 and 3: rate × 16)
• k = 3.37 × 10^6 dm6 mol–2 s–1
• common errors 4.65 × 10^9 mol–3 dm9 s–1 (CO second order) and 2446 mol–1 dm3 s–1 (CO zero order) score 4 with ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Many candidates made some attempt at this unusual twist on a Q = mcΔT calculation, but only a minority of candidates produced a fully correct answer. A temperature of 84.9 oC was seen much more often than 52.4 oC. The calculation of n(AgNO3) was performed well. Common errors were not dividing the moles by 2, not recognising the need to calculate the energy released per mole of AgNO3, confusing the mass of water with the mass of the silver nitrate solid or of using an energy in kJ alongside a value of c based on J.

Notes from the mark scheme and report

• n(AgNO3) = 0.0400
• energy = 678 × 0.0400/2 = 13.56 kJ (2 mol AgNO3 per ∆H)
• ∆T = 32.4
• add 20.0 gives 52.4 °C to 3 sig figs
• common errors 32.4 (∆T only) and 84.9 (no ÷2) score 3

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Although there were a few well explained, correct answers, this question proved challenging for most candidates. Candidates needed to link the changes as a proportion (e.g. half) to be given the mark. Candidates often only considered the change to energy or mass, but not both, e.g. dealt only with the decrease in moles reacting leading to a smaller temperature rise.

Notes from the mark scheme and report

• same maximum temperature AND half the energy/moles AND half the mass/volume

Skills: M0.2 Use ratios, fractions and percentages; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Almost all candidates calculated the titres correctly, but a significant number were penalised for recording 22.70 as 22.7. A significant number also used this result and the trial titre to derive their mean value. There were also very small numbers of candidates who used all 4, or all of titres 1, 2 and 3, to calculate their mean.

Notes from the mark scheme and report

• titres to 2 d.p. (22.70 not 22.7)
• mean from concordant 22.45 and 22.55 = 22.50
• DO NOT ALLOW use of trial
• Links to the legacy coursework tasks and PAG practice question sets can be found on OCR Interchange. Exam hints for students can be found at: https://www.ocr.org.uk/Images/592305-exam-hints-for-students.pdf.

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Candidates made good progress with this calculation, many gaining 4 or 5 marks, including error carried forward from incorrect titres. Common errors included, in various combinations: not converting the final answer into mg, not converting volume to dm3, missed ratio, multiplying the moles in 10cm3 acid by 10 instead of 25 and/or wrong Mr. Responses to Question 19 (a) (ii) often featured rows of figures and random sums without a single word about what the figures, or sums, were set to calculate. Candidates should remember to provide written indications of what it is they are working out – presenting the calculations without any annotations can make it harder for error carried forward marks to be given if there is an error in their calculation. The exemplar here shows a good use of annotation. There is a clear indication of the mathematical process so that the error carried forward is easily identified and the candidate gains the method marks.

Notes from the mark scheme and report

• ÷2 (dibasic)
• × 25 to scale 10.0 cm3 to 250.0 cm3
• × 118
• ÷4 tablets and convert to mg = 498
• common errors 996 (no ÷2), 19.9 (no × 25)
• 503 from titre 22.725 scores 5 by ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

The use of 2,4-dinitrophenylhydrazine as a test for the carbonyl group is well known by candidates at this level. The majority of the cohort correctly identified this test and the subsequent analysis of the melting point of the products as a method of identifying each compound. Lower ability candidate responses made reference to analysis of the boiling points or omitted the reference to analysis of the melting points.

Notes from the mark scheme and report

• 2,4-DNP(H) AND orange/yellow/red precipitate
• take melting point of the derivative
• compare with known values
• DO NOT ALLOW boiling points or chromatograms

Skills: PAG6 Synthesis of an organic solid; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Almost all candidates were able to correctly describe the use of Tollens’ reagent as a test for an aldehyde fictional group

Notes from the mark scheme and report

• Tollens' reagent AND silver mirror
• ALLOW acidified dichromate turning green

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Few candidates knew the test for a phenol group. Frequent incorrect responses involved the production of a gas with a carbonate or stating that bromine water is decolourised but failing to state that a white precipitate is also formed. A significant number of candidates also stated that the reaction with sodium hydroxide confirms the presence of the phenol group.

Notes from the mark scheme and report

• indicator shows acidity AND no reaction with carbonate
• ALLOW bromine giving white precipitate
• ALLOW FeCl3 giving violet/purple colour

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

Showed limited knowledge and understanding of practical scenarios and drawing labelled diagrams of apparatus, e.g. • Question 3 (a): Diagram of reflux • Question 3 (a): Purification of an organic liquid There was no evidence that any time constraints had led to a candidate underperforming or of scripts where there were no responses to many questions.

Notes from the mark scheme and report

• reflux diagram: condenser above flask, water in at bottom and out at top
• mass = 6.10 × 1.268 = 7.73 g, yield 45.2% (35.6% if density omitted)
• separating funnel - organic layer is the BOTTOM layer (denser), dry with named anhydrous salt, redistil at 102 °C
• bare 'separate, dry, redistil' not detailed

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

This question asked candidates to explain the different rates from a novel experiment carried out at 50ºC and 90ºC, and to predict the ∆H sign for the forward reaction. Candidate explanations for the rates were often superficial, solely in terms of greater energy at 90ºC. Many responses referred neither to the different frequency of collisions nor the greater number of particles exceeding the activation energy at 90ºC. Most candidates predicted that ∆H would have a negative sign. Candidates were expected to link the evidence from the absorbance data in the graph to less [Co(H2O)6]2+ being present at 90ºC. When experimental information has been presented, candidates are advised to look for the evidence responsible in their explanations.

Notes from the mark scheme and report

• at 90 °C faster rate AND more frequent collisions
• more particles have energy ≥ Ea
• equilibrium [Co(H2O)6]2+ is lower at 90 °C so forward reaction exothermic/∆H negative
• IGNORE more successful collisions

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme, with the levels and indicative points

Examiner's comment

This question was assessed by level of response (LoR). Candidates were required to process raw experimental results to determine one enthalpy change, and then to determine a second enthalpy change by using an energy cycle. Levels were determined by the accuracy of the candidates’ processing of the results, calculations and use of the energy cycle. Marks within a level were determined by communication. This question discriminated extremely well. Level 3 candidates used the mass of solution as 106.6 g with mc∆T to obtain an energy change of 8.24 kJ. They then divided this value by the moles of CaCl2 that reacted (0.0835 mol) to obtain the enthalpy change of –98.7 kJ mol–1. Finally, they constructed an energy cycle which they then used to obtain the second enthalpy change of –1566 kJ mol–1. Level 2 candidates determined the first enthalpy change but may have used the approximate mass of 100 g for the mass of solution from the experimental method. Their energy cycle was often incorrect or absent, with the second enthalpy change calculated incorrectly. Level 1 candidates often calculated the initial energy change using mc∆T but made little further correct progress. Less successful responses used the solid mass of calcium chloride (9.28 g) instead of the mass of the solution in their mc∆T calculation. Overall, mathematical skills were displayed well but some basic errors were made, particularly with subtractions. This may have been the result of mis-keying values into a calculator and believing the answer displayed. calcium chloride and the first enthalpy change using a correct method. This response has not been given marks for the communication strand of Level 3 because there is nothing to indicate what the numbers refer to. The response could be summarised as a mass of numbers scrawled across the page. Unfortunately, this is the pattern of many responses. So, this response was given 3/6 marks.

Notes from the mark scheme and report

• q = 106.6 × 4.18 × 18.5 = 8.24 kJ (mass of final solution)
• n(CaCl2) = 9.28/111.1 = 0.0835
• ∆solH = –98.7
• ∆hydH(Ca2+) = –2223 + (–98.7) – 2(–378) = –1566
• 2 × hydration of Cl–
• one small error allowed e.g. using 97.32 g or 115.88 g for m

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations; PAG3 Enthalpy determination

Mark scheme

Notes from the mark scheme and report

• mix each halogen with a different halide
• Cl2 + bromide orange, Br2 + iodide violet, I2 no change
• ionic equation
• IGNORE addition of halogen to same halide

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Notes from the mark scheme and report

• labelled condenser above a flask
• second mark (flask and heat labelled) only if first mark awarded
• ALLOW water in and water out as condenser label

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Notes from the mark scheme and report

• n(Sc) = 0.27/45 or n(O) = 0.144/16
• Sc2O3
• NO ECF

Skills: M0.2 Use ratios, fractions and percentages; PAG1 Moles determination

Mark scheme

Notes from the mark scheme and report

• heat to constant mass
• IGNORE 'heat for longer' with no link to constant mass

Skills: PAG1 Moles determination

Mark scheme

Notes from the mark scheme and report

• colourless to pink (alkali in burette)

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• all titres to 2 d.p. ending in 0 or 5
• DO NOT ALLOW 1 decimal place

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• to estimate the titre/rough idea of end point

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• mean of concordant titres only (18.20 and 18.30) = 18.25

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• 2 × 0.05 = 0.1 uncertainty in a titre
• ALLOW ECF from (b)(i) or use of other titre values

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• 1:2 ratio acid:NaOH
• scale × 10 to 250 cm3
• M = 132 then subtract 90 for two COOH and divide by 14
• whole number required
• ALLOW ECF throughout and from (b)(iii)

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations; PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• titre less
• because glutaric acid solution is diluted/less concentrated

Skills: PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Notes from the mark scheme and report

• mass of solution 103.01 g from weighings (using 100 g cannot reach Level 3)
• q = mc∆T = 2153 J
• n(MgCO3) = 0.0500
• ∆H2 = –43.06 with sign
• ∆H1 = ∆H2 – ∆H3 = +93.04 kJ mol–1

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.4 Solve algebraic equations; PAG3 Enthalpy determination

Mark scheme

Notes from the mark scheme and report

• smooth best-fit curve
• tangent at t = 50 s
• gradient about 0.44 cm3 s–1
• DO NOT ALLOW interpolation (direct reading from graph)
• ALLOW ECF from incorrectly drawn tangent

Skills: M3.2 Plot two variables from experimental or other data; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Notes from the mark scheme and report

• advantage: no loss of gas (before syringe connected)
• disadvantage: loss in mass (of H2) very small for a 2 d.p. balance
• IGNORE easier to set up

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme, with the levels and indicative points

Notes from the mark scheme and report

• separating funnel – organic layer is the TOP layer (lower density)
• dry with anhydrous MgSO4/CaCl2
• distil collecting at 50 °C
• mass = 7.70/74.0 × 0.76 × 92.5 = 7.3 g
• inverse ratio 100/76 gives 12.7 g
• IGNORE washing with carbonate/water

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Notes from the mark scheme and report

• starch AND blue-black to colourless
• IGNORE 'clear'

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Question 18(d): This was a percentage by mass calculation.

Notes from the mark scheme and report

• n(Cu2+) = n(S2O3 2–) (2Cu2+ : I2 : 2S2O3 2–)
• 3 SF (1.35%)
• ECF from ratio in (b)
• ECF needs a calculated mass of Cu

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• lower AND smaller titre/less I2 produced

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• the same AND burette volume is measured by difference

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• any two: make up to a standard volume and titrate portions
• repeat and use mean of concordant titres
• change thiosulfate concentration to give a suitable titre
• 3 d.p. balance

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Question 19(a)(i): Most candidates were able to draw a circuit containing a voltmeter and the chromium half-cell but very few candidates included the H+ in the MnO4-/Mn2+ cell. 19(a)(ii) and (b)(i): Most candidates were able to construct the equation for the overall cell reaction and to use oxidation numbers to explain disproportionation. .

Notes from the mark scheme and report

• complete circuit with voltmeter AND labelled salt bridge
• Cr electrode in Cr3+
• Pt electrode in MnO4– AND H+ AND Mn2+
• electrodes must dip into solutions

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Question 21(a)(i): Most candidates found the order with respect to S2O32- by comparing experiments 1 and 2.

