Equation questions

Mark scheme

Examiner's comment

This was well answered by the majority of candidates. Where the mark was missed, it was either through incorrect products or incorrect balancing of oxygen.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates were able to write the ionic equation. Common incorrect responses included giving the full equation, word equation or using the spectator ions i.e. Na+ + Cl- → NaCl

Notes from the mark scheme and report

• H+ + OH– → H2O
• IGNORE state symbols

Mark scheme

Examiner's comment

More than half of the candidates gave the correct equation. However, many lost the mark for missing or incorrect state symbols. In some responses it was hard to differentiate between (g) and (s) – candidates need to make sure their handwriting is clear so credit can be given. Some other errors that were seen included: losing 2 electrons, incorrect charges, gaining electrons or using Mg rather than Ca.

Notes from the mark scheme and report

• Ca+(g) → Ca2+(g) + e–
• starts from Ca+ not Ca
• IGNORE states on the electron

Mark scheme

Examiner's comment

The majority gave a correctly balanced equation here. The most common error was an incorrect number of oxygen molecules. A few candidates gave the equation for propane rather than pentane.

Notes from the mark scheme and report

• C5H12 + 8O2 → 5CO2 + 6H2O
• DO NOT ALLOW multiples (one mole of pentane)

Mark scheme

Examiner's comment

Generally well known, with most recognising that HBr is formed as a product. Incorrect responses sometimes had 2 Br instead of Br2 or included other radicals in their response. Some candidates confused propane and pentane, despite being given them in the table. Some started with a longer C chain, breaking the chain and substituting with Br to give bromopropane as state in the question, e.g. C4H10 + Br2 → C3H7Br + CH3Br. Some used structural formulae rather than the molecular formula which was asked for.

Notes from the mark scheme and report

• C3H8 + Br2 → C3H7Br + HBr

Mark scheme

Examiner's comment

Most candidates were given this mark. However, there were multiple errors made which included the wrong element, missing or incorrect state symbols, and the formation of a negatively charged ion. Misconceptions included that a non-metal would form a negative ion, and the sulfur existed as S8 rather than an atom. Candidates are advised to make sure that state symbols are clearly written and the (s) and (g) can be easily distinguished by the reader. The convention is to use lower case letters of normal size, e.g. S(s) or S(g).

Notes from the mark scheme and report

• S(g) → S+(g) + e–
• IGNORE state symbol on the electron

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

Most candidates were unable to gain credit here. The most common incorrect response given was C6H14 + 6O2 → 6CO + 7H2O which doesn’t balance in terms of oxygen. A significant number recognised that this was not balanced so gave C6H14 + 6.5O2 → 6CO + 7H2O but this misses the crucial 1:6 ratio of fuel/hexane to oxygen required in the question. Some gave H2 as a product of incomplete combustion, e.g. C6H14 + 6.5O2 → 6CO + 6H2O + H2. Candidates need to be reminded that C or CO will form in a limited supply of O2 but not H2. Candidates seemed unaware that multiple C products of combustion can be formed in one reaction. A few gave O2 as a product as well as a reactant.

Notes from the mark scheme and report

• C6H14 + 6O2 → 5CO + C + 7H2O (or other CO/CO2/C combinations in a 1:6 ratio)
• DO NOT ALLOW H2 as a product

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

The majority were able to get the mark for the 2,4,6-tribromophenol structure, with some candidates using the relative molecular mass given to confirm their answer. Many were also able to obtain the mark for the balanced equation. Common errors seen in balancing included omitting HBr as a product or only having 1.5 Br2. Some gave H2 as a product. Lower attaining candidates struggled, often replacing the -OH with a -Br group but not accounting for the molar mass.

Notes from the mark scheme and report

• 2,4,6-tribromophenol only (DO NOT ALLOW other positions)
• 3Br2 and 3HBr
• second mark dependent on first

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates found all aspects of this question extremely difficult, one of the most testing’s on this examination paper. There was important information in the flowchart to aid candidates: the ‘green solution’ indicated that Cr3+ was one of the products, and the formation of a white precipitate with Ba2+(aq) confirmed the presence of SO42–(aq). The reacting ions were provided in the flowchart. Together this information provided a framework for the equation: SO32–(aq) + Cr2O72– → Cr3+ + SO42–(aq). The equation then needed to be balanced, ideally using oxidation numbers, H2O added as a product, and finally balancing. Similar problems would be an ideal way of preparing candidates for writing equations for redox reactions.

Notes from the mark scheme and report

• 3SO32– + Cr2O72– + 8H+ → 2Cr3+ + 3SO42– + 4H2O
• DO NOT ALLOW other Cr or S species (Cr2+, Cr(OH)3, [Cr(OH)6]3–)
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates found this question difficult with few being given 2 marks. 1 mark was available for showing an equation with BH4– and OH– as reactants and NaBO2 and H2O as products. The equation often included O2 as a reactant and 4e–. Only higher attaining candidates were able to combine the equation successfully. This question was a very good discriminator.

Notes from the mark scheme and report

• NaBH4 + 8OH– → NaBO2 + 6H2O + 8e–
• 1 mark if correct but reversed
• species common to both sides must be cancelled

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates were able to balance this straightforward equation.

Notes from the mark scheme and report

• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This question discriminated very well. Most candidates were given a first mark by showing correct structures for the organic reactant and its product, 2-methylpropan-2-ol. The question asked for an equation for alkaline hydrolysis and candidates were expected to use an alkali. Acceptable responses would include NaOH/KOH and NaBr/KBr, or OH– and Br–. Equations including H2O and HBr were not given marks, a common error for alkaline hydrolysis. Candidates are advised to carefully read the requirements in the question. For the most successful responses, candidates often underline these to draw their attention.

Notes from the mark scheme and report

• structures of both organic compounds with OH– on left and Br– on right
• DO NOT ALLOW H2O and HBr (question specifies aqueous alkali)
• DO NOT ALLOW unbalanced charges

Mark scheme

Examiner's comment

Many candidates correctly gave the balanced equation here. However, some didn’t balance but had the correct formula. A few gave Mg2 as a reactant or MgO2 as a product. Some had O2 on both sides of the equation.

Notes from the mark scheme and report

• 2Mg + O2 → 2MgO
• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

More than half of candidates scored both marks here. Errors seen included missing or incorrect state symbols, especially (s), but also (aq) was seen. Some had electrons on the left hand side of the equation, i.e. ‘B+ + e- → Be2+’. Some included negatively charged ions and occasionally the wrong element was used, e.g. Mg or Be.

Notes from the mark scheme and report

• B+(g) → B2+(g) + e–
• state-symbol mark dependent on correct equation except for listed near-misses (e.g. B(g) → B2+(g) + 2e–)

Mark scheme

Examiner's comment

Most students scored the equation mark, although some presented unbalanced equations. Nearly all candidates used the ionic equation. Explanations were well argued with most candidates using the order of reactivity. A few considered the oxidising power of the halogen. Candidates are advised to ensure that both comparisons are clearly made, and it is obvious which of the two halogens the response is referring to. It is also important that candidates can distinguish between a halogen and a halide. Some candidates explained in terms of electronegativity and displacement.

Notes from the mark scheme and report

• ionic equation required (DO NOT ALLOW full equation)
• compare reactivity/oxidising strength of Cl2, Br2 and I2
• care with -ide/-ine
• 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)

Mark scheme

Examiner's comment

Some candidates coped well with this question which was based on the AS part of the specification and gained all three marks. Common errors were for unbalanced equations in reaction 1 or adding H2 to the product of reaction 2. Reaction 3 was often, incorrectly, considered as: redox, halogenation, nucleophilic substitution or a precipitation reaction.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols
• Reaction 3 = neutralisation/acid–base
• Regular practice writing formulae and balancing chemical equations will help to consolidate these concepts, avoiding basic errors such as giving formula of group 2 hydroxide as BaOH.
• We have produced a topic exploration pack to assist with learning about the reaction of group 2 elements and their compounds: Teach Cambridge (ocr.org.uk)

Mark scheme

Examiner's comment

Most candidates were successful in constructing this equation. Common errors included the backwards reaction and some with unbalanced Li+. Candidates should also check their equations, as a few included an erroneous C in the formula e.g. LiCoCO2.