Notes from the mark scheme and report

• expts 1 and 2: [S2O3 2–] halves, rate halves so first order
• expts 2 and 3: [S2O3 2–] ×¼ and [H+] halves, rate ×¼ so zero order in H+

Skills: M0.2 Use ratios, fractions and percentages; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Question 22(b)(ii): This question asked the candidates to calculate the enthalpy change and then to deduce the value for the enthalpy change of neutralisation.

Notes from the mark scheme and report

• mass = 80.0 g (total volume)
• divide by n(H2SO4) (DO NOT ALLOW n(NaOH))
• ∆neutH = ∆H1/2
• – sign required

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Notes from the mark scheme and report

• same temperature rise (13 °C): double the energy spread over double the volume

Skills: M0.2 Use ratios, fractions and percentages; PAG3 Enthalpy determination

Mark scheme

Notes from the mark scheme and report

• dissolve in hot solvent
• minimum amount
• cool AND filter AND dry (all three)
• DO NOT ALLOW adding a drying agent
• IGNORE hot filtration

Skills: PAG6 Synthesis of an organic solid

Mark scheme, with the levels and indicative points

Notes from the mark scheme and report

• D no C=O, oxidised so primary/secondary alcohol, saturated, 3 carbon environments with C–O at 73 (alicyclic alcohol)
• E and F ketones (not oxidised so not aldehydes)
• E quartet 2.4 + triplet 1.1 (CH3CH2CO)
• F heptet 2.6, singlet 2.1 (CH3CO), doublet 1.1

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Q1(d), Q1(e), Q5(a)(i) • Unclear drawing of diagrams and labelling of laboratory apparatus: Q2(a) • Had difficulty explaining chemical ideas, often repeating the same points several times, e.g. Q2(a): Candidates were asked to draw a labelled diagram for a familiar practical set up.

Notes from the mark scheme and report

• closed system with reaction vessel and gas syringe or collection over water in a measuring cylinder
• DO NOT ALLOW delivery tube below reaction mixture
• DO NOT ALLOW beaker, volumetric flask or measuring cylinder as reaction vessel

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Q1(a) Partial pressure calculation Q1(d) Formula determination from percentage composition by mass Q2(b) Converting mass and volume into moles Q3(b) Unstructured titration calculation Q5(a)(ii) Ideal gas calculation Q5(b)(i) Calculation of the pH of a weak acid • Produced clear and concise responses for a Level of Response question based on a familiar context i.e., Q4(b) rates by continuous monitoring and graphical analysis. • Had difficulty when answering questions requiring application of knowledge and understanding on questions set in novel and unfamiliar contexts, e.g. Q2(b): Candidates were required to analyse the experimental results to obtain a conclusion. Q2(d): Candidates found it much more difficult to evaluate the unfamiliar scenarios in Q2(c) and Q2(d) than the stock mole calculation in Q2(b).

Notes from the mark scheme and report

• n(H2) = 152/24000 = 6.33 × 10–3
• n(Eu) = 0.988/152 = 6.50 × 10–3
• ratio 1:1 so Equation 2
• conclusion only if the two amounts are approximately equal
• ALLOW ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; PAG1 Moles determination

Mark scheme

Examiner's comment

Q2(c), Q2(d): Predicting the effect of modifications to an experiment Q5(c): Interpreting information in a Level of Response question provided in an unfamiliar contexts • Had difficulty when writing equations for unfamiliar reactions, e.g. Q2(c): Candidates needed to evaluate the effect on the results and conclusion of the apparatus getting hot. Q2(d): Candidates found it much more difficult to evaluate the unfamiliar scenarios in Q2(c) and Q2(d) than the stock mole calculation in Q2(b).

Notes from the mark scheme and report

• gas volume would be larger (than at RTP)
• ratio H2 : Eu would be larger (ALLOW Equation 3 linked to ratio > 1)
• IGNORE effect on rate

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Q2(c), Q2(d): Predicting the effect of modifications to an experiment Q5(c): Interpreting information in a Level of Response question provided in an unfamiliar contexts • Had difficulty when writing equations for unfamiliar reactions, e.g. Q2(d): Candidates found it much more difficult to evaluate the unfamiliar scenarios in Q2(c) and Q2(d) than the stock mole calculation in Q2(b).

Notes from the mark scheme and report

• precipitates EuOH, Eu(OH)2, Eu(OH)3 have different molar masses/formulae
• Equation 2 gives Eu(OH)2, M = 186, i.e. 1.86 g
• or M = mass of precipitate/0.01

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Q3(a): Candidates were required to describe how they would prepare 250 cm3 of a 0.150 mol dm–3 standard solution. Q3(b): As with Question 2, candidates had more success with the mole calculation in Q3(b) than the descriptive response in Q3(a).

Notes from the mark scheme and report

• mass Ba(OH)2 = 0.0375 × 171.3 = 6.42 g
• dissolve in distilled water in a beaker
• transfer solution AND washings to 250 cm3 volumetric flask
• make up to the mark AND invert
• DO NOT ALLOW round-bottom or conical flask as the volumetric flask

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Q1(a) Partial pressure calculation Q1(d) Formula determination from percentage composition by mass Q2(b) Converting mass and volume into moles Q3(b) Unstructured titration calculation Q5(a)(ii) Ideal gas calculation Q5(b)(i) Calculation of the pH of a weak acid • Produced clear and concise responses for a Level of Response question based on a familiar context i.e., Q4(b) rates by continuous monitoring and graphical analysis. • Had difficulty when answering questions requiring application of knowledge and understanding on questions set in novel and unfamiliar contexts, e.g. Q3(b): As with Question 2, candidates had more success with the mole calculation in Q3(b) than the descriptive response in Q3(a). See also comments made for Q1(e) and Q3(b).

Notes from the mark scheme and report

• n(D) in 25 cm3 = 2 × 3.525 × 10–3
• ×4 for 100 cm3
• M = 114 so C5H9COOH
• any 2 cis isomers with different groups on each C of C=C
• common errors: scaling omitted (M = 456), no × 2 (M = 228), 25.00 used as titre (M = 100.8)
• 1 mark for 2 trans isomers

Skills: M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Q4(a) required candidates to identify that pent-1-ene was in great excess.

Notes from the mark scheme and report

• (large) excess of pent-1-ene / its concentration is much greater so effectively constant

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme, with the levels and indicative points

Examiner's comment

Q1(a) Partial pressure calculation Q1(d) Formula determination from percentage composition by mass Q2(b) Converting mass and volume into moles Q3(b) Unstructured titration calculation Q5(a)(ii) Ideal gas calculation Q5(b)(i) Calculation of the pH of a weak acid • Produced clear and concise responses for a Level of Response question based on a familiar context i.e., Q4(b) rates by continuous monitoring and graphical analysis. • Had difficulty when answering questions requiring application of knowledge and understanding on questions set in novel and unfamiliar contexts, e.g. Q4(b) is a Level of Response question which is commented on after this section. Q5(c) is a Level of Response question which is commented on after this section Level of response questions Q4(b): Candidates were required to analyse the graphical results of a rates investigation to determine the reaction order, rate and rate constant.

Notes from the mark scheme and report

• tangent at t = 0 gives initial rate 4.5–6.5 × 10–6 mol dm–3 s–1
• constant half-life of about 2500 s (two half-lives shown) so first order
• k = rate/[I2] or ln2/t½ with units s–1

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Q4(c)(ii) Candidates needed to suggest monitoring the change in rate with changing pent-1-ene concentration while keeping the iodine concentration constant.

Notes from the mark scheme and report

• keep [I2] constant or use a large excess of I2
• monitor [pent-1-ene] over time or how [pent-1-ene] affects rate
• ALLOW initial rates approach with different pent-1-ene concentrations

Skills: PAG10 Rates of reaction – initial rates; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• within 0.1 cm3
• DO NOT ALLOW incorrect units e.g. dm3

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• DO NOT ALLOW 4.3 × 10–3 for n(HNO3) but ECF for remaining marks
• 1:2 ratio Ba(OH)2:HNO3

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

23(a) This question required candidates to calculate the moles of Ba(OH)2 in 100 cm3, to scale this value up to 1 dm3 for the concentration of Ba(OH)2 and finally to multiply this value by 2 to obtain the concentration of OH– ions of 0.454 mol dm–3. This part discriminated extremely well. Lower-attaining candidates would often calculate an incorrect molar mass of Ba(OH)2 and scale up by either 10 or 2. The commonest incorrect answer seen was 0.227, half of the correct answer by omitting the ×2 stage. Most candidates did supply their final answer to the required 3 significant figures. As with all calculations, candidates should round once, at the end of the calculation. Rounding of intermediate values can lead to significant rounding errors. Sometimes, intermediate values were rounded to 2 significant figures when the final value required 3. This use of maths will almost certainly lead to a rounding error in the final answer. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. Many candidates produced largely unreferenced numbers. 23(b) Many candidates seemed to be unfamiliar with the term ‘concordant titres’. Acceptable responses were ‘within 0.1 cm3 and within 0.05 cm3. Common incorrect answers for this important term included ‘the closest titres’, ‘similar titres’, ‘repeated titres’ and ‘within 0.5 cm3’. 23(c) Many candidates were able to calculate the amount of HNO3 in the titration as 4.28 × 10–3 mol. Most candidates were credited for the amount of Ba(OH)2 as 2.14 × 10–3 mol, half the calculated amount of HNO3. Candidates then need to scale up this value by 1000/25 to obtain the concentration as 0.0856 mol dm–3. Unlike Q23(a), all intermediate calculations gave values to 3 significant figures. Candidates found this stock titration calculation easier than the more unfamiliar 23(a), with many scoring all 3 marks. Discrimination was extremely good, but about a third of candidates did not receive any marks. Candidates should be encouraged to practise stock titration calculations as part of their preparation for the examinations. Candidates should show clear working so that credit can be given for such responses by applying error carried forward. As with Q23(a), many candidates produced largely unreferenced numbers. 23(d) Candidates often find questions linked to practical work challenging. The two reaction routes involving Group 2 reactions are drawn directly from the specification: the reaction of barium with water, and the reaction of barium with oxygen, followed by water. Most candidates found this difficult, and the one-step route using barium and water was seen far more often than the two-step oxygen then water approach. Only the highest-attaining candidates were able to suggest two routes, with scores of 3/4 or 4/4 marks being comparatively rare. As with 21(c)(ii) and 25c(iii), candidates had difficulty in interpreting provided information. OCR support Our new practical question packs, split up by PAG, provide additional opportunities for students to practise questions relating to practical work. They are available on Interchange: https://interchange.ocr.org.uk/Downloads/H432_H433_Chemistry_PAG_ Practice_Question_Sets.zip

Notes from the mark scheme and report

• Route 1 Ba + 2H2O
• Route 2 via BaO then water
• ALLOW multiples
• 1 mark for both reactants O2 then H2O in route 2 without equations

Mark scheme, with the levels and indicative points

Examiner's comment

1(a) A considerable number of candidates did not score the mark here despite knowing that it was an oxidation reaction. They either omitted to give the number of electrons lost or gave their justification using oxidation numbers. This question highlights the importance of reading the question carefully and answering the question asked, not one that may have been seen previously. 1(b)(i) Most candidates were able to make some progress with this calculation. Common errors included omitting to multiply by two to find the number of moles of HCl or not multiplying their volume in dm3 by 1000 to give an answer in cm3. 1(b)(ii) More candidates struggled with this part of calculation. A significant number of candidates used the number of moles rounded to 3 SF but then gave a final answer to 4 SF, giving an answer of 78.48 cm3. This was given credit as correct calculation had been carried out for one mark. A couple of candidates correctly used the ideal gas equation and were given credit for this, but made it more complicated than necessary for one mark. 1(b)(iii) Very few candidates managed to gain marks here. Most did not recognise that HCl was the limiting reagent in the reaction and suggested that a greater volume of H2 would be produced, as same mass of Mg would give a greater number of moles. 1(c)* Very few candidates managed to score full marks for this question. Even the highest-attaining candidates struggled with writing balanced chemical equations. The most successful candidates used ionic equations with state symbols in their responses. A large proportion of candidates gave unnecessary details such as testing for CO2 using limewater or the colours of other silver halide precipitates. The best responses broken down their response to cover each test in turn, giving clear and concise details for each.