Notes from the mark scheme and report

• DO NOT ALLOW uncancelled species

Mark scheme

Examiner's comment

This question differentiated well, with a wide mix of responses given. Just over 40% scored all 3 marks. Equation: The most common error was to omit HBr as a product. Some gave H2 or just Br as a product. Some misunderstood the question and attempted to give either a partial mechanism, such as the propagation step or a complete mechanism, rather than the overall equation. Some gave molecular formula rather than structures as asked for. Those using structural or displayed formula were more prone to errors, such as missing hydrogens or incorrect chain length, than those that were confident using skeletal formula. Candidate should avoid giving equations in two formats, e.g. skeletal and structural because slips in one will lose marks. Mechanism: Many candidates were able to identify the radical substitution mechanism, but a significant number did not score here. Most common incorrect responses were electrophilic or nucleophilic substitution but there were also those that thought it was an addition reaction. Bond fission: A significant number identified this as heterolytic, even if the recognised mechanism was radical. Some struggled with the spelling or even suggested homogeneous or heterogenous.

Notes from the mark scheme and report

• overall equation (CH3)3CH + Br2 → (CH3)3CBr + HBr (IGNORE mechanism steps)
• radical substitution
• homolytic fission
• Many struggled to identify the mechanism and then to link to bond fission. Try to introduce key terminology early on in teaching organic chemistry so that it can then revisited with each topic. Relevant mechanism terminology: - substitution / addition / elimination - electrophile / nucleophile / radical - homolytic / heterolytic

Mark scheme

Examiner's comment

This question differentiated between candidates well with the full range of marks seen. Some couldn’t identify the correct reagent to use to form an acyl chloride i.e. SOCl2, so lost 2 marks for the first step. Use of HCl as an alternative was common. A significant number used SOCl2 but struggled to balance the equation. Many made errors with structures used, for example missing the CH3 side chain, adding CH3 to 3-position instead 4, inserting an additional CH2 group or using a much easier structure, e.g. ethanoic acid. None of these structures were given marks as they would not form Ester F. Some used R- instead to simplify, for which credit was given, but only if it was clear what structure R represented. Another common error was to include an additional O in the acyl chloride group i.e. COOCl not COCl. The most common error made in Step 2 was to omit the formation of HCl or to use H2O instead. A few lost the final mark for a connectivity error on ethanol i.e. OHCH2CH3.

Notes from the mark scheme and report

• SOCl2 in step 1 giving acyl chloride + SO2 + HCl
• acyl chloride + ethanol gives ester F + HCl
• DO NOT ALLOW incorrect connectivity on OH (ECF on later structures)

Mark scheme

Examiner's comment

The majority of candidates were able to give a suitable tri-substituted product, with many showing the structure although not asked for in the question. Many were also able to give a correct balanced equation too. Some were unsure how phenylamine would react showing the reaction with the amine group or only giving a monosubstituted product. Some didn’t form HCl as another product, reacting phenyl amine with 1.5 Cl2 instead. Others gave hydrogen as the product.

Notes from the mark scheme and report

• 2,4,6-trichlorophenylamine (ALLOW any trichloro structure) + 3HCl with 3Cl2
• 1 mark for a balanced equation using Br2

Mark scheme

Examiner's comment

Another fairly challenging question, however most secured at least one mark for giving an equation for the reaction of sulfuric acid with sodium carbonate. Less confident candidates struggled to gain any marks as they were unable to give correct formula for sodium sulfate, giving NaSO4 for example. Although many attempted the equation showing the reaction of compound G with sodium carbonate, only some correctly identified that only the carboxyl group would react, not the phenol. A small minority of students were able to balance the second equation gaining all 3 marks.

Notes from the mark scheme and report

• Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O
• only the COOH of G reacts (phenol OH unchanged) giving –COONa
• 2 G per Na2CO3 (ALLOW 1:1 forming NaHCO3)
• ALLOW ionic equations

Mark scheme

Examiner's comment

Candidates were required to write a straightforward ionic equation that they would have encountered many times during the A Level Chemistry course. It was surprising that only just over half the candidates produced an equation that could be given. Common errors included the following. • Omission of state symbols or incorrect state symbols, especially (aq) in AgCl(s). • Inclusion of nitrate ions or use of AgNO3 instead of Ag+. • An equation using Cl2 and forming AgCl2. Some candidates used the ideal gas equation to determine the moles of hydrogen, choosing suitable values for temperature and pressure. This approach was allowed, although the exercise would have wasted candidate time compared to the much simpler division by 24 for using RTP, which is stated in the question.

Notes from the mark scheme and report

• ALL 3 state symbols required

Mark scheme

Examiner's comment

Most candidates were given a mark for one of their half-equations. The half-equation for the oxidation of iron(II) ions was correct more often than that for ethanedioate ions. Strangely, a significant number of half-equations started with Fe metal instead of Fe2+ ions. The mark scheme allowed for variations in the oxidation of ethanedioate ions, with several alternative equations being given from the correct equation forming CO2 receiving a mark.

Notes from the mark scheme and report

• Fe2+ → Fe3+ + e–
• C2O4 2– → 2CO2 + 2e–
• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

Candidates found this question quite challenging, with only about one-third writing a correct equation. The question gave the reactants and products with only the formula of phosphorus(V) oxide having to be worked out. The actual reaction does produce P4O10 but P2O5 was shown in almost all equations, and this was acceptable. Various incorrect formulae were seen for phosphorus(V) oxide including PO, PO2, P5O, HPO, etc. Unfortunately a significant number of candidates could not balance the equation, despite using correct formulae.

Notes from the mark scheme and report

• 4PH3 + 8O2 → P4O10 + 6H2O
• ALLOW P2O5 version and multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This question generated a wide range of responses, testing many important chemical skills. Candidates often used oxidation numbers correctly to show that silver is reduced and phosphorus oxidised, with silver being the easier. Hydrogen was sometimes incorrectly chosen for oxidation. The oxidation number change of +1 to 0 for silver was usually correct although +9 and +11 were common errors for silver in AgNO3, presumably by choosing the oxidation number of nitrogen as –3 or – 5. Candidates usually recognised that phosphorus started with an oxidation number of –3 but the oxidation number of +5 was a common error in H3PO3. Balancing the equation was the most difficult part of this question with numbers being added almost at random. It is easier to balance equations for redox reactions by balancing the oxidation number changes first.

Notes from the mark scheme and report

• 6AgNO3 + PH3 + 3H2O → 6Ag + H3PO3 + 6HNO3
• Ag reduced +1 to 0, P oxidised –3 to +3
• + signs required
• 1 mark if both changes correct but oxidised/reduced omitted or reversed
• IGNORE electrons
• Ag+ and NO3– are among the common ions that students should know (see also Question 4 (c) (i)).
• For a NO3– ion to have a charge of 1–, the oxidation number of nitrogen must be +5. By choosing –5, the charge on NO3 would be –11 and silver would have an oxidation number of +11. This is completely unrealistic and should be rejected as it points to a serious error.

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This question again required candidates to construct an equation. Candidates were provided with the formula of all species reactants and products except for that of ammonium chloride. Candidates are expected to know that the ammonium ion is NH4+ but many incorrect equations showed NH3Cl. About half the candidates were able to construct a correctly balanced equation with the ‘12’ balancing number for HCl being the hardest part. This links back to the ‘assessment for learning’ callout added to Question 4 (b) (ii) in this report. As with other questions requiring equations to be written, this question differentiated very well. Writing formulae and balancing equations are fundamentals for mastering chemistry and candidates are advised to practise these skills throughout the course.

Notes from the mark scheme and report

• 3PCl5 + 3NH4Cl → P3N3Cl6 + 12HCl
• ALLOW multiples
• The specification states the following.
• Formulae and equations 2.1.2(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+ This section will be studied at the start of the two-year course and form the backbone for chemical literacy. For success in chemistry, the common ions should be learnt.

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This question was answered well by candidates, with most showing correct structures for the organic reactant and its product, propan-1-ol, and skeletal formulae mostly used. The question asked for an equation for alkaline hydrolysis and candidates were expected to use an alkali. Acceptable answers would include NaOH/KOH and NaBr/KBr, or OH– and Br–. Equations including H2O and HBr were not given a mark, a common error for alkaline hydrolysis.