Notes from the mark scheme and report

• acid then fizzing for carbonate
• Ba2+(aq) white precipitate for sulfate
• AgNO3(aq) cream precipitate for bromide
• sequence carbonate then sulfate then halide
• equations ionic or full, IGNORE state symbols

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

2 (a) Most candidates managed to gain at least one mark for this question having learnt the definition. Some lost a mark for stating one mole of substance gives one mole of product, clearly confusing their definitions. 2(b) Nearly all candidates managed to gain at least one mark for correctly calculating the moles of cyclohexane burnt and the majority also used q = mc∆T. However, very few candidates managed to gain all 4 marks, most frequently losing the final mark for SF and sign. Other common errors included using the mass of cyclohexane in calculation of q, using Gas Constant R rather than specific heat capacity of water, or not using ∆T but using 293 K instead. 2(c)(i) A significant number of candidates struggled to correctly calculate the % uncertainties for the apparatus given or did not back up their claims with any calculations. Even the highest-attaining candidates often did not account for two measurements for temperature, or tried to calculate the % uncertainty for each temperature measurement taken. 2(c)(ii) Most candidates gained at least one mark here from one of the three acceptable responses. There was some evidence of candidates being confused here by the use of average bond enthalpies to find values of enthalpy of combustion. Others discussed human errors, need for repeat measurements or precision of equipment, demonstrating a lack of understanding with regard to the language used in scientific measurement. 2(c)(iii) This was a challenging question. Some candidates gained one mark for reduced accuracy due to increased heat loss or water evaporation. The responses given demonstrated that candidates often do not understand the factors that influence the accuracy in scientific experiments. Some indicated that repeating results would improve accuracy. Another common response was that a longer time would allow for complete combustion, failing to understand that degree of combustion is due to the availability of oxygen, not the time taken.

Notes from the mark scheme and report

• –2680: 3 SF AND negative sign (+2680 scores 3)
• mass of water 200 g not mass of fuel in q = mc∆T
• common error –7.02 scores 3
• ALLOW ECF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Question 2(c) demonstrated that some candidates are unsure on what is meant by accuracy and the impact that uncertainties have.

Notes from the mark scheme and report

• two thermometer readings so 1/20 × 100 = 5% vs 2/200 × 100 = 1%
• 1 mark if 0.05 and 0.01 given with correct statement

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Question 2(c) demonstrated that some candidates are unsure on what is meant by accuracy and the impact that uncertainties have.

Notes from the mark scheme and report

• heat loss to surroundings / incomplete combustion / non-standard conditions
• IGNORE reference to evaporation

Skills: PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Question 2(c) demonstrated that some candidates are unsure on what is meant by accuracy and the impact that uncertainties have.

Notes from the mark scheme and report

• less accurate from greater heat losses vs more accurate from smaller % uncertainty in temperature change or mass of fuel
• ALLOW error for uncertainty

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most candidates drew a circuit containing a voltmeter and the silver half cell but very few candidates included the H+ in the MnO4-/Mn2+ cell.

Notes from the mark scheme and report

• voltmeter and labelled salt bridge
• Ag in 1 mol dm–3 Ag+
• Pt in H+/MnO4–/Mn2+ (1 mol dm–3 or equimolar)
• IGNORE pressure
• 1 mark for both half-cells if concentrations omitted

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Most candidates used incorrect ideas about reaction going to completion or the methanol not being limiting.

Notes from the mark scheme and report

• IGNORE 'methanol does not run out/not limiting'

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Very few candidates were given full marks. Higher-attaining students calculated one half life in range but very few could come up with a second half life as the graph did not allow another successive half life to be obtained. Higher-attaining candidates chose alternative half lives from the data given.

Notes from the mark scheme and report

• two half-lives shown on graph (102–110 min) AND statement that half-life is constant
• values on the graph.
• Further guidance on rates of reaction can be found at: https://www.ocr.org.uk/Images/371956-experiments-on-rates-of-reaction.doc

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question required the candidate to draw a line of best fit and then draw a tangent at t=0. Many candidates did not draw a line of best fit, and many did not get a tangent in the acceptable range. Very few candidates processed the gradient by using the correct subtraction on the y axis (scale was from 1 to 5) or by using the 10-3 on the axis label. Electrode potentials

Notes from the mark scheme and report

• tangent at t = 0 (gradient 2.9–4.0 × 10^–5) OR k = ln 2/t½ then rate = k × 5.0 × 10^–3
• note 10^–3 on y-axis
• ALLOW ECF from t½

Skills: M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

This question required the candidate to explain the reactivity of the halogens given experimental observations. Higher-attaining candidates were able to explain the observations with ionic equations and explain the reactivity in terms of gaining electrons. Some candidates did not associate the colour with the halogen and linked it with the halide ion, but then did explain the trend in reactivity due to the ability to gain electrons. Lower-attaining candidates explained the reaction in terms of displacement (which was ignored) and they did not proceed with ionic equations or describe the ability to gain electrons. Key teaching and learning points – comments on improving performance Candidates could improve their performance in this examination by: • Practising the skills required to complete multiple choice questions. • Showing clearly structured working in their calculation questions so that methods are understood and followed. • Practising linking experimental observations with theoretical understanding to improve the quality of descriptions and explanations. • Making sure that they use the appropriate terminology. • Developing their ability to write ionic equations. • Improving their ability to process experimental results from graphical data. Guidance on using this paper as a mock This paper should be used in conjunction with H432/02 and H432/03. Candidates should sit the paper under examination conditions in the allotted time. The marked paper should be reviewed, in conjunction with the mark scheme, by the student to identify the terminology and structure required within the questions to achieve full credit. Internal moderation will add validity to the credit given. If any of your students’ results are not as expected, you may wish to consider one of our review of results services. For full information about the options available visit the OCR website. If university places are at stake you may wish to consider priority service 2 reviews of marking which have an earlier deadline to ensure your reviews are processed in time for university applications. Our priority is supporting you and your students this autumn and to support you as you prepare for summer 2021 exams. We’ll update our website information regularly with resources, guidance and key information. • Teachers • Students • Exams officers • Assessment specialists We are sending a weekly roundup to tell you about important updates. You can also sign up for your subject specific updates. If you haven’t already, sign up here. Attend one of our popular CPD courses to hear directly from a senior assessor or drop in to a Q&A session. All our courses for the academic year 2020-2021 are being delivered live via an online platform, so you can attend from any location. Please find details for all our courses on the relevant subject page on our website or visit OCR professional development. ExamBuilder is the question builder platform for a range of our GCSE, A Level, Cambridge Nationals, Cambridge Technicals and Functional Skills qualifications. See the full list of available qualifications in the sign up form. ExamBuilder is free for all OCR centres with an Interchange account and gives you unlimited users per centre. We need an Interchange username to validate the identity of your centre's first user account for ExamBuilder. If you do not have an Interchange account please contact your centre administrator (usually the Exams Officer) to request a username, or nominate an existing Interchange user in your department. Review of results

Notes from the mark scheme and report

• orange = bromine, no reaction with Cl–
• violet = iodine formed
• Br2 + 2I– → 2Br– + I2
• reactivity/oxidising power decreases down group: larger radius/more shielding, less nuclear attraction for gained electron
• DO NOT ALLOW losing electrons/ionisation energy
• IGNORE explanations in terms of displacement
• rather than electron affinity. The colour of the organic layer was also associated with the halide ion rather than the halogen.
• Further guidance can be found in the AS Level delivery guide ‘Theme: Patterns’ (Group 2 and Group 17): https://www.ocr.org.uk/Images/231740- patterns.pdf Links to the legacy coursework tasks and PAG practice question sets can be found on OCR Interchange.

Skills: PAG4 Qualitative analysis of ions

Mark scheme, with the levels and indicative points

Notes from the mark scheme and report

• B = FeSO4, D = Fe(OH)2, G = BaSO4
• C = CrCl3, E = Cr(OH)3, F = [Cr(NH3)6]3+, H = AgCl
• ionic equations for D–H
• Level 3 needs at least six identified and equations mostly correct

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Question 19(b) Candidates who found this question difficult often did not give a response that would identify all three of the functional groups (aldehyde, ketone and alkene).

Notes from the mark scheme and report

• each mark needs compound/group, reagent AND observation
• F: Tollens' gives silver mirror
• G: bromine decolourised, 2,4-DNP gives orange precipitate, no change with Tollens'
• IGNORE 2,4-DNP with F

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

Level of response questions Question 20(a) This question differentiated well.

Notes from the mark scheme and report

• mass of cyclopentanol 7.90 g (errors: inverse ratio 3.24 g, ignoring yield 5.06 g)
• neutralise e.g. Na2CO3
• separating funnel with organic layer on top
• dry with anhydrous salt e.g. MgSO4
• redistil at about 44 °C

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

1a(i) Most candidates labelled some of the apparatus correctly and identified the reflux technique. A significant number showed water flowing in the wrong direction and ‘distillation’ was given as the name of the technique. The ‘condenser’ was sometimes labelled incorrectly, e.g. ‘condensation tube, ‘distillation tube’ and ‘water jacket’. Only just over half the candidates were given both marks. 1a(ii) Many diagrams were incomplete and it was comparatively rare for both of the two available marks to be given. Important labels were often missing. Some candidates drew diagrams of other techniques, such as distillation. Many responses were not credited with marks and this question was often omitted. Candidates need practice in recognising practical techniques and in drawing acceptable diagrams. 1b(i) Candidates were expected to explain the different boiling points of three amines. Most candidates linked increased branching with less surface contact between molecules, leading to weaker induced dipole interactions (London forces). Fewer candidates linked the decreasing boiling points to less energy being required to break the intermolecular bonds. The highest performing candidates recognised the large difference between the boiling points of the primary and tertiary amines. They then identified the cause: primary amines form hydrogen bonds but tertiary amines do not. This question was answered reasonably well with fewer candidates than in the past describing relative strengths of covalent bonds, a common misconception. 1(b)(ii) Most candidates rearranged the ideal gas equation correctly to make n the subject. They then substituted correct values for p, V, R and T into the equation, with correct unit conversions. Most candidates then calculated n correctly as 2.32 × 10–3 and combined this value with the mass of 0.202 g to derive the molecular formula as 87. Lower-attaining candidates often made an error in their conversion of cm3 into m3, by multiplying by 10–3 rather than 10–6. This error resulted in a value of n as 2.32 and the unrealistic molecular mass of 8.7 × 10–3, from which a molecular formula is impossible. Candidates in this position are advised to check back at the unit conversions in the working. Very few candidates did not convert 100ºC into 373 K. Candidates with the correct molecular formula of 87 usually suggested C5H13N as the molecular formula. A significant number quoted C5H11NH2. This response was not credited as it is not a ‘molecular’ formula. There are several possible structures of C5H13N with three 13C NMR peaks and any were credited. Error carried forward was applied from a different molecular mass for both the molecular formula and a structure. This emphasises the importance of working within calculation; credit will always be given for a correct method. Instead of using the ideal gas equation, some candidates used the molar gas constant at room temperature and pressure, 24.0 dm3, which gives a molecular mass of 67. This simplified approach could only gain the final 2 marks for this question: a molecular formula (with an error carried forward value of 67, e.g. C4H5N) and a structure of an amine with this molecular formula and 3 peaks in the 13C NMR spectrum. 1(c) Candidates were supplied with information about an unfamiliar reaction of an amino acid and asked to predict a possible equation. Many candidates suggested H2O as one product, being the difference in the formula of the amino acid and the C3H3NO cyclic organic product. Any cyclic structure of C3H3NO that met the bonding rules for C, H, N and O was credited. Examples included a 4-membered ring lactam and substituted cyclopropenes. A significant number of candidates showed an equation for the reaction of two molecules of the amino acid to form 2 H2O and a cyclic dipeptide. Although chemically feasible, the dipeptide could not be credited because the molecular formula was C3H3NO in the question. This error could have been avoided if the information in the question had been used.