Notes from the mark scheme and report

• structures required (DO NOT ALLOW C3H7Br/C3H7OH)
• OH– and Br– or KOH and KBr
• DO NOT ALLOW H2O and HBr
• DO NOT ALLOW K–OH covalent

Mark scheme

Examiner's comment

This question required candidates to recognise the reaction as being ‘acid–base’ and to interpret a formula from a name containing a Roman numeral. Candidates identifying the formula of copper(II) oxide as CuO were normally able to complete the equation. A reasonably large number identified the copper compounds as CuO2 and CuCl. Overall, most candidates produced a correct equation.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols and charges

Mark scheme

Examiner's comment

Most candidates were able to construct a balanced equation for the combustion of heptane. Most were aware that CO2 and H2O would be the products although some generated CO, C6H12 or unusual compounds such as C7H14O. The hardest part was the formula of heptane itself with use of hexane instead being a common error; candidates who made this error were given 1 mark, provided that their equation was balanced.

Notes from the mark scheme and report

• 11O2
• ALLOW multiples
• 1 mark ECF for a balanced combustion equation of a different alkane

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This question discriminated very well at the top end of the ability range, but many ignored the instruction to use skeletal formula and obtained no marks as a result. Of those that did use skeletal formula, many placed the dot on the wrong carbon atom or produced 1- brombutane, rather than 2-bromobutane, stated in the question. A mark was still available by ECF for misplaced or absent dots or formation of 1-brombutane with dots. This question was aimed to be demanding and so it proved to be.

Notes from the mark scheme and report

• skeletal formulae required (no credit for molecular formulae)
• radical dot on C2
• 1 mark ECF if dots omitted/misplaced or for forming 1-bromobutane correctly

Mark scheme

Examiner's comment

Most candidates scored the mark for the correct observation. A few said what would happen rather than what they would see, e.g. gas is formed, pH would increase, mass lost or even reference to the ‘squeaky pop’ test. Many struggled to give the correct balanced equation with either CaO being given as a product or incorrect balancing. Many did not have a gas produced but then had bubbling as an observation.

Notes from the mark scheme and report

• Ca + 2H2O → Ca(OH)2 + H2
• observation must be observable (fizzing, solid disappears, white ppt): IGNORE gas formed, temperature or pH change, 'turns cloudy'
• DO NOT ALLOW colour change
• OCR has some resources to help support the understanding of balancing symbol equations such as this delivery guide for Atoms and equations.

Mark scheme

Examiner's comment

Less than half the candidates gained credit for this challenging question. There was lots of information to process. Many struggled to give the correct formula for the products, e.g. NaNO3, Ba2SO4, or had issues with balancing. Some tried to involve the NaCl in the reaction, either recognising that it didn’t react (acceptable on the mark scheme) or forming barium chloride or even Cl2. Lots of candidates lost the mark for state symbols as they left Ba(NO3)2 as (s), not recognising that in step 1 the mixture was dissolved in water so should now be (aq).

Notes from the mark scheme and report

• BaSO4(s) with other species (aq)
• ALLOW ionic equation
• state symbol mark dependent on balanced equation

Mark scheme

Examiner's comment

Most candidates did not score either mark here, despite the structures for B and C being given in the table below for (iii). Many thought this was oxidation, showing [O] in equations and giving a carbonyl product. Many had alkenes but still with the -OH present. Some attempted to use structural or displayed formulae but errors were made in giving the correct number of H atoms. For those that did have the correct structure, they often did not give an equation, added the acid as a reactant, or missed off the water as a product.

Notes from the mark scheme and report

• D = 3-methylpent-1-ene (no E/Z) + H2O
• DO NOT ALLOW H+ or [O] as reactants
• ALLOW 3-methylpent-2-ene for the balancing mark only

Mark scheme

Examiner's comment

This was often answered correctly but some candidates gave the incorrect formulae for Sr(NO3)2 and either no other product or H2 gas.

Notes from the mark scheme and report

• DO NOT ALLOW H2CO3 for H2O + CO2 (a gas is produced)
• ALLOW multiples

Mark scheme

Examiner's comment

Ionic equations still present candidates with a challenge. A few candidates scored the mark but many candidates gave a full equation or one that contained a mismatch of spectator ions as well as the correct ions.

Notes from the mark scheme and report

• Mg + 2H+ → Mg2+ + H2
• ALLOW multiples

Mark scheme

Examiner's comment

Very few candidates were able to produce the balanced overall equation; a few had the correct reactants and products but not balanced. Candidates are advised to look for the information contained within the question. The formulas were given, and it was stated that the solution was acidified, leaving only water to be identified. Some candidates approached this through two half equations whereas others used oxidation numbers to balance their equations.

Notes from the mark scheme and report

• 3V3+ + Cr2O7 2– + 2H+ → 3VO2+ + 2Cr3+ + H2O
• first mark for all species
• DO NOT ALLOW electrons in final answer

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most responses gained at least 1 mark for this question giving the correct molecular formula of C6Br14. However many hadn’t assimilated that when a hydrogen atom is substituted in an alkane it requires one mole of a halogen and produces one mole of the hydrogen halide. So many gave this incorrect equation instead: C6H14 + 7Br2 → C6Br14 + 7H2. Some lost marks for C5H14 or for use of structural formulae.

Notes from the mark scheme and report

• C6Br14
• C6H14 + 14Br2 → C6Br14 + 14HBr (HBr, not H2)
• 1 mark if balanced but not in molecular formulae

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Approximately half the candidates gained both marks here but just over a third gained no credit. A very common error was C6H12 + 9O2 → 6CO2 + 6H2O missing the need for an alcohol group. Another common error was balancing with 9O2 i.e. not deducting O from alcohol from their count of O atoms. Some struggled to determine the correct number of Hs when a single C=C bond is introduced so gave C6H12OH or C6H13OH instead. Lower attaining candidates did not understand what happens during complete combustion. For example, they used [O] instead of molecular oxygen or didn’t have CO2 and water as the products. Some used structural formula which made it easier to get the correct formula of the reactant but often made it trickier to balance the equation.

Notes from the mark scheme and report

• C6H11OH (C6H12O) + 8½O2 → 6CO2 + 6H2O
• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates were required to write a balanced equation for an acid–base reaction. As with Question 4 (b) (ii), candidates needed to write formulae from what should have been common ions, but the formulae for aluminium oxide and aluminium nitrate were often incorrect. In the equation, the reactants and products were sometimes unbalanced, or incorrectly balanced. A common error was H2 instead of H2O as the second product. The question was an excellent discriminator.

Notes from the mark scheme and report

• Al2O3 + 6HNO3 → 2Al(NO3)3 + 3H2O
• 1 mark for any three species correct
• DO NOT ALLOW more than 4 species
• ALLOW ionic equation

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates added ‘4’ before HCl and H+, and 3 before e– to gain this mark. Where an error was made, it invariably was with the number of electrons, usually 4e–.

Notes from the mark scheme and report

• Au + 4HCl → 4H+ + AuCl4– + 3e–

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates found the equation hard, despite this reaction being specification content and the inclusion in the earlier part of the stem of ‘NaClO’ as one product. The correct response required candidates to realise that NaCl would be a product and to balance the resulting equation. Some did not add the balancing ‘2’ before NaOH, and many selected HCl as the second product, a compound that would react further with NaOH to produce NaCl. The explanation worked the same whether NaCl or HCl had been identified as the second product. There were some excellent responses, providing the correct oxidation number changes, linking these to the species involved and identifying the changes as either oxidation or reduction. Two explanation marks were available with marks not being given for omission of one of the three features described above. This exemplar has been included to emphasise the points made above. It was only possible to award this response 1/3 marks. The equation shows the common error of the second chlorine-containing product being HCl and not NaCl: 0 marks The candidate has identified the oxidation number changes and has linked these to the correct species. The last statement in brackets is correct but the candidate has not communicated which oxidation number change is oxidation and which is reduction: 1/2 marks

Notes from the mark scheme and report

• DO NOT ALLOW Cl2 + NaOH → NaClO + HCl
• signs required on oxidation numbers
• 1 of 2 redox marks if changes correct but oxidation/reduction assigned the wrong way round
• IGNORE electron loss/gain

Mark scheme

Examiner's comment

Despite oxidation of alcohols being a topic that has been assessed many times, candidates found this question very tricky with a majority failing to score any marks. Candidates needed to correctly identify this as a secondary alcohol which, when oxidised, will become a ketone. A wide range of errors were seen, including the following: Other errors included forming C=C or removing COOH. Approximately half of those that got the correct structure were also able to balance the equation gaining both marks. Some forgot that another product was formed or gave H2 instead. Candidates were given the structure and told to use [O] to represent the oxidising agent. However, some attempted to use structural or molecular formula and/or adding in dichromate to their answers.

Notes from the mark scheme and report

• secondary alcohol group oxidised to ketone HOOCCH2COCOOH
• COOH groups unchanged

Mark scheme

Examiner's comment

Around half of all candidates did not score this mark. The most common error was giving SrO as the product rather than the hydroxide. Other errors included incorrect balancing (missing 2 on H2O, SrOH as the formula of the hydroxide and no hydrogen formed (often giving H2O instead)).

Notes from the mark scheme and report

• Sr + 2H2O → Sr(OH)2 + H2
• IGNORE state symbols
• Regular practice writing formulae and balancing chemical equations will help to consolidate these concepts, avoiding basic errors such as giving formula of group 2 hydroxide as SrOH.

Mark scheme

Examiner's comment

Candidates always find radical mechanisms tricky and this one had the added complexity of forming 2- bromo isomer. However, a majority of students still gained marks. Many candidates formed the incorrect radical removing H from C-1 i.e. CH3CH2CH2CH2• therefore scoring only 1 mark. Some responses were a little messy making it very easy to miss off a dot or H or Br. Many candidates reacted with Br• in the first step but added Br to the radical intermediate (as well as forming HBr). Candidates should always check equations so that they balance in terms of atoms.

Notes from the mark scheme and report

• radical must be CH3CH2•CHCH3 (1 mark if CH3CH2CH2CH2• used or both equations correct with molecular formulae)
• dots required

Mark scheme

Examiner's comment

Some candidates coped well with this question which was based on the AS part of the specification and gained both marks. More candidates gained 1 mark through writing one half equation, usually the oxidation of magnesium. Common errors were for chlorine to featuring in the reduction half equation and the lack of electrons in their answers. Very few candidates mixed up the oxidation and reduction equations.

Notes from the mark scheme and report

• Mg → Mg2+ + 2e–
• 2H+ + 2e– → H2
• only species that change
• 1 mark if both correct but labelled the wrong way round

Mark scheme

Examiner's comment

This question was answered well, with many candidates giving one of the equations in the ‘ALLOW’ part of the mark scheme. Those candidates who did not gain this mark gave full equations or missed out state symbols.

Notes from the mark scheme and report

• Ba2+(aq) + 2H+(aq) + SO4 2–(aq) + 2OH–(aq) → BaSO4(s) + 2H2O(l)
• ALLOW H+ + OH– → H2O OR Ba2+ + SO4 2– → BaSO4 with state symbols

Mark scheme

Examiner's comment

This was another very challenging question using an unfamiliar mineral. Most candidates identified a formula of salts containing both magnesium and calcium, or carbonates of the separate elements. Only the most successful candidates were able to give the correct formula. Common errors, for those who solved the formulae, were the use of “4”HCl in balancing and the absence of state symbols.

Notes from the mark scheme and report

• Mg3Ca(CO3)4(s) + 8HCl(aq) → 3MgCl2(aq) + CaCl2(aq) + 4H2O(l) + 4CO2(g)
• balancing/state symbols mark dependent on correct formulae

Mark scheme

Examiner's comment

Most students scored this mark, although several gave no response.

Notes from the mark scheme and report

• Fe2+ + 2OH– → Fe(OH)2
• ALLOW complexed formulae
• IGNORE state symbols

Mark scheme

Examiner's comment

Although a spread of marks across the full available range was seen, a good proportion of candidates gained 3 or 4 marks. Most candidates were able to produce a balanced equation, but candidates should take care cancelling out any species present on both side of the equation, e.g. the hydroxide ions. A common error within the formula of iron (III) hydroxide was to place the number of hydroxide ions within the brackets, e.g. Fe(OH3). Candidates are advised to read the instructions contained within the equation and to use or comment on all the data presented. When commenting on electrode potentials, candidates should avoid the use of higher/lower as these phrases are meaningless due to the negative signs involved.

Notes from the mark scheme and report

• brown ppt is Fe(OH)3/Fe(OH)2 oxidised
• E of system 1 more negative than system 2
• system 1 shifts left (or system 2 right)
• 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3
• IGNORE lower/higher
• DO NOT ALLOW uncancelled species

Mark scheme

Examiner's comment

This question proved challenging for candidates. The first equation was often answered correctly, although some candidates used sodium hydroxide rather than potassium hydroxide in their response. The second equation was frequently incorrect. Candidates frequently missed a hydrogen from the structure for methanolic acid or did not recognise that hydrogen was a product. Many candidates did not account for magnesium having a 2+ charge when working out the product. For the third equation, the majority of candidates identified that carbon dioxide and water would be produced but were unable to give the correct formula of the salt as they did not interpret the information given regarding the R group.

Notes from the mark scheme and report

• 2HCOOH + Mg → (HCOO)2Mg + H2
• both COOH groups of aspartic acid react with excess Na2CO3 giving H2O AND CO2 (ALLOW H2CO3)
• DO NOT ALLOW –COO–K covalent bond or missing charge (penalise once)

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates were able to score at least 1 mark for this question. Common errors included candidates producing two water molecules or failing to balance [O]. A significant proportion of candidates did not score any marks, frequently due to the organic product having too many carbon atoms in it.

Notes from the mark scheme and report

• product C6H5CH2COOH (reflux)
• 2[O] and H2O

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates were asked to predict an unfamiliar equation from provided information and to draw structures of square planar complexes containing an amino acid. Candidates found the structures easier than the equation, with many drawing 3D structures with 2 out-wedges and 2 in-wedges and attaching the NH2 and COO groups correctly. It was also common to see a ‘criss-cross’ orientation, looking down on the complex, which is easier to draw. Many candidates connected the NH2 and COO groups next to, and across from, each another in the isomers. A common error was for candidates to rotate their first structure, to produce a second drawing of the first structure. Less successful responses often tried to attach NH2 and COO groups but with no CH2 between the groups to produce a cyclic attachment. A minority of candidates ignored ‘square planar’ and drew tetrahedral structures instead. The equation proved to be very difficult, the commonest error being omission of the ‘2’ balancing numbers for H2NCH2COOH and CH3COOH. The formulae for ethanol or propanoic acid were also often seen for ethanoic acid. Candidates are advised to check all formulae and then to check balancing, the golden rules for successfully constructing all equations.

Notes from the mark scheme and report

• 2H2NCH2COOH + Cu(CH3COO)2 → Cu(H2NCH2COO)2 + 2CH3COOH
• cis and trans square planar isomers with ligands shown as bidentate rings bonded through N and O of COO
• 1 mark if both drawn as tetrahedral
• IGNORE charges

Skills: M4.2 Visualise and represent 2-D and 3-D forms including 2-D representations of 3-D objects

Mark scheme

Examiner's comment

Candidates were required to write a balanced equation from provided reactants and products and to use oxidation numbers to illustrate disproportionation. In the equation, the reactants and products were sometimes unbalanced, or incorrectly balanced. A common error was to balance the equation after first adding O2 as an extra reactant. Illustrating disproportionation proved to be easier with the oxidation number changes from the initial +1 being required. Otherwise, more successful responses sometimes missed out on marks if they omitted detail. For example, the oxidation number changes were stated but the candidate omitted to state which change was oxidation and which was reduction. The best responses identified Ca(ClO3)2 as the oxidation product and CaCl2 as the reduction product. One unexpected error on many scripts was for calcium to be identified as the element undergoing disproportionation, with oxidation number changes from +6 to +14 and +2. It was difficult to see why this happened, with Ca forming +2 compounds, but the error was common.