Notes from the mark scheme and report

• water in at bottom and out at top AND condenser
• pear-shaped/round-bottom flask AND reflux
• DO NOT ALLOW conical flask, volumetric flask or beaker

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

1a(i) Most candidates labelled some of the apparatus correctly and identified the reflux technique. A significant number showed water flowing in the wrong direction and ‘distillation’ was given as the name of the technique. The ‘condenser’ was sometimes labelled incorrectly, e.g. ‘condensation tube, ‘distillation tube’ and ‘water jacket’. Only just over half the candidates were given both marks. 1a(ii) Many diagrams were incomplete and it was comparatively rare for both of the two available marks to be given. Important labels were often missing. Some candidates drew diagrams of other techniques, such as distillation. Many responses were not credited with marks and this question was often omitted. Candidates need practice in recognising practical techniques and in drawing acceptable diagrams. 1b(i) Candidates were expected to explain the different boiling points of three amines. Most candidates linked increased branching with less surface contact between molecules, leading to weaker induced dipole interactions (London forces). Fewer candidates linked the decreasing boiling points to less energy being required to break the intermolecular bonds. The highest performing candidates recognised the large difference between the boiling points of the primary and tertiary amines. They then identified the cause: primary amines form hydrogen bonds but tertiary amines do not. This question was answered reasonably well with fewer candidates than in the past describing relative strengths of covalent bonds, a common misconception. 1(b)(ii) Most candidates rearranged the ideal gas equation correctly to make n the subject. They then substituted correct values for p, V, R and T into the equation, with correct unit conversions. Most candidates then calculated n correctly as 2.32 × 10–3 and combined this value with the mass of 0.202 g to derive the molecular formula as 87. Lower-attaining candidates often made an error in their conversion of cm3 into m3, by multiplying by 10–3 rather than 10–6. This error resulted in a value of n as 2.32 and the unrealistic molecular mass of 8.7 × 10–3, from which a molecular formula is impossible. Candidates in this position are advised to check back at the unit conversions in the working. Very few candidates did not convert 100ºC into 373 K. Candidates with the correct molecular formula of 87 usually suggested C5H13N as the molecular formula. A significant number quoted C5H11NH2. This response was not credited as it is not a ‘molecular’ formula. There are several possible structures of C5H13N with three 13C NMR peaks and any were credited. Error carried forward was applied from a different molecular mass for both the molecular formula and a structure. This emphasises the importance of working within calculation; credit will always be given for a correct method. Instead of using the ideal gas equation, some candidates used the molar gas constant at room temperature and pressure, 24.0 dm3, which gives a molecular mass of 67. This simplified approach could only gain the final 2 marks for this question: a molecular formula (with an error carried forward value of 67, e.g. C4H5N) and a structure of an amine with this molecular formula and 3 peaks in the 13C NMR spectrum. 1(c) Candidates were supplied with information about an unfamiliar reaction of an amino acid and asked to predict a possible equation. Many candidates suggested H2O as one product, being the difference in the formula of the amino acid and the C3H3NO cyclic organic product. Any cyclic structure of C3H3NO that met the bonding rules for C, H, N and O was credited. Examples included a 4-membered ring lactam and substituted cyclopropenes. A significant number of candidates showed an equation for the reaction of two molecules of the amino acid to form 2 H2O and a cyclic dipeptide. Although chemically feasible, the dipeptide could not be credited because the molecular formula was C3H3NO in the question. This error could have been avoided if the information in the question had been used.

Notes from the mark scheme and report

• Buchner flask with ONE side arm and Buchner/Hirsch funnel sealed on top
• label from side arm indicating reduced pressure (to pump/vacuum)
• DO NOT ALLOW just 'flask' or 'funnel' as labels

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

3(a)(i) Most candidates analysed the raw practical results supplied to correctly calculate the energy change as 10,241 J using q = mc∆T, and the amount of propane burnt as 0.0550 mol. Candidates usually determined the energy change per mole of propane as 1862 kJ. The question asked for the enthalpy change of combustion to 3 significant figures, which gives –1860 kJ mol–1. The mark for this final value was often not given because either the – sign was omitted or the answer was not quoted to the required 3 significant figures. Candidates are advised to read the requirements of each question more closely to eliminate such errors. A double-check on significant figure requirements is recommended before moving on to the next question. Lower-attaining candidates often extracted the wrong data from the results table. Instead of the mass of water (100 cm3), the mass of propane burnt (0.242 g) was often used in the q = mc∆T calculation. Candidates are advised that the thermometer measures the temperature change of the material in which it is placed (the water). 3(a)(ii) One mark was available for identifying two reasons for the difference in enthalpy change. It was expected that candidates would respond with ‘heat loss’ and ‘incomplete combustion’. Most candidates suggested heat loss but the other reason often suggested was equipment error, e.g. the thermometer did not reading correctly’, ‘the balance gave incorrect readings’. Candidates are advised that responses of faulty apparatus are unlikely to be creditworthy. 3(b) This question was assessed by LoR. Level 3 candidates calculated the enthalpy change for Equation 3.1 correctly as –2054 kJ mol–1. They linked 4 × ∆vapH with the enthalpy change for Equation 3.1 in an energy cycle and calculated the enthalpy change of combustion as –2054 – (4 × 40.65) = –2216.6 kJ mol–1. Most Level 3 candidates drew out bonds in the molecules involved in Equation 3.1, which allowed them to visually count the bonds broken and bonds made. This strategy is recommended to all candidates for any calculation based on bond enthalpies and is much more likely to generate the correct enthalpy change than visualising the relevant bonds solely from the equation. Level 2 candidates often derived the correct enthalpy change for Equation 3.1 but then tried to link this value with ∆vapH instead of 4 × ∆vapH. Other Level 2 candidates calculated ∆H correctly for either the bonds broken or bonds made and combine this value with 4 × ∆vapH. A common error was the omission of the 2 C–C bonds in propane, use of 3 C–C, instead of 2 C–C, bonds, or use of 4, rather than 8 O–H bonds. These errors could have been avoided by drawing out the bonds as described above. Level 1 candidates typically attempted to use the bond enthalpies to calculate energies for bonds broken and bonds made but made several errors. They then combined this value with ∆vapH. This question discriminated extremely well across different abilities.

Notes from the mark scheme and report

• common errors (2 marks by ECF): m = 0.242 g (mass of propane) used in mc∆T, ∆T taken as 297.5 K
• –1862 scores 2 (not 3 SF)
• DO NOT ALLOW c = 4.2
• – sign AND 3 SF required

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

3(a)(i) Most candidates analysed the raw practical results supplied to correctly calculate the energy change as 10,241 J using q = mc∆T, and the amount of propane burnt as 0.0550 mol. Candidates usually determined the energy change per mole of propane as 1862 kJ. The question asked for the enthalpy change of combustion to 3 significant figures, which gives –1860 kJ mol–1. The mark for this final value was often not given because either the – sign was omitted or the answer was not quoted to the required 3 significant figures. Candidates are advised to read the requirements of each question more closely to eliminate such errors. A double-check on significant figure requirements is recommended before moving on to the next question. Lower-attaining candidates often extracted the wrong data from the results table. Instead of the mass of water (100 cm3), the mass of propane burnt (0.242 g) was often used in the q = mc∆T calculation. Candidates are advised that the thermometer measures the temperature change of the material in which it is placed (the water). 3(a)(ii) One mark was available for identifying two reasons for the difference in enthalpy change. It was expected that candidates would respond with ‘heat loss’ and ‘incomplete combustion’. Most candidates suggested heat loss but the other reason often suggested was equipment error, e.g. the thermometer did not reading correctly’, ‘the balance gave incorrect readings’. Candidates are advised that responses of faulty apparatus are unlikely to be creditworthy. 3(b) This question was assessed by LoR. Level 3 candidates calculated the enthalpy change for Equation 3.1 correctly as –2054 kJ mol–1. They linked 4 × ∆vapH with the enthalpy change for Equation 3.1 in an energy cycle and calculated the enthalpy change of combustion as –2054 – (4 × 40.65) = –2216.6 kJ mol–1. Most Level 3 candidates drew out bonds in the molecules involved in Equation 3.1, which allowed them to visually count the bonds broken and bonds made. This strategy is recommended to all candidates for any calculation based on bond enthalpies and is much more likely to generate the correct enthalpy change than visualising the relevant bonds solely from the equation. Level 2 candidates often derived the correct enthalpy change for Equation 3.1 but then tried to link this value with ∆vapH instead of 4 × ∆vapH. Other Level 2 candidates calculated ∆H correctly for either the bonds broken or bonds made and combine this value with 4 × ∆vapH. A common error was the omission of the 2 C–C bonds in propane, use of 3 C–C, instead of 2 C–C, bonds, or use of 4, rather than 8 O–H bonds. These errors could have been avoided by drawing out the bonds as described above. Level 1 candidates typically attempted to use the bond enthalpies to calculate energies for bonds broken and bonds made but made several errors. They then combined this value with ∆vapH. This question discriminated extremely well across different abilities.

Notes from the mark scheme and report

• any two for 1 mark: heat loss to surroundings, incomplete combustion (IGNORE incomplete 'reaction'), evaporation of water
• IGNORE evaporation of C3H8

Skills: PAG3 Enthalpy determination

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• ALL fizzing has stopped
• ALL the metal has dissolved ('metal disappears' not quite enough)
• IGNORE constant mass and no temperature increase

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• common errors: mean of all 3 titres 27.35 (5 marks)
• no ÷2 for n(M) gives Fe
• no subtraction from 0.210 gives Tb
• no ×10 scale-up gives Zn
• omitting titration calculation scores zero
• DO NOT ALLOW Ga or Sc (3+ ions only)
• mean from concordant 27.30 and 27.20

Skills: M0.2 Use ratios, fractions and percentages; M1.2 Find arithmetic means; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• mass loss 2.75 g ÷ 44 = 0.0625 mol

Skills: M0.0 Recognise and make use of appropriate units in calculations; PAG1 Moles determination

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• mass of X2CO3 = 14.57 g
• M = 233.12
• subtract 60 and halve gives 86.6 so Rb
• DO NOT ALLOW Sr (wrong carbonate formula)
• ECF for X only if Group 1 metal or Ag

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; PAG1 Moles determination

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• reweigh to constant mass
• IGNORE 'leave for longer' or 'wait until fizzing stops' without link to constant mass

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

4(a)(i) This question required candidates to write an overall equation and half equations for oxidation and reduction. Many candidates made errors within one or more equations. The overall equation was often written without state symbols, despite the question instruction ‘with state symbols’. The oxidation half equation was more likely to be correct than the reduction half equation, which often used Cl instead of H+. When H+ was used, the half equation was often unbalanced or electrons had been omitted. It is recommended that candidates carefully use the chemical information in the question. 4(a)(ii) Most candidates identified that all the metal would have reacted when it had all disappeared and that gas bubbles from the reaction would have stopped. Some responses did not emphasise that these observations would have stopped and this prevented credit being given. 4(a)(iii) The ionic equation for neutralisation of an acid with an alkali was well known and this question was answered correctly by most candidates. 4(a)(iv) Candidates were presented with information about a back titration, a technique that they would be unlikely to have encountered during their course. The question stem to 4a(iv) suggested a three-step strategy. Many candidates followed this guidance and were credited with many of the available marks. Marks were given for a correct method (by error carried forward) even if there was an error or omission in the multi-step calculation. This emphasises the importance of clear working. Most candidates determined the correct mean titre of 27.25 cm3. A few candidates did take the mean of all three titres rather than the closest. Most calculated that 8.72 × 10–3 mol of NaOH reacted with the same number of moles of HCl in the titration and then scaled up the HCl by a factor of 10 to 8.72 × 10–2 mol in the 250 cm3 volumetric flask. These steps are standard for many titration calculations and gave a route to three of the six available marks. The more difficult back titration steps then followed and the higher-attaining candidates recognised the need to subtract this amount of HCl from the original amount of HCl used to react with metal M. These candidates then divide this value by two to find the moles of M that reacted (from the 1 : 2 stoichiometry of M : HCl). The correct calculation then gave a relative atomic mass of M as 112 and its identity as cadmium. It was common for candidates to omit the division by two and to arrive at a relative atomic mass of 56 for iron. The mark scheme shows the variety of metals that candidates identified from their calculations, the errors made, and the error carried forward marks that resulted. Many lower-attaining candidates did not follow the 3 steps in the stem, using only the original amount of HCl and ignoring the titration. This approach was not credited with marks. A large range of marks was seen, and the question discriminated extremely well. 4(b)(i) This straightforward mole calculation required candidates to determine the mass of CO2 as 2.75 g from the table of results and to divide this value by the molar mass of CO2 to arrive at 0.0625 mol. Most candidates were successful. As with other questions providing raw experimental results, lower-attaining candidates often chose the wrong data from the supplied results. 4(b)(ii) Many candidates used their value from 4b(i) with the mass of X2CO3 from the results table to obtain a value for the molar mass (233.13 from 14.57 ÷ 0.0625). Higher-attaining candidates obtained a value for the atomic mass of X (86.56 from 233.13) and identified X as the Group 1 metal rubidium. Some candidates did not recognise the significance of the formula of X2CO3 and identified X instead as strontium (Ar = 87.6). Lower-attaining candidates often used incorrect data from the supplied information and made little progress with this calculation. 4(c)(i) Most candidates suggested that the mass of the flask and contents could be reweighed until the mass no longer changed. An alternative creditworthy suggestion was to attach a gas syringe and to measure the gas volume until it no longer changed. Vague suggestions such as ‘leave for longer’ were not credited with marks. 4(c)(ii) Candidates found this question challenging. Success required candidates to appreciate that some CO2 dissolving would result in less mass of CO2 being recorded as lost, meaning that less moles of CO2 had apparently been formed and so less moles of X2CO3 had reacted. Using n = m/M, this would show that the calculated molar mass of X2CO3 appears to be greater. Only the highest-attaining candidates successfully navigated this question.

Notes from the mark scheme and report

• mass/moles of CO2 lost would be smaller so n(X2CO3) appears smaller
• calculated molar mass would be greater

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Question 5 related an unfamiliar mathematical relationship with the y = mx + c equation for a straight line. Use of y = mx + c is a mathematical requirement of A level chemistry and candidates were expected to apply their understanding of the Arrhenius equation (part of the specification) which makes use of this straight-line equation in a novel context. Many candidates were unable to use this mathematical concept. In contrast, higher-attaining candidates were able to be credited with marks throughout question 5. 5(a) Candidates were expected to complete values for 1/T and ln Kp from supplied T and Kp values. Candidates were supplied with an example and this enabled most candidates to obtain both available marks. 5(b) Most candidates were aware that a decrease in Kp with increasing temperature signals that the forward reaction is exothermic. The question also asked for the effect on the equilibrium position. A significant number of candidates omitted this part of the question. Candidates are advised to check back to all the requirements in a question. 5(c) Candidates were required to plot a graph using their calculated values from 5(a). The axes for the graph had been provided. It was expected that the plotting of 5 points, with a best-fit straight line, would be straightforward. Many candidates plotted one or more points incorrectly, particularly the point at 1/T = 1.25 × 10–3. Candidates then needed to recognise that the gradient is equal to –∆H/R from Equation 5.1, to measure the gradient, and then to determine ∆H. Most candidates recognised that the gradient needed to be measured but its value was then not taken any further. The higher-attaining candidates correctly multiplied the gradient by R but did not always convert the calculated J mol–1 value into kJ mol–1, or to express their value to 3 significant figures, as required in the question. Significantly, nearly a third of candidates did not collect any of the four available marks. The question was an excellent discriminator. 5(d) Mathematically able candidates used the y = mx + c equation for a straight line with the supplied mathematical relationship (Equation 5.1) to identify the y intercept as ∆S/R. They then stated that ∆S could be determined by multiplying the value of the y intercept by R. Many candidates found the mathematical requirements of parts 5(c) and 5(d) difficult. Responses for 5(d) were often in terms of the gradient instead of ‘intercept’.

Notes from the mark scheme and report

• mark by row
• 1/T to 2 SF or more
• ln Kp whole numbers 105, 86, 72, 60
• DO NOT ALLOW whole-number errors e.g. 85 for 86

Skills: M0.1 Recognise and use expressions in decimal and ordinary form; M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

Question 5 related an unfamiliar mathematical relationship with the y = mx + c equation for a straight line. Use of y = mx + c is a mathematical requirement of A level chemistry and candidates were expected to apply their understanding of the Arrhenius equation (part of the specification) which makes use of this straight-line equation in a novel context. Many candidates were unable to use this mathematical concept. In contrast, higher-attaining candidates were able to be credited with marks throughout question 5. 5(a) Candidates were expected to complete values for 1/T and ln Kp from supplied T and Kp values. Candidates were supplied with an example and this enabled most candidates to obtain both available marks. 5(b) Most candidates were aware that a decrease in Kp with increasing temperature signals that the forward reaction is exothermic. The question also asked for the effect on the equilibrium position. A significant number of candidates omitted this part of the question. Candidates are advised to check back to all the requirements in a question. 5(c) Candidates were required to plot a graph using their calculated values from 5(a). The axes for the graph had been provided. It was expected that the plotting of 5 points, with a best-fit straight line, would be straightforward. Many candidates plotted one or more points incorrectly, particularly the point at 1/T = 1.25 × 10–3. Candidates then needed to recognise that the gradient is equal to –∆H/R from Equation 5.1, to measure the gradient, and then to determine ∆H. Most candidates recognised that the gradient needed to be measured but its value was then not taken any further. The higher-attaining candidates correctly multiplied the gradient by R but did not always convert the calculated J mol–1 value into kJ mol–1, or to express their value to 3 significant figures, as required in the question. Significantly, nearly a third of candidates did not collect any of the four available marks. The question was an excellent discriminator. 5(d) Mathematically able candidates used the y = mx + c equation for a straight line with the supplied mathematical relationship (Equation 5.1) to identify the y intercept as ∆S/R. They then stated that ∆S could be determined by multiplying the value of the y intercept by R. Many candidates found the mathematical requirements of parts 5(c) and 5(d) difficult. Responses for 5(d) were often in terms of the gradient instead of ‘intercept’.

Notes from the mark scheme and report

• all points plotted with best-fit straight line
• gradient ±57 000 to ±63 000
• ∆H = –gradient × 8.314
• answer in kJ mol–1 to 3 SF with – sign (range –480 to –530)

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; M3.2 Plot two variables from experimental or other data; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Examiner's comment

Question 5 related an unfamiliar mathematical relationship with the y = mx + c equation for a straight line. Use of y = mx + c is a mathematical requirement of A level chemistry and candidates were expected to apply their understanding of the Arrhenius equation (part of the specification) which makes use of this straight-line equation in a novel context. Many candidates were unable to use this mathematical concept. In contrast, higher-attaining candidates were able to be credited with marks throughout question 5. 5(a) Candidates were expected to complete values for 1/T and ln Kp from supplied T and Kp values. Candidates were supplied with an example and this enabled most candidates to obtain both available marks. 5(b) Most candidates were aware that a decrease in Kp with increasing temperature signals that the forward reaction is exothermic. The question also asked for the effect on the equilibrium position. A significant number of candidates omitted this part of the question. Candidates are advised to check back to all the requirements in a question. 5(c) Candidates were required to plot a graph using their calculated values from 5(a). The axes for the graph had been provided. It was expected that the plotting of 5 points, with a best-fit straight line, would be straightforward. Many candidates plotted one or more points incorrectly, particularly the point at 1/T = 1.25 × 10–3. Candidates then needed to recognise that the gradient is equal to –∆H/R from Equation 5.1, to measure the gradient, and then to determine ∆H. Most candidates recognised that the gradient needed to be measured but its value was then not taken any further. The higher-attaining candidates correctly multiplied the gradient by R but did not always convert the calculated J mol–1 value into kJ mol–1, or to express their value to 3 significant figures, as required in the question. Significantly, nearly a third of candidates did not collect any of the four available marks. The question was an excellent discriminator. 5(d) Mathematically able candidates used the y = mx + c equation for a straight line with the supplied mathematical relationship (Equation 5.1) to identify the y intercept as ∆S/R. They then stated that ∆S could be determined by multiplying the value of the y intercept by R. Many candidates found the mathematical requirements of parts 5(c) and 5(d) difficult. Responses for 5(d) were often in terms of the gradient instead of ‘intercept’.

Notes from the mark scheme and report

• extrapolate to y-intercept
• intercept = ∆S/R so ∆S = R × intercept
• ALLOW substituting ln Kp, 1/T and gradient into equation 5.1
• x-intercept loses first mark

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Examiner's comment

Most candidates added correct titres for the three titrations. However, an error made by a quarter of candidates was to omit the zero as the second decimal place in the first and third titres. This should have been usual practice from candidate experience of practical work and has also been highlighted as a common error in previous exam series.

Notes from the mark scheme and report

• titres must be to 2 decimal places: DO NOT ALLOW 24.2 or 24.3

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates identified that the first and third titres were concordant and calculated the mean titre that should be used as 24.25 cm3. About a third of candidates calculated the mean of all 3 titres as 24.10 or 24.12 cm3. Normal practice in titrations would be to select the closest titres.

Notes from the mark scheme and report

• use concordant titres only
• DO NOT ALLOW mean of all three titres (24.10/24.12)
• The Practical Skills handbook contains guidance on correct practice for recording titration results and calculating average titre values in Appendix 4: Measurements, which can be shared with students: https://www.ocr.org.uk/Images/208932-chemistry-practical-skills- handbook.pdf.

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many candidates were able to calculate the amount of Na2CO3 in the titration as 0.00375 mol although a common error was to calculate the amount of Na2CO3 in the 250 cm3 volumetric flask as 0.0375 mol. Most candidates were credited for the amount of HCl: twice their calculated amount of Na2CO3. Candidates then need to scale up this value by 1000/mean titre to obtain the concentration as 0.309 mol dm–3, and to quote the answer to 3 significant figures. Many candidates scaled up using 50.0, the burette volume, rather than their mean titre, resulting in a concentration 0.15 or 1.5 mol dm–3. A significant number also rounded their value to 2 rather than 3 significant figures. Candidates are advised to show clear working so that credit can be given for such responses by applying error carried forward. The working shown in this response is clear. Many candidates working was more jumbled, with unreferenced numbers common.