Notes from the mark scheme and report

• 3Ca(ClO)2 → 2CaCl2 + Ca(ClO3)2
• Cl reduced +1 to –1 and Cl oxidised +1 to +5
• 1 of 2 redox marks if changes correct but oxidation/reduction mis-assigned or not stated
• signs required
• IGNORE ionic charges such as Cl5+

Mark scheme

Examiner's comment

Candidates needed to interpret the information provided and to use this as the basis for their redox equation. The clue of a yellow product proved to be very difficult to interpret as being sulphur. The equation then required H+ to be added as a reactant (‘acidified’ in the information) and H2O as the other product. Balancing required use of oxidation numbers. Candidates found this equation very difficult and relatively few correct equations were seen. The mark scheme did allow 1 mark for any correct five species but the correct equation proved to be challenging in demand. Question 3 (a*) This question was assessed by level of response (LoR). Candidates were required to describe key features in a procedure to prepare a pure organic liquid, including a labelled diagram for reflux, a calculation of the percentage yield and the procedural steps for purification. Levels were determined using these three features. Marks within a level were determined by communication. This question discriminated extremely well. Level 3 candidates would draw a clear diagram with all key items labelled and the set up being capable of being used safely. The percentage yield calculation would be correct, producing a percentage yield close to 45.2%. The steps for the purification: use of a separating funnel, drying and redistillation would be described in the correct order and with some detail. Level 2 candidates would have obtained some of the features required for Level 3 but there would be some key omissions or errors. The diagram may have been drawn clearly but labelling may have been incomplete or a thermometer with bung may have been inserted into the top of the condenser, a very hazardous arrangement. The calculation would be attempted but with some errors, such as omitting to use the density, or using a mixture of moles and masses. The purification steps may have been described but in the wrong order. Purification steps would be incomplete, perhaps only including distillation. Level 1 candidates often drew a diagram resembling a tube above a flask, with water often flowing in the wrong direction. The percentage yield may have been a simple mass ratio with no moles being used. A significant number of candidates described the purification steps for an organic solid, including recrystallisation. The preparation of an organic liquid is a key practical procedure that will have been experienced by students during their A Level studies (PAG 5). The overall standard of drawing diagrams was poor, an area that needs improvement.

Notes from the mark scheme and report

• 5H2S + 2MnO4– + 6H+ → 2Mn2+ + 5S + 8H2O
• first mark for any five correct species (yellow precipitate is sulfur)
• IGNORE electrons and state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Most candidates attempted to write an equation for the combustion of the 1:1 molar mixture of octane and ethanol. The formulae of C8H18 and C2H5OH were usually seen although some candidates combined these as a ‘mixture formula’ of C10H24O (which was accepted). The balancing of the equation using 15½O2 was the hardest part of the equation and many different balancing numbers for O2 were seen (10CO2 and 12H2O where usually correct). Less successful responses often attempted a combustion equation using octane OR ethanol, but not both. This is not an easy equation to construct, and the context was novel. Overall candidates made a good attempt at this question.

Notes from the mark scheme and report

• C8H18 + C2H5OH + 15½O2 → 10CO2 + 12H2O
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• 3KClO4 + 8Al → 3KCl + 4Al2O3
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

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

• 2Br2 and 2HBr needed to balance
• correct structure of 1,3-dibromo-2-methylpropane

Mark scheme

Notes from the mark scheme and report

• skeletal formulae required for the first mark
• (CH3)3COH + HCl → (CH3)3CCl + H2O

Mark scheme

Notes from the mark scheme and report

• B = (CH3)2CHCH2OH
• reflux gives carboxylic acid so 2[O] and H2O
• ALLOW ECF for product and equation if B is a different primary or secondary alcohol

Mark scheme

Notes from the mark scheme and report

• Mg3+(g) → Mg4+(g) + e–
• state symbols required

Mark scheme

Notes from the mark scheme and report

• CuCO3 and balanced equation giving CO2
• ALLOW other copper(II) compounds giving a gas e.g. CuSO3

Mark scheme

Notes from the mark scheme and report

• 2Cu2+(aq) + 4I–(aq) → 2CuI(s) + I2(aq)
• state symbols required

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• 5Cr + 3MnO4– + 24H+ → 5Cr3+ + 3Mn2+ + 12H2O
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• H2 + 2OH– → 2H2O + 2e–
• ALLOW multiples

Mark scheme

Examiner's comment

Question 20(a)(iii): Candidates who found this question difficult often did not recognise the functional groups present in the reacting molecule.

Notes from the mark scheme and report

• cyclic diester (two ester links, three rings) from 2 molecules
• 2H2O to balance

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. 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. Q1(d) Candidates were required to analyse provided information to identify three unknown compounds and to write an equation.

Notes from the mark scheme and report

• A = Ca3N2 (formula required), B = NH3, C = Ca(OH)2
• Ca3N2 + 6H2O → 2NH3 + 3Ca(OH)2
• 1 of 2 if B and C labels wrong way round
• ECF for CaO as C

Skills: M0.2 Use ratios, fractions and percentages

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. Q1(e): Candidates needed to use their knowledge of reactions of carboxylic acids to construct two equations. See also comments made for Q1(e) and Q3(b).

Notes from the mark scheme and report

• 2:1 acid:Na2CO3 giving salt + CO2 + H2O
• 3 acid + Al → (CH3CH(OH)COO)3Al + 1½H2
• alcohol OH does not react
• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Q4(c)(i) required candidates to suggest equations for a two-step mechanism.

Notes from the mark scheme and report

• slow step must have CH3CH2CH2CH=CH2 + I2 as reactants
• two steps must add up to the overall equation
• second mark dependent on correct slow step
• ALLOW electrophilic addition mechanism

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. Q5(a)(i) required candidates to write half equations for a redox reaction.

Notes from the mark scheme and report

• reduction Na+ + e– → Na
• oxidation 2N3– → 3N2 + 2e–
• 1 mark if both correct but wrong way round
• ALLOW multiples

Mark scheme

Examiner's comment

Q1(f) Outlining a mechanism for an unfamiliar reaction Q5(b)(iii) Predicting the equation for an unfamiliar organic reaction • Did not consider what a question is asking or read the information, e.g. Q5(b)(iii) Candidates were expected to interpret information for an unfamiliar organic reaction and to write a balanced question. In Q5(b)(iii), 2-methylbutanoic acid is CH3CH(CH3)CH2COOH.

Notes from the mark scheme and report

• structure of 2-methylbutanoic acid
• amine is CH3CH2CH(CH3)NH2 (butan-2-amine)
• CO2 AND N2 as products
• DO NOT ALLOW ECF from unbranched species
• common error: CO2 + H2 from using NH3

Mark scheme

Examiner's comment

22(a)(i) Candidates answered this part well, showing good knowledge and understanding of electron configuration. When incorrect, it was usually errors with the number of electrons in the 3d and 4p subshells. 22a(ii) Most candidates were able to construct a correct balanced equation for this unfamiliar equation. The commonest error was use of Br instead of Br2 in the equation. 22(b) Structure and bonding continues to be a difficult concept for many candidates. This part did discriminate extremely well, with high-attaining candidates identifying that the compound would have a giant ionic lattice with fixed ions in the solid state which become mobile in the liquid state to conduct electricity. It was common for ‘ionic’ or ‘giant’ to be omitted in the name of the lattice. Most candidates used the ideal gas equation, which was usually rearranged correctly, with substituted values for p, V, R and T being added. A common mistake was for the volume in cm3 to be multiplied by 10–3 rather than 10–6 for the necessary conversion into m3. Low-attaining candidates often did not convert 100 ºC into 373 K. Unfortunately, candidates making an error in unit conversions then obtained an unrealistic molar mass for A and were then unable to derive a possible molecular formula. Well-prepared candidates carried out the calculation well with clear working. It was very encouraging to see the number of candidates able to predict the molecular formula of A as BrF5 from the calculated molar mass of 174.6 g mol–1. There were, however a significant number of incorrect responses, suggesting that some candidates found this type of question challenging. Some candidates used the molar gas volume at room temperature and pressure of 24.0 dm3 mol–1. Two of the five marks were still available for this response.

Notes from the mark scheme and report

• P4 not P
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

25(a) This part was generally well-answered with well-prepared candidates scoring all three marks. The most common errors resulted from the products of reaction with HBr. The same structure was often drawn, reversed in one of the boxes. Candidates should take care to check that the atoms in a structure have the correct number of bonds. Some candidates added HBr across the C=C double bond but left the C=C bond intact in their final structures, showing carbon atoms with 5 bonds instead of 4. 25(b) As with 25(a), this question rewarded the well-prepared candidate and discriminated well. Conversely, many mechanisms showed little resemblance to the accepted mechanism for electrophilic addition. Mechanisms were often seen showing curly arrows going in the wrong direction and between the wrong bonds and atoms, with incorrect charges and dipoles, and partial charges used where full charges were required. Many candidates were aware that members of a homologous series have the same functional group and react in similar ways. A second mark was available for stating that the formula of successive members increases by CH2. It is important to stress ‘successive’ in communicating this information. 25(c)(ii) This part required candidates to apply their understanding of a ‘general formula’ in a novel context. Many candidates analysed the provided formulae for the alkynes homologous series to derive the correct general formula of CnH2n–2. There was no real pattern in the incorrect responses which usually contained a mixture of numbers for the H atom, e.g. CnH2n, CnHn+1. 25(c)(iii) Although attempted by most candidates, comparatively few responses could be credited. The key to success was again to use the information provided: the formation of a saturated compound. The commonest response seen showed addition of 1 Br2 molecule to form the unsaturated CH3CBr=CHBr instead of the saturated CH3CBr2–CBr2 by addition of 2 Br2. This question was one of the most difficult on the exam paper. 25(c)(iv) Most candidates achieved 1 of the available 2 marks for drawing the structure of but-2-yne, CH3C≡CCH3. The structure of but-1-yne was then usually drawn in the other box despite it being provided already in the table at the start of part (c). A variety of creditworthy structures were seen, including H2C=CH– CH=CH2, cyclobutene and isomers of methylcyclopropane. 25(c)(v) It was encouraging to see the many correct structures drawn from the unfamiliar 2,5-dimethylhept-3-yne. Most candidates positioned the C≡C group and the two substituted CH3 groups correctly. The commonest error was showing a main stem with 6, rather than 7 C atoms. 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