Notes from the mark scheme and report

• 3 sig figs required
• 1:2 ratio Na2CO3:HCl
• ALLOW ECF from mean titre (0.311 from mean of all three titres)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.2 Change the subject of an equation; PAG2 Acid–base titration

Mark scheme

Examiner's comment

The calculations here should have reflected practical work carried out by candidates. Candidates were expected to realise that the pipette volume involves one measurement requiring the uncertainty of ±0.04 provided being used. As the volume measured by a burette uses two measurements, the uncertainty of ±0.05 must then be doubled to obtain the percentage uncertainty. It was very common for the burette value to be obtained using 0.05 rather than the doubled 0.10; some candidates doubled both uncertainties. Another common error was to use the volume of the burette of 50 cm3 rather than the volume of solution measured in the burette.

Notes from the mark scheme and report

• burette uncertainty counted twice (2 × 0.05) because two readings
• 1 mark if both correct but ×100 omitted both times
• ALLOW burette volume of 50 cm3
• The Practical Skills handbook contains guidance on calculating uncertainties in Appendix 4: Measurements, which can be shared with students: https://www.ocr.org.uk/Images/208932-chemistry-practical-skills- handbook.pdf.

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

This question was best discriminator of the paper and rewarded the well-prepared candidates who were competent in writing equations. Most candidates were given the 2 marks for the bromide test with silver nitrate and the related equation (usually shown ionically). Many found the test for the ammonium ion more challenging to describe. The alkaline nature of ammonia was well-known and the indicator colour change to blue was often seen. Many candidates omitted the NaOH reagent and tested the compound with indicator, thinking that the ammonium ion itself is alkaline. Few candidates were able to write the equation for the ammonium test. Lower attaining students often outlined electrolysis as a test and many candidates wrote about the carbonate and sulfate tests prior to the halide test.

Notes from the mark scheme and report

• NaOH (NOT ammonia) and heat, damp indicator paper turns blue
• AgNO3 gives cream precipitate
• IGNORE nitric acid
• ionic or full equations allowed
• ALLOW displacement by Cl2 giving orange solution (DO NOT ALLOW precipitate)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Most students managed to gain some marks on this question. The most common error was rounding to 6, something they have been taught to do for water of crystallisation. This caused them to lose a mark as the question asked for two significant figures. Many rounded too early so a variety of responses were seen.

Notes from the mark scheme and report

• use final constant mass 17.58 g
• answer must be to 2 SF (5.8)
• 5.83 scores 2
• ALLOW ECF from n(SrCl2) and n(H2O)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG1 Moles determination

Mark scheme

Examiner's comment

The majority of candidates answered this correctly, the main incorrect answer was “to achieve constant mass”.

Notes from the mark scheme and report

• to make sure ALL the water has been removed
• IGNORE just 'to weigh to constant mass'

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Most candidates identified either using a larger mass or a more accurate balance, not many stated both. The most common incorrect answers involved heating for longer or taking less measurements.

Notes from the mark scheme and report

• balance reading to 3 or more decimal places
• use a larger mass
• IGNORE lid on crucible, repeating/averaging, weighing straight after heating

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG1 Moles determination

Mark scheme, with the levels and indicative points

Examiner's comment

Most candidates focused on removing the water of crystallisation, often going to great depths of explanation, with apparatus diagrams, of how to remove it. Some candidates then went on to explain how to make a standard solution and could be given marks. Calculations for the mass required were often correct or could be given some marks. This in a response of a common approach to this question, it was given 4 marks. Despite calculating the mass properly, the candidate then put all their efforts into describing removing the water of crystallisation and left out the valuable fine detail required for a Level 3 response.

Notes from the mark scheme and report

• mass 25.63 g of Mg(NO3)2•6H2O (14.83 g if water of crystallisation omitted)
• dissolve in distilled water, transfer with washings to 250 cm3 volumetric flask, make up to mark with bottom of meniscus on mark, invert to mix
• IGNORE removing water of crystallisation

Skills: M0.0 Recognise and make use of appropriate units in calculations; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

The question asked for the measurement AND observation, many students did not answer both parts and therefore did not gain any marks. “How fast” and “how long” were not given as they did not detail the measurement.

Notes from the mark scheme and report

• TIME for PRECIPITATE to appear - both required

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Most candidates obtained this mark, some lost the mark because they did not use a linear scale or provide units.

Notes from the mark scheme and report

• linear x scale AND label Time / s AND points plotted ±1 small square
• ALLOW up to 3 plotting errors

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Nearly all the candidates obtained this mark

Notes from the mark scheme and report

• point at 80 s circled

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Nearly all the candidates obtained this mark

Notes from the mark scheme and report

• smooth curve through all points EXCEPT the 80 s point

Skills: M3.2 Plot two variables from experimental or other data; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Many candidates wrote the colour change the wrong way around, or thought that a gas would be evolved, or wrote “clear” instead of “colourless”. A large proportion merely stated what type of reaction it was, rather than what they would observe.

Notes from the mark scheme and report

• (orange to) colourless/decolourised
• IGNORE 'colour change'

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

This question discriminated well and most were able to attempt to calculate a mass and explain the purification steps with some fine detail. It was evident that most candidates were aware of the apparatus required but the logic in the order was sometimes out of sequence. Drying agents were mentioned, some of the examples used were incorrect and errors were made with the boiling points if candidates mentioned distillation. This in a response of an excellent approach to this question, it was given 6 marks. The correct mass has been calculated and all of the fine detail, except distilling at 63°C

Notes from the mark scheme and report

• 0.0500 mol hex-1-ene needs 0.0800 mol hexan-1-ol = 8.16 g
• partly correct: 5.1 g (% yield omitted), 3.19 g (% yield inverted)
• separating funnel (upper organic layer), dry with anhydrous MgSO4/CaCl2, distil collecting fraction at 63 °C
• incorrect purification method not creditworthy

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Only a very small minority of candidates appreciated that a larger titre reduces percentage error in titre values. Most erroneously described a reading of 1.35 cm3 as being less accurate than a reading of 13.5(0) cm3. The accuracy of these is equal in the same scaled apparatus.

Notes from the mark scheme and report

• dilute solution gives a larger titre and so a smaller percentage error/uncertainty

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Many candidates coped well with this multi-step calculation. The common errors were: • determining the mass of C12H22FeO14 in two tablets instead of just one tablet as required in the question • determining the mass of Fe in a tablet instead of the mass of C12H22FeO14 • failing to convert from grams to milligrams

Notes from the mark scheme and report

• ×5 for Fe2+
• ×10 scaling to 250 cm3
• ÷2 for one tablet and convert g to mg
• common errors 602 mg (not halved) and 37.7 mg (55.8 used instead of 445.8) score 4
• 3 SF

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Examiner's comment

The second Level of Response question in the paper was also answered very well. Almost all candidates determined the order with respect to H+ to be first order and gave suitable explanations. A very high proportion of candidates determined the order with respect to CH3COCH3 to be first order and related this to the direct proportionality shown on the graph. The zero order with respect to Br2 proved a little more problematic with many candidates just giving an order with no attempted explanation. Having determined orders, nearly all candidates were able to give a corresponding rate equation and could calculate a value for the rate constant, albeit with frequent omission of units and forgetting that the initial rates given were in terms of 10–5. An example of a complete answer achieving L3 (6 marks) is given. This candidate gave each order clearly followed by a concise explanation of each choice. The rate equation (based on the orders given) is clearly stated, the calculation clearly shows working and correct units are given.

Notes from the mark scheme and report

• zero order in Br2 (straight line concentration–time, constant gradient)
• first order in CH3COCH3 (straight line through origin)
• first order in H+ (doubling doubles rate)
• rate = k[CH3COCH3][H+]
• k = 3.9 × 10–5 dm3 mol–1 s–1

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; PAG10 Rates of reaction – initial rates; PAG9 Rates of reaction – continuous monitoring

Mark scheme

Examiner's comment

Almost all candidates were able to describe a suitable test to distinguish between geraniol and citronellal and most scored both marks. The most common choice was to use Tollens’ reagent to identify the aldehyde group in citronellal by the formation of a silver mirror. Other candidates opted for 2,4- dinitrophenylhdrazine to differentiate the carbonyl compound from the alcohol.

Notes from the mark scheme and report

• Tollens' reagent: silver mirror with citronellal
• ALLOW 2,4-DNP orange precipitate with citronellal
• IGNORE acidified dichromate (reacts with both)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Candidates are confident in tackling titration analysis and the majority of responses were well structured. Most candidates used the results to determine the Mr of the unknown amino acid and hence identify it as leucine. However, other approaches were also seen and these were given full credit if correct. A common error was incorrect scaling to determine the amount of amino acid used to make the 250 cm3 solution. A significant number of responses simply multiplied the number of moles of acid by ten rather than using a factor of 250/21.30. Error carried forward marks were available and so the majority of candidates scored 3 or 4 marks.

Notes from the mark scheme and report

• amino acid solution is in the burette: n in 250 cm3 = 3.75 × 10^-3 × 250.0/21.30
• M = 131 so leucine, with working (R = 57 or Mr) to justify
• ALLOW ECF

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Candidates are familiar with the calculation of Rf values and this question was answered very well. Although almost all candidates showed appropriate working as part of their response some candidates gave the final answer to only one significant figure and did not receive credit. Candidates were expected to give their answer to a number of significant figures appropriate to the measurements they had made, so a minimum of two significant figures was expected. The mark scheme allowed a response within a range, rather than a specific value, and most candidates scored in this part.

Notes from the mark scheme and report

• 0.33–0.35
• ALLOW 0.3

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

A LEVEL CHEMISTRY A Examiners’ report H432/02 Summer 2019 series Version 1 H432 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 2 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 3 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 6 Question 5 ............................................................................................................................................ 7 Question 10 .......................................................................................................................................... 8 Question 12 .......................................................................................................................................... 9 Question 13 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 10 Question 15 ........................................................................................................................................ 11 Section B overview ................................................................................................................................. 12 Question 16 (a) (i) ............................................................................................................................... 12 Question 16 (a) (ii) .............................................................................................................................. 12 Question 16 (b) (i) ............................................................................................................................... 13 Question 16 (b) (ii) .............................................................................................................................. 14 Question 16 (b) (iv) ............................................................................................................................. 14 Question 16 (b) (v) .............................................................................................................................. 15 Question 17 (a) (i) ............................................................................................................................... 17 Question 17 (a) (ii) .............................................................................................................................. 19 Question 17 (b) (i) ............................................................................................................................... 20 Question 18 (a) (i) ............................................................................................................................... 21 Question 18 (a) (ii) .............................................................................................................................. 22 Question 18 (b) ................................................................................................................................... 23 Question 18 (c) ................................................................................................................................... 23 Question 18 (d) ................................................................................................................................... 24 Question 18 (e) (i) ............................................................................................................................... 24 Question 18 (e) (ii) .............................................................................................................................. 26 Question 19 (a) (i) ............................................................................................................................... 27 Question 19 (a) (ii) .............................................................................................................................. 29 Question 19 (b) (i) ............................................................................................................................... 30 Question 19 (b) (iii) ............................................................................................................................. 32 Question 19 (b) (iv) ............................................................................................................................. 34 Question 20 (a) (i) ............................................................................................................................... 36 Question 20 (a) (ii) .............................................................................................................................. 37 Question 20 (a) (iii) ............................................................................................................................. 37 Question 20 (b) (i) ............................................................................................................................... 38 Question 21 ........................................................................................................................................ 39 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 17 (b) (i) Candidates are familiar with the calculation of Rf values and this question was answered very well.

Notes from the mark scheme and report

• glycine: matches leu and gly in solvent W AND ala and gly in solvent X, so same Rf in both solvents

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Candidates found this part extremely difficult. The question was aimed to stretch and challenge. Many candidates followed on directly from part (a), stating in simple terms that the alkaline solution needed to be neutralised to remove hydroxide ions. However, candidates were expected to recognise that the alkaline conditions would lead to benzoate ions rather than benzoic acid being present in the mixture. The mixture is acidified to protonate the benzoate. The hint in the question was about making the benzoic acid appearing when acid is added.

Notes from the mark scheme and report

• H+ protonates benzoate/carboxylate ion to form benzoic acid
• IGNORE responses only about neutralising the alkali/carbonate

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

Many candidates produced thorough responses, showing that they had encountered recrystallisation as a technique in their practical work. Most candidates were aware that the impure product is dissolved in a minimum volume of hot solvent, although ‘minimum’ was sometimes omitted. The subsequent stages were sometimes incomplete or in the wrong order. Many were aware that the hot solution can be passed through fluted filter paper to remove solid impurities. (This is beyond the specification requirements for A Level but good practice). Most candidates were aware of the need to filter (usually under reduced pressure) but the necessary cooling stage to form the crystals was sometimes omitted. Finally, many responses omitted the need to dry the crystals. Candidates did sometimes dry the crystals by adding an anhydrous salt (e.g. CaCl2 or MgSO4), a clear confusion with drying an organic liquid. Others described the purification of an organic liquid for their response, including use of a separating funnel, drying and distillation.

Notes from the mark scheme and report

• dissolve in the minimum quantity of hot solvent
• cool AND filter AND dry (all three needed)
• DO NOT ALLOW use of a drying agent
• IGNORE hot filtration

Skills: PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

The mark scheme catered for a wide range of responses that were judged to be worthy of credit. Responses that form part of normal safe laboratory practice, such as keeping bags under benches, tying hair back, etc., were not given. Candidates were expected to identify a precaution for a chemical hazard. As with GCSE, general words to describe hazards, such as harmful and dangerous, were not given. Acceptable chemical hazards would include those encountered in hazard signs such as flammable (for methane) or toxic (for lead or lead compounds). The best responses suggested that naked flames should be kept away from flammable methane or that gloves should be worn when using toxic lead. Other suggested use of a fume cupboard or good ventilation when using flammable or toxic materials. Responses such as ‘face masks’ gained no credit.

Notes from the mark scheme and report

• definite safety issue AND matching precaution (toxic lead compounds – gloves or fume cupboard
• flammable methane – keep away from flame)
• 'harmful'/'dangerous' too vague
• IGNORE safety glasses, lab coat

Mark scheme

Examiner's comment

For one modification, many candidates suggested heating to constant mass. A second modification proved to be elusive for many. Other acceptable modifications included using powdered lead oxide to increase the surface area, bubbling the escape gases through lime water to identify when no more CO2 was being produced in the reaction, and passing an inert gas through the apparatus during cooling to prevent re-oxidation of the lead from oxygen in the air. Some suggestions would have created safety issues, such as working in a closed system. A significant number of candidates suggested collecting the gases evolved and that the reaction was complete when the gas volume no longer increased. Presumably the suggestion came from practical experience of rates experiments involving gas collection. This is unfeasible though when there is a constant flow of methane gas.

Notes from the mark scheme and report

• heat to constant mass
• spread out/powder the oxide
• pass methane or inert gas (not air) while cooling
• excess methane
• IGNORE 'heat for longer' without constant mass
• IGNORE repeats/averages

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

Most candidates were able to work out the masses of Pb and O atoms in the lead oxide from the supplied experimental results, and to then attempt to find the molar ratio of Pb : O. The 1 : 1.33 ratio was not always identified and a significant number of candidates approximated this to PbO or Pb2O3 instead of scaling up the ratio to 3 : 4 to show that the formula is Pb3O4 .

Notes from the mark scheme and report

• masses Pb 3.132 g and O 0.322 g
• Pb3O4 must come from masses
• NO ECF from incorrect masses

Skills: M0.2 Use ratios, fractions and percentages; PAG1 Moles determination

Mark scheme

Examiner's comment

Most candidates identified the phenol functional group but a satisfactory test proved to be far more difficult. Many candidates identified that there would be no effervescence with sodium carbonate but it was common that the pH being less than 7 was often not seen. Neutralisation with sodium hydroxide was often seen but with no observation. Although not in the specification, the reaction with bromine to produce a white precipitate was often seen as an acceptable alternative test for the phenol group.

Notes from the mark scheme and report

• phenol (DO NOT ALLOW alcohol or hydroxide
• IGNORE hydroxyl/OH)
• acidic to indicator/pH < 7 AND no reaction with carbonate
• ALLOW bromine giving white precipitate

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme, with the levels and indicative points

Examiner's comment

This question was assessed by level of response (LoR). Candidates were required to process raw experimental results to determine one enthalpy change, and then to determine a second enthalpy change by constructing and using an energy cycle. Levels were determined by the accuracy of the candidates’ processing, calculations and use of the energy cycle. This question discriminated extremely well. Level 3 candidates used the mass of solution as 50.7 g with mc∆T to obtain an energy change of 2861 J. They then divided this value by the moles of CuSO4 that reacted (0.05 mol) to obtain the enthalpy change of –57.22 kJ mol–1. Finally, they constructed an energy cycle which they then used to obtain the second enthalpy change of –65.65 kJ mol–1. Level 2 candidates determined the first enthalpy change but may have used the approximate mass of 50 g for the mass of solution from the experimental method. Their energy cycle was often incorrect or absent, with the second enthalpy change calculated incorrectly. Level 1 candidates often calculated the initial energy change using mc∆T but made little further correct progress. A significant number of lower ability candidates used the solid mass of copper sulfate in their calculation. Overall, mathematical skills were displayed well but some basic errors were made, particularly with subtractions. This may have been the result of mis-keying values into a calculator and believing the answer displayed. An example was the mass of the solution (74.13 – 23.43) being seen as 49.7 rather than 50.7. A significant number of candidates added or subtracted the mass of copper sulfate from 50.7 or 50 for their value of m in mc∆T, using for example 57.98 and 42.72. This limited the level that they could reach. initial energy change using mc∆T, the first enthalpy change using the energy change with the moles of copper sulfate, and the second enthalpy change including a clear energy cycle. Numbers are clearly shown and unrounded until the final values. This is an excellent response.

Notes from the mark scheme and report

• mass of solution 50.70 g (not 50.0) and ∆T 13.5 °C
• n(CuSO4) = 7.98/159.6 = 0.0500 mol
• ∆solH = –57.22 kJ mol–1 with – sign
• ∆rH = ∆solH(CuSO4) – ∆solH(CuSO4•5H2O) = –65.65
• energy cycle expected

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.1 Translate information between graphical, numerical and algebraic forms; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Just over half the candidates obtained the correct temperature change of 1°C. A common error was a temperature change of 0.5°C, the result of not considering that two temperature readings are made when calculating a temperature change.

Notes from the mark scheme and report

• two readings so 0.2/20 × 100 = 1.0 °C
• IGNORE direction of temperature change

Skills: M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; M2.2 Change the subject of an equation; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Candidates often struggle with questions based on practical work. There were many random responses to this question, with relatively few candidates identifying that solid magnesium phosphate could be obtained by filtration, followed by drying.

Notes from the mark scheme and report

• filter to obtain the solid, then dry
• IGNORE 'leave to crystallise' (already solid)
• IGNORE distil if product or H2 is distilled

Mark scheme

Examiner's comment

Candidates are well-versed with the relationship q = mc∆T and most were able to calculate that 2.926 kJ of energy was released in this reaction. It was also common to see the amount of AgNO3 correctly calculated as 1.28 × 10–3 mol. Candidates were expected to determine the amount of energy released from 1 mol AgNO3 as 229 kJ and finally to multiply this value by 2 for the molar quantities in the equation to match the ‘enthalpy change of reaction’. It was common to see –229 given as the final answer but this was rarely multiplied by 2. The question also required the final answer to be given to an appropriate number of significant figures. Many candidates seemed to be unaware that this reflects the least significant figure provided in the data, in this case 3 significant figures. The exemplar shows a typical response for 3 of the available 4 marks. Many omitted the negative sign in their ∆H value to consider the exothermicity of the reaction. Candidates are also advised to only round at the end of a multi-step calculation. Rounding of intermediate values introduces rounding errors in the final answer. Answer = –457 kJ mol–1

Notes from the mark scheme and report

• 3 SF and – sign needed
• ±229 scores 3 (not doubled for 2 mol AgNO3)
• mass used in q is 25.0 g of solution
• ALLOW ECF throughout

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Most candidates recognised that silver nitrate and chloride ions react together to form a white precipitate, but many did not make the link between this observation and whether any silver nitrate was left unreacted. Many candidates did not give a correct equation, with missing or incorrect state symbols being common. This question discriminated extremely well.

Notes from the mark scheme and report

• state symbols required in equation
• observation must be linked to conclusion (white precipitate = not all reacted)

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

Many candidates gave a good description of standard conditions or stated 1 mol dm-3, but that did not answer the question so no marks could be credited.

Notes from the mark scheme and report

• solution of known concentration

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

Most candidates were able to accurately record the burette readings and made the correct subtractions. Despite the examination question requesting the mean titre to be recorded to the accuracy of the burette, many candidates did not do this. A common error was taking a mean of all three readings instead of only the concordant results; this led the candidates to give an answer of 26.58 which lost them 2 marks.

Notes from the mark scheme and report

• readings to 2 d.p. ending in 0 or 5
• mean from concordant titres only (26.45)
• DO NOT ALLOW 26.5
• ALLOW ECF from incorrect burette readings

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

A good attempt by many candidates but some did not know how to calculate this or did not multiply by 2.

Notes from the mark scheme and report

• two readings so 2 × 0.05
• ALLOW ECF from (c)(i) or use of any titre

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Although there were some excellent descriptions of the correct processes, such as inverting the apparatus to ensure mixing and then making the solution up to the mark, many candidates could not name a volumetric flask.

Notes from the mark scheme and report

• volumetric flask instead of beaker
• IGNORE graduated flask

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

This was a good discriminator. Many candidates were credited the first two marks, and higher ability candidates went on to obtain 3 or 4 marks. Common errors were not rounding to a whole number at the end and not scaling up by 250/27.30. Many candidates did not scale up at all, obtaining an answer of 1081, or merely multiplied by 250/25 and obtained a molar mass of 108.

Notes from the mark scheme and report

• acid A is in the burette so scale by 250.0/27.30
• 1:2 ratio A:NaOH
• common ECF answers 108, 1081, 59, 114, 99 (3 marks)

Skills: M0.2 Use ratios, fractions and percentages; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG2 Acid–base titration

Mark scheme, with the levels and indicative points

Examiner's comment

Candidates were not prepared to answer this type of question and the diagrams were hard to give credit to. Many had significant safety implications such as open beakers of butan-1-ol being heated by a Bunsen burner. Most mis-read the question and just outlined the method for purification and struggled to recall the practical details. Very few candidates mentioned the use of anhydrous salts, referring instead to ‘boiling off’ the water. This candidate was credited 4 marks for this level 2 answer. Although they have drawn distillation apparatus instead of reflux, they have considered the boiling point of the product, detailed using a separating funnel, a drying agent and that the lower organic layer would be drawn off first.

Notes from the mark scheme and report

• reflux with water in at bottom of condenser and open system
• separating funnel – organic layer is the lower (denser) layer
• dry with anhydrous salt
• redistil collecting at 102 °C

Skills: PAG5 Synthesis of an organic liquid

Mark scheme, with the levels and indicative points

Examiner's comment

Most candidates were able to use the information in the table to determine the order of reaction with respect to Fe3+ ions and I– ions, which were 1 and 2 respectively. Most candidates were then able to use this knowledge and further information from the table to determine the rate constant, including units (22.5 dm6 mol–1 s–1). Many candidates appreciated that the mechanism involved a stoichiometric ratio of 1 : 2 with respect to Fe3+ ions and I– ions, but only the more able were able to suggest a suitable possible two step mechanism. Common errors in proposing a mechanism included equations such as Fe3+(aq) + 2I–(aq)  Fe2+(aq) + ½I2(aq) + I–(aq) which gives a net ratio of 1 : 1 for the reactants. or Fe3+(aq) + 2I–(aq)  Fe2+(aq) + I2(aq) + e– which although introducing a correct ratio of reactants also introduces a ‘floating’ electron which in all reality would have attached itself to one of the product species. In questions such as 17a where the quality of extended responses is assessed candidates need to be aware of the need for explanation of their answers. For instance, just giving a correct order of a species in the rate equation is not as strong an answer as one which explains how altering, say doubling, the concentration of a reactant effects the overall rate of reaction, leading to determination of the order. This exemplifies how a concise response can gain full marks on a Level of Response question. The candidate has clearly identified which data they have referred to in the table, and explained the conclusions that can be drawn. The rate equation is then clearly shown, along with subsequent working to determine the value of the rate constant. Finally, the suggested mechanism is given, along with an indication of which is the slow step. The line breaks clearly identify each stage in the response, and it fully satisfies the level 3 criteria in the mark scheme.