• 2Br2 needed to form a saturated product
• ALLOW C3H4 for propyne
• ALLOW H3CCBr2CHBr2

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

3(a) More able candidates scored well in this question, setting out their answers in a logical order. They often first discussed Ca and its bonding and structure, linking this to the physical properties and then doing the same for Br. A number of candidates discussed the chemical properties of Ca and Br, such as their ability to bond with other elements, ionisation energy and their reactivity based on their position in the periodic table. Some candidates gave a good description of metallic bonding but then went on to discuss melting point in terms of intermolecular forces. 3(b)(i) Most candidates were able to give the correct diagrams for ionic bonding, although care needs to be taken that diagrams are well drawn with both charges given. Some gave diagrams for covalent bonding. 3(b)(ii) It was important to answer the question asked. A number of responses lost marks for describing the general trend down group 2 without making reference at all to calcium and barium. Most candidates managed to score at least one mark here but a considerable proportion missed the second marking point explaining that nuclear attraction was less in Ba. 3(c)(i) This was dependent on candidates’ ability to give correct formula of aluminium hydroxide, Al(OH)3. Writing correct formulae is an important skill in chemistry, which should be practised regularly in different topics to help candidates master the skill. 3(c)(ii) Most candidates were able to recognise that water had the higher boiling point due to hydrogen bonding. However, it was evident in a number of responses that they were unclear that this is an example of an intermolecular force and doesn’t just refer to the covalent bond formed between O and H. Most avoided discussing the intermolecular forces present in H2Se even although both London forces and permanent dipole-dipole interactions were accepted as they may not have come across this specific example and had not been given electronegativity data. 3(d)(i) Very few candidates scored this mark. Although a number of candidates did give sodium bromate as the answer (with the omission of the oxidation state), many other answers were seen suggesting candidates are not aware of naming conventions for inorganic compounds. 3(d)(ii) This is the first time in a reformed chemistry AS paper that the question space has been left unstructured for oxidation number changes. The highest-attaining candidates set out their responses clearly, dealing with changes for oxidation and reduction separately, and giving the correct oxidation numbers. Some struggled to obtain an oxidation state of Br in NaBrO3 as +5, suggesting +1 instead.

Notes from the mark scheme and report

• Al2Se3 + 6H2O → 2Al(OH)3 + 3H2Se

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

3(a) More able candidates scored well in this question, setting out their answers in a logical order. They often first discussed Ca and its bonding and structure, linking this to the physical properties and then doing the same for Br. A number of candidates discussed the chemical properties of Ca and Br, such as their ability to bond with other elements, ionisation energy and their reactivity based on their position in the periodic table. Some candidates gave a good description of metallic bonding but then went on to discuss melting point in terms of intermolecular forces. 3(b)(i) Most candidates were able to give the correct diagrams for ionic bonding, although care needs to be taken that diagrams are well drawn with both charges given. Some gave diagrams for covalent bonding. 3(b)(ii) It was important to answer the question asked. A number of responses lost marks for describing the general trend down group 2 without making reference at all to calcium and barium. Most candidates managed to score at least one mark here but a considerable proportion missed the second marking point explaining that nuclear attraction was less in Ba. 3(c)(i) This was dependent on candidates’ ability to give correct formula of aluminium hydroxide, Al(OH)3. Writing correct formulae is an important skill in chemistry, which should be practised regularly in different topics to help candidates master the skill. 3(c)(ii) Most candidates were able to recognise that water had the higher boiling point due to hydrogen bonding. However, it was evident in a number of responses that they were unclear that this is an example of an intermolecular force and doesn’t just refer to the covalent bond formed between O and H. Most avoided discussing the intermolecular forces present in H2Se even although both London forces and permanent dipole-dipole interactions were accepted as they may not have come across this specific example and had not been given electronegativity data. 3(d)(i) Very few candidates scored this mark. Although a number of candidates did give sodium bromate as the answer (with the omission of the oxidation state), many other answers were seen suggesting candidates are not aware of naming conventions for inorganic compounds. 3(d)(ii) This is the first time in a reformed chemistry AS paper that the question space has been left unstructured for oxidation number changes. The highest-attaining candidates set out their responses clearly, dealing with changes for oxidation and reduction separately, and giving the correct oxidation numbers. Some struggled to obtain an oxidation state of Br in NaBrO3 as +5, suggesting +1 instead.

Notes from the mark scheme and report

• sodium bromate(V)

Mark scheme

Examiner's comment

5(a) Generally this was well answered. However, some candidates only gave two responses where three were required, presumably because it was worth two marks. 5(b) A significant number of candidates did not gain the mark here as they incorrectly balanced the number of oxygens, forgetting that the alcohol contains an O atom i.e. 6.5 O2 given rather than 6. 5(c) Nomenclature in organic chemistry takes lots of practice. Approximately half of the candidates did not score this mark. It was more challenging as they needed to work with skeletal structures but evidence of drawing out displayed formula was often seen. 5(d)(i) Most candidates were able to recall the formula to calculate atom economy, however a number made errors in working out Mr values. However, some left this blank or just gave an answer without any working. 5(d)(ii) Mechanisms were often seen showing curly arrows going in the wrong direction and between the wrong bonds and atoms, charges and dipoles were often incorrect, and partial changes used where full charges were required. Writing mechanisms is an important skill in organic chemistry so it is vital that time is spent practising writing them out and fully understanding the significance of the curly arrow. 5(d)(iii) Many candidates gave the correct mechanism here, with common incorrect responses being other types of mechanism, substitution only, or isomerism. 5(e) Very few candidates gained both marks here. It was not enough here to just state faster/slower without specifying which haloalkane they were referring to. It was also important to specify the C-X bond not just vague reference to the chlorine or bromine bond. Many described the difference in bond polarity so had the wrong order. Some were more general and discussed the reactivity of bromine and chlorine themselves. 5(f)* This was a challenging problem-solving question, relying on candidates to make use of all of the information provided to determine the structure. Very few candidates made no attempt at all at this question. The most common error was incorrectly identifying the peak at 3000 cm-1 as O-H from a carboxylic acid, despite it being indicated on the data sheet that this would be a broad peak. It suggested that candidates were not familiar with looking at spectra and understanding what data it gave. Candidates should also be reminded when answering these types of question that they should give the structures, not just molecular formulae, where possible. 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

• 6O2 → 4CO2 + 5H2O
• DO NOT ALLOW [O]

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• Mg + 2CH3COOH → (CH3COO)2Mg + H2
• Mg 0 to +2 oxidised
• H +1 to 0 reduced
• 1 mark for correct oxidation numbers wrongly linked to redox

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

Examiner's comment

Question 18(a)(i) Many candidates did not correctly balance this equation or missed water as a product entirely.