Notes from the mark scheme and report

• Fe3+ first order and I– second order supported by quantitative comparison of experiments
• k = 22.5 dm6 mol–2 s–1
• two steps must add up to overall equation, slow step matches rate equation, each step balanced for species and charge with no electrons

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG10 Rates of reaction – initial rates

Mark scheme

Examiner's comment

Higher ability candidates realised that the gradient was equivalent to –Ea/R and determined a gradient within the range ± 800 to ± 1040, depending upon the line drawn. Credit was given to gradients of ± 0.800 to ± 1.040 resulting from calculations which omitted 10–3 on the x-axis. The negative gradient was multiplied by R to determine Ea with a value rounded to 3 significant figures. Common errors were omission of 10–3 in the calculation which led to Ea values between 6.65 and 8.65 J mol–1, or not rounding to 3 significant figures.

Notes from the mark scheme and report

• gradient in range –800 to –1040 (K)
• Ea = –gradient × 8.314
• 3 sig figs AND use of 10^–3 on x-axis
• omission of 10^–3 can score first 2 marks
• ALLOW lines that do not reach the y-axis

Skills: M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M3.2 Plot two variables from experimental or other data; M3.3 Determine the slope and intercept of a linear graph

Mark scheme

Examiner's comment

It is clear candidates are not as experienced at filling in titration tables as might be expected. Every value in a titration table should be recorded to a second decimal place to an accuracy of ± 0.05 cm3. The average titre should be calculated by averaging concordant titres, i.e. those within 0.10 cm3 of each other.

Notes from the mark scheme and report

• all titres to 2 d.p.
• mean from concordant titres only (23.75 and 23.85) = 23.80

Skills: M1.2 Find arithmetic means; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Candidates are unfamiliar with determination of percentage uncertainty. Marks were credited for any percentage uncertainty calculation correctly determined from any titre value, as many opted to choose the trial value as titre 1 or used an average titre.

Notes from the mark scheme and report

• 2 × 0.05 / 23.40 × 100 = 0.43
• 2 d.p. required
• ALLOW ECF from (i)

Skills: M0.2 Use ratios, fractions and percentages; M1.3 Identify uncertainties in measurements and use simple techniques to determine uncertainty when data are combined; PAG2 Acid–base titration

Mark scheme

Examiner's comment

Only the higher ability candidates realised starch needed to be added close to the end-point and this made the resulting colour change (blue-black to colourless) easier to see. The common error was to assume this was an acid-base titration and indicators such as methyl orange or phenolphthalein should be added.

Notes from the mark scheme and report

• add starch near the end point
• blue/black to colourless
• IGNORE 'clear'
• IGNORE colorimetry

Skills: PAG2 Acid–base titration

Mark scheme

Examiner's comment

A significant number of candidates did not draw a simple circuit. Most candidates scored the mark for the Ni electrode but fewer scored the mark for the Pt electrode in the I–(aq)/I2(aq) mixture. The last mark, for the conditions (including concentrations of 1.00 mol dm–3), was almost universally scored even by those who omitted the solutions in their diagram. Common errors were: diagrams with electrodes not entering solutions; salt bridges not labelled or not entering solutions; making solid iodine an electrode; and assuming iodine to be a gas and adapting a standard hydrogen electrode to accommodate this.

Notes from the mark scheme and report

• voltmeter and labelled salt bridge in complete circuit
• Pt electrode in I2(aq)/I–(aq)
• Ni in Ni2+(aq)
• 1 mol dm–3 AND 298 K
• DO NOT ALLOW I2(s)/(g)/(l)
• DO NOT ALLOW '1 mol' for concentration
• IGNORE pressure

Skills: PAG8 Electrochemical cells

Mark scheme

Examiner's comment

Most candidates were able to draw a suitable diagram to show the apparatus required for reflux but some included a stopper on top of the condenser. Many of the diagrams were labelled appropriately but common errors included incorrect direction of water flow or omission of the ‘flask’ label. A small but significant proportion of candidates drew a diagram showing distillation.

Notes from the mark scheme and report

• round-bottom/pear-shaped flask AND upright condenser
• labels: water in at bottom and out at top, heat
• DO NOT ALLOW conical flask/beaker
• DO NOT ALLOW distillation
• DO NOT ALLOW stopper on top of condenser

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Examiner's comment

Almost all candidates were able to correctly describe the use of bromine as a test for an unsaturated chain.

Notes from the mark scheme and report

• both reagent and observation required: bromine (water) decolourised

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Almost all candidates were able to correctly describe the use of Tollens’ reagent as a test for an aldehyde functional group.

Notes from the mark scheme and report

• both reagent and observation required: Tollens' reagent and silver mirror (ALLOW black/grey ppt)

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

The use of 2,4-dinitrophenylhydrazine as a test for the carbonyl group is well known by candidates at this level. The majority of the cohort correctly identified this test and the subsequent analysis of the melting point of the products as a method of identifying each compound. Lower ability candidate responses made reference to analysis of the boiling points of the cinnamaldehyde and methylcinnamaldehyde as a means of identification.

Notes from the mark scheme and report

• 2,4-DNP(H) gives orange/yellow/red precipitate
• take melting point of the derivative
• compare with known values
• DO NOT ALLOW boiling points or chromatograms

Skills: PAG6 Synthesis of an organic solid; PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

The graph was generally well drawn although many didn’t realise that a tangent needed to be drawn. A few tried to draw a straight line of best fit, rather than a curve. Most candidates were able to construct a correct graph, assigning the axes correctly and making good use of the graph paper. Points were usually plotted correctly and most candidates made a good attempt to draw a curved line of best fit. Candidates then needed to draw a tangent at 500 seconds and to measure its gradient to calculate the rate. Lower ability candidates did not draw a tangent to their curve and just calculated the volume formed at 500 seconds.

Notes from the mark scheme and report

• axes labelled with units, linear scales using at least half the grid
• smooth curve through origin not point-to-point lines
• tangent at t = 500 ± 100 s
• rate 0.037–0.047
• using x and y coordinates at 500 s (33/500 = 0.066) scores zero

Skills: M3.1 Translate information between graphical, numerical and algebraic forms; M3.2 Plot two variables from experimental or other data; M3.5 Draw and use the slope of a tangent to a curve as a measure of rate of change; PAG9 Rates of reaction – continuous monitoring

Mark scheme, with the levels and indicative points

Examiner's comment

This part assessed practical aspects of a two-stage organic synthesis. Overall, candidates responded well, and this part was discriminating. Many candidates produced well-structured responses although lower ability candidates do have problems with constructing a cohesive response. Most candidates identified the correct reagents (Sn and concentrated HCl) and the intermediate (4- aminophenol), which was usually shown as its structure. Able candidates usually showed that 11.5 g of 4-nitrophenol is needed for the synthesis. A common error used the ‘inverse percentage’ ratio of 40/100, resulting in an incorrect mass of 1.84 g. Candidates are recommended to check whether a calculated answer looks sensible. Looking at the structures and with a percentage yield of 40%, 1.84 g does not look to be enough of the starting chemical. Some lower ability candidate responses assumed that 5.00 g was 40% of the required mass and responded with 5.00 × 100/40 = 12.5 g. There were some good descriptions of purification, although finer details such as using a minimum volume of hot solvent, washing with cold solvent, and drying) were often omitted. Candidates needed to respond with more than just ‘recrystallisation’. In the purification, common errors were showing the correct steps but in the wrong order and use of a drying agent such as CaCl2 (confusion with part of the purification of an organic liquid). These candidates seemed unaware that adding a solid drying agent to an organic solid would result in impure paracetamol rather than purifying. In comparison, Exemplar 12 is much less detailed: concentrated HCl has not been shown as a reagent for step 1, the candidate has not shown that they know how to carry out a percentage yield calculation, and the purification is confused, and lacks detail.

Notes from the mark scheme and report

• 5.00/151 = 0.0331 mol, ×100/40 = 0.0828 mol, × 139 = 11.5 g (inverted ratio gives 1.84 g)
• Sn + conc HCl then NaOH
• intermediate 4-aminophenol
• recrystallise: dissolve in MINIMUM volume of HOT solvent, cool, filter, wash with cold solvent, dry
• 'recrystallisation' alone is not a detailed description

Skills: M0.2 Use ratios, fractions and percentages; M2.2 Change the subject of an equation; PAG6 Synthesis of an organic solid

Mark scheme

Examiner's comment

The direct determination of an enthalpy change is a common practical procedure. Most candidates were able to calculate the energy change in the reaction as 2.09 kJ, with comparatively few not adding the two 50 cm3 volumes in their calculation. Common errors were using the mass or moles of succinic acid in the mc∆T calculation or to use 278 K, rather than 5ºC for ∆T. Most candidates calculated that 0.02 mol of succinic acid had reacted. The best responses recognised that the enthalpy change of neutralisation (i.e. formation of 1 mol of H2O) was required and that 0.02 mol succinic acid produced 2 × 0.02 = 0.04 mol H2O. 2.09 kJ was then scaled up to give the correct ∆H value of –52.25 kJ mol–1. Example 1 follows this approach and each stage in the working can be followed easily. Most candidates scaled up using 0.02 mol to obtain –104.6 kJ mol–1 and then thought that this was the final answer. Lower ability candidates often calculated the moles of succinic acid as 0.02 but also calculated the moles of NaOH as 0.05 mol. They often then went on to scale up their energy change by 0.05 or 0.07. Many responses were disorganised and poorly presented with unsubstantiated numbers across the answer space. Imperfect calculations may be partially credited by applying error carried forward but this is only possible if sense can be made of the response. Compare the responses in Exemplar 13 and Exemplar 14. In Exemplar 14, there is no labelling to show what each stage refers to. It looks as if the candidate has calculated the moles of NaOH rather than succinic acid and has used this in the subsequent stage. Error carried forward has been applied generously on this assumption. Answer = –52.25 kJ mol–1.

Notes from the mark scheme and report

• –52.3 from two cancelling errors (50 g instead of 100 g AND no ÷2) scores only 2
• succinic acid limiting, 2 mol H2O per mol acid
• DO NOT ALLOW fewer than 3 SF
• – sign required

Skills: M0.0 Recognise and make use of appropriate units in calculations; M1.1 Use an appropriate number of significant figures; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; PAG3 Enthalpy determination

Mark scheme

Examiner's comment

Candidates found this part difficult and only higher ability candidates identified that a titration could easily determine the concentration of succinic acid. The answers seen covered most of the techniques encountered in the course. Candidates should consider the information provided in a practical context to arrive at an informed response rather than what sometimes seemed to be a guess.

Notes from the mark scheme and report

• titration
• IGNORE type of titration

Skills: PAG2 Acid–base titration

Mark scheme

Notes from the mark scheme and report

• DO NOT ALLOW hydrogen for bubbles
• molar mass must be used at least once in reasoning
• names or KCO3/NaSO4 score only one of the two identification marks

Skills: PAG4 Qualitative analysis of ions