Notes from the mark scheme and report

• K2Cr2O7 AND acid AND reflux
• 4[O] and 2H2O

Skills: M0.2 Use ratios, fractions and percentages

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

• M(s) + 2HCl(aq) → MCl2(aq) + H2(g) - state symbols required in overall equation only
• M → M2+ + 2e–
• 2H+ + 2e– → H2
• half-equations must only contain species that change
• 1 mark if both correct but oxidation/reduction swapped

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

• H+ + OH– → H2O
• IGNORE state symbols

Mark scheme

Examiner's comment

6(a) Candidates were expected to use their knowledge and understanding of bond angles and shapes. Most candidates were given both available marks. A significant number responded with incorrect names of shapes, or chose the wrong shape, such as pyramidal for the O–C–O bond angle in the carboxylic acid. Candidates are recommended to learn this basic area of chemistry. 6(b) Most candidates completed a correct equilibrium equation and assigned the correct acid-base pairs. A significant number of candidates used ethanoic acid instead of water in the equation. The examiners allowed error carried forward in this case for the acid-base pairs. Candidates found it much more difficult to explain whether the sulfonic acid had a lower pH value. The higher-attaining candidates answered concisely. They usually identifying that the sulfonic acid would have a lower pH as the pKa value was lower (or the Ka value greater), leading to more dissociation than ethanoic acid. Lower-attaining candidates often produced unfocussed and lengthy responses, sometimes relating increased dissociation in an acid to a higher, rather than a lower, pH. Some candidates approached their explanation mathematically, calculating pH values for ethanoic acid and sulfonic acid from their concentrations and pKa values. If correct, this approach was fully credited. 6(c) Most candidates showed a good understanding and appreciation of drawing curly arrows. It must be stressed that curly arrows that do not start from a lone pair, negative charge or a bond cannot be credited. Lower-attaining candidates often drew imprecisely positioned curly arrows, curly arrows in the wrong direction or to the wrong atoms. For their response to be credited with marks, candidates should position curly arrows to ensure credit when outlining reaction mechanisms. 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

• CH3SO2OH + H2O ⇌ CH3SO2O– + H3O+ with pairs labelled
• CH3SO2OH dissociates more/stronger acid (IGNORE 'more acidic')
• student correct AND lower pKa/higher Ka/greater [H+]
• ALLOW pH calculations for both acids

Skills: 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

Most candidates were given 1 of the 2 available marks for showing the formula of one correct product, Ca(OH)2 or NH3. The best answers identified both products and were then able to balance the equation. Common errors included ‘CaO’ as a product and incorrect compounds of nitrogen (see the two responses above). This part discriminated very well.

Notes from the mark scheme and report

• 1 mark for Ca(OH)2 OR NH3 as a product
• ALLOW NH4OH for NH3

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Only the highest attaining candidates were able to write the correct equation. Although most candidates did identify the organic product as C4Cl10, the other product was usually seen as 5H2 rather than 10HCl.

Notes from the mark scheme and report

• 10 Cl2 and 10 HCl
• ALLOW structural formulae

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Nearly half of the candidates did not answer this question correctly, mainly because of incorrect balancing or the formation of strontium oxide instead of strontium hydroxide.

Notes from the mark scheme and report

• Sr + 2H2O → Sr(OH)2 + H2
• IGNORE state symbols

Mark scheme

Examiner's comment

A LEVEL CHEMISTRY A Examiners’ report H432/01 Summer 2019 series Version 1 H432 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 3 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 7 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 8 Question 8 ............................................................................................................................................ 8 Question 9 ............................................................................................................................................ 9 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 13 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Section B overview ................................................................................................................................. 12 Question 16 (a) ................................................................................................................................... 12 Question 16 (b) ................................................................................................................................... 12 Question 16 (c) (i) ............................................................................................................................... 12 Question 16 (c) (ii) .............................................................................................................................. 13 Question 16 (d) (i) ............................................................................................................................... 14 Question 16 (d) (ii) .............................................................................................................................. 15 Question 16 (e) (i) ............................................................................................................................... 16 Question 16 (e) (ii) .............................................................................................................................. 16 Question 17 (a) ................................................................................................................................... 17 Question 17 (b) ................................................................................................................................... 19 Question 18 (a) (i) ............................................................................................................................... 20 Question 18 (a) (ii) .............................................................................................................................. 20 Question 18 (a) (iii) ............................................................................................................................. 21 Question 18 (a) (iv) ............................................................................................................................. 22 Question 18 (a) (v) .............................................................................................................................. 22 Question 18 (b) (i) ............................................................................................................................... 22 Question 18 (b) (ii) .............................................................................................................................. 23 Question 18 (c) (i) ............................................................................................................................... 24 Question 18 (c) (ii) .............................................................................................................................. 24 Question 19 (a) (i) ............................................................................................................................... 25 Question 19 (a) (ii) .............................................................................................................................. 25 Question 19 (a) (iii) ............................................................................................................................. 26 Question 19 (b) (i) ............................................................................................................................... 26 Question 19 (b) (ii) .............................................................................................................................. 27 Question 19 (b) (iii) ............................................................................................................................. 27 Question 19 (b) (iv) ............................................................................................................................. 28 Question 19 (c) (i) ............................................................................................................................... 28 Question 19 (c) (ii) .............................................................................................................................. 29 Question 20 (a) ................................................................................................................................... 29 Question 20 (b) (i) ............................................................................................................................... 30 Question 20 (b) (ii) .............................................................................................................................. 30 Question 20 (b) (iii) ............................................................................................................................. 31 Question 20 (b) (iv) ............................................................................................................................. 32 Question 20 (b) (v) .............................................................................................................................. 32 Question 20 (c) ................................................................................................................................... 33 Question 21 ........................................................................................................................................ 34 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Question 16 (c) (i) Question 16 (c) (ii) These two sub-questions were well answered.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

A LEVEL CHEMISTRY A Examiners’ report H432/01 Summer 2019 series Version 1 H432 For first teaching in 2015 www.ocr.org.uk/science Contents Introduction .............................................................................................................................................. 4 Paper 1 series overview ........................................................................................................................... 5 Section A overview ................................................................................................................................... 6 Question 2 ............................................................................................................................................ 6 Question 3 ............................................................................................................................................ 6 Question 4 ............................................................................................................................................ 7 Question 5 ............................................................................................................................................ 7 Question 6 ............................................................................................................................................ 8 Question 8 ............................................................................................................................................ 8 Question 9 ............................................................................................................................................ 9 Question 10 .......................................................................................................................................... 9 Question 11 ........................................................................................................................................ 10 Question 13 ........................................................................................................................................ 10 Question 14 ........................................................................................................................................ 11 Question 15 ........................................................................................................................................ 11 Section B overview ................................................................................................................................. 12 Question 16 (a) ................................................................................................................................... 12 Question 16 (b) ................................................................................................................................... 12 Question 16 (c) (i) ............................................................................................................................... 12 Question 16 (c) (ii) .............................................................................................................................. 13 Question 16 (d) (i) ............................................................................................................................... 14 Question 16 (d) (ii) .............................................................................................................................. 15 Question 16 (e) (i) ............................................................................................................................... 16 Question 16 (e) (ii) .............................................................................................................................. 16 Question 17 (a) ................................................................................................................................... 17 Question 17 (b) ................................................................................................................................... 19 Question 18 (a) (i) ............................................................................................................................... 20 Question 18 (a) (ii) .............................................................................................................................. 20 Question 18 (a) (iii) ............................................................................................................................. 21 Question 18 (a) (iv) ............................................................................................................................. 22 Question 18 (a) (v) .............................................................................................................................. 22 Question 18 (b) (i) ............................................................................................................................... 22 Question 18 (b) (ii) .............................................................................................................................. 23 Question 18 (c) (i) ............................................................................................................................... 24 Question 18 (c) (ii) .............................................................................................................................. 24 Question 19 (a) (i) ............................................................................................................................... 25 Question 19 (a) (ii) .............................................................................................................................. 25 Question 19 (a) (iii) ............................................................................................................................. 26 Question 19 (b) (i) ............................................................................................................................... 26 Question 19 (b) (ii) .............................................................................................................................. 27 Question 19 (b) (iii) ............................................................................................................................. 27 Question 19 (b) (iv) ............................................................................................................................. 28 Question 19 (c) (i) ............................................................................................................................... 28 Question 19 (c) (ii) .............................................................................................................................. 29 Question 20 (a) ................................................................................................................................... 29 Question 20 (b) (i) ............................................................................................................................... 30 Question 20 (b) (ii) .............................................................................................................................. 30 Question 20 (b) (iii) ............................................................................................................................. 31 Question 20 (b) (iv) ............................................................................................................................. 32 Question 20 (b) (v) .............................................................................................................................. 32 Question 20 (c) ................................................................................................................................... 33 Question 21 ........................................................................................................................................ 34 Introduction Our examiners’ reports are produced to offer constructive feedback on candidates’ performance in the examinations. Those who stated orange received credit Question 18 (b) (i) Many candidates did not realise that when transition metal ions are formed, the first electrons removed from atoms are the 4s electrons and so wrote 2s2 2p6 3s2 3p6 3d1 4s2. Question 18 (b) (ii) Most candidates did not state that the direction of reaction of redox equilibria is dependent on the relative negativity/positivity of the standard electrode potentials.

Notes from the mark scheme and report

• Fe + 4H+ + 2VO2+ → Fe2+ + 2H2O + 2V3+
• IGNORE state symbols
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

This equation presented problems for many candidates, despite the question asking for an equation between carbonate ions and water. An acceptable equation had to be ionic and needed to produce OH– (for the alkaline solution) and either HCO3– or CO2. Many candidates wrote an equation with H+ instead of H2O, with lower ability candidates showing the carbonate ion with the wrong charge as CO3–. Many candidates wrote full equations despite the question asking for an ionic equation. Candidates do need to read the instructions in the question.

Notes from the mark scheme and report

• CO32– + H2O → OH– + HCO3–
• ionic equation required
• IGNORE equations with H+ or H3O+ (question asks for reaction with H2O)
• IGNORE state symbols

Mark scheme

Examiner's comment

This part discriminated well with many candidates being able to write a correct equation using their knowledge of the oxidation of alcohols. Mistakes usually resulted in the balancing with either [O] instead of 2[O] or 2H2O instead of H2O. Written equations always need to be checked for the atoms balancing.

Notes from the mark scheme and report

• C6H5CH2OH + 2[O] → C6H5COOH + H2O
• ALLOW molecular formulae

Mark scheme

Examiner's comment

This equation proved to be very testing and only a minority of candidates were able to write a balanced equation for this reaction forming Pb, CO2 and H2O. Many different and implausible products were seen such as O2, H2, and CH3OH.

Notes from the mark scheme and report

• 4Pb2O3 + 3CH4 → 8Pb + 3CO2 + 6H2O (ALLOW versions forming CO or C)
• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates found this equation extremely difficult with only a small number of candidates writing a correct equation. The problem lies with balancing the oxidation numbers and charges. Many wrote an equation with a 1:1 ratio or 1:2 ratio for [Fe(H2O)6]2+ : Cl2. An equation balanced in oxidation number and charge required a 2:1 ratio. When writing equations for redox reactions, candidates are recommended to check that oxidation changes and charges are balanced as well as atoms.

Notes from the mark scheme and report

• 2[Fe(H2O)6]2+ + Cl2 → 2[Fe(H2O)6]3+ + 2Cl–
• must be balanced by charge and oxidation number
• ALLOW multiples

Mark scheme

Examiner's comment

Candidates needed to interpret the information in the flowchart and to use this as the basis for their redox equation. The clue that a yellow solid is a product proved to be very difficult to interpret as being sulfur. The equation then required H2O to be added as the other product. Candidates found this equation difficult and relatively few correct equations were seen. As with 6a(iii), many equations were not balanced by oxidation number or charge. Some candidates omitted this part entirely.

Notes from the mark scheme and report

• 5H2S + 2MnO4– + 6H+ → 2Mn2+ + 5S + 8H2O
• 1 mark for all six correct species or Mn and S species balanced 5:2
• IGNORE extra electrons for 1st mark

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Candidates were expected to identify a suitable reagent for this reaction, with most choosing magnesium oxide, hydroxide or carbonate. Credit was also given for using a soluble magnesium salt such as its sulfate, chloride or nitrate. The correct equation often followed, but errors sometimes appeared in the form of incorrect formulae, such as MgOH for magnesium hydroxide. The exemplar shows a good clear response, using MgO as the reagent.

Notes from the mark scheme and report

• MgO, Mg(OH)2 or MgCO3 (or a soluble Mg salt)
• no ECF from incorrect formula
• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

Most candidates were able to write a correctly balanced equation for this reaction.

Notes from the mark scheme and report

• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

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 correctly drew the structure of compound D but comparatively few were able to construct a correct equation. For this equation, candidates needed to apply their knowledge and understanding of monosubstitution of alkanes to substitution of three H atoms by three Cl atoms. This task proved to be one of the most difficult questions on this paper. The exemplar shows an excellent response. The candidate has drawn a trisubstituted structure that fits the molar mass of 175.5 g mol–1 and a correct equation for its formation. Many attempts at this equation showed H2 as the second product rather than HCl.

Notes from the mark scheme and report

• any trichloro isomer of A
• molecular formulae required in equation (C5H12 + 3Cl2 → C5H9Cl3 + 3HCl)
• no ECF from incorrect structure of D

Mark scheme

Examiner's comment

The majority of candidates obtained 1 or 2 marks on this question. The most common errors seen were identifying the gas as H2S or incorrect balancing.

Notes from the mark scheme and report

• H2Se
• DO NOT ALLOW H2S for gas B but ALLOW ECF for equation
• IGNORE state symbols

Mark scheme

Examiner's comment

Around 50% of candidates answered this correctly. The most common error was not balancing the equation, and many did not know what an ionic equation was.

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

A good attempt by many candidates but some lost marks by having the wrong state of bromine, even though the question stated it was a liquid changing to a gas. Many added water or oxygen, some confused the equation with bond enthalpy and answers such as Br2 (l)  2Br(g) were commonly seen.

Notes from the mark scheme and report

• Br2(l) → Br2(g)
• state symbols essential

Mark scheme

Examiner's comment

Most candidates correctly drew the repeat unit and were credited with one mark, but many placed the n position in the wrong place on the left-hand side of the equation or forgot to write it in at all.

Notes from the mark scheme and report

• displayed repeat unit with side bonds and brackets
• n on both sides (subscript on RHS)
• DO NOT ALLOW ECF from incorrect repeat unit

Mark scheme

Examiner's comment

Most candidates got the equation correct. Occasionally a number was omitted causing incorrect stoichiometry and occasionally the equation was reversed.

Notes from the mark scheme and report

• H2O2 + 2H+ + 2Fe2+ → 2Fe3+ + 2H2O
• ALLOW multiples
• IGNORE state symbols

Mark scheme

Examiner's comment

This question proved difficult for candidates. Although many candidates were able to identify the correct organic product, only the higher ability candidates were able to construct an appropriate balanced equation. A common error was to omit the role of the acid; this is shown in Exemplar 2 below. Lower ability candidates appeared not to recognise this reaction and suggested an alkoxide salt, rather than a haloalkane as the organic product. This type of response was seen frequently by examiners. The candidate has drawn the correct structure of the haloalkane formed and scores the first mark. However, the response fails to recognise that the reaction occurs under acidic conditions and omits the sulfuric acid from the equation.

Notes from the mark scheme and report

• DO NOT ALLOW equations that form NaOH
• ALLOW Na2SO4 version, H+ for H2SO4, or equation with HCl

Mark scheme

Examiner's comment

The majority of candidates were able to provide a correct equation for the incomplete combustion of heptane.

Notes from the mark scheme and report

• product must be CO and/or C with H2O
• ALLOW multiples and mixtures with CO2

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

Many candidates found this demanding question very difficult. Some were able to deduce that propanone was the carbonyl compound in the reaction. Only the most able recognised that water was a by-product of this reaction.

Notes from the mark scheme and report

• carbonyl compound is propanone
• H2O as the other product
• structures required

Mark scheme

Examiner's comment

This part discriminated extremely well with many candidates obtaining either both marks or zero marks. Candidates needed to select the correct redox pairs, combine them and cancel H+ and H2O. Many otherwise correct equations did not receive credit as they contained basic balancing errors. As a final step in constructing an equation, candidates are recommended to check the formulae of their species and that the equation is balanced.

Notes from the mark scheme and report

• 2MnO4– + 5H2O2 + 6H+ → 2Mn2+ + 8H2O + 5O2
• first mark for balanced but uncancelled H+/e–
• 1 mark only for 2H2O2 → 2H2O + O2

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

In this part, candidates needed to apply their knowledge and understanding of ligand substitution and equilibrium to a novel situation. The best equations used Cl– ions to form CoCl4 2–. Some candidates used HCl instead and then H+ was often omitted in the equation. As with 2b, candidates are recommended to check that their completed equations are balanced.

Notes from the mark scheme and report

• ALLOW reverse equation
• IGNORE state symbols

Mark scheme

Examiner's comment

Candidates were required to derive the equation from which the supplied Kc expression had been written. Overall, this part was answered well but some candidates struggled with the brackets or used CH2COOH2 for succinic acid.

Notes from the mark scheme and report

• (CH2COOH)2 + 2C2H5OH ⇌ (CH2COOC2H5)2 + 2H2O
• ALLOW → and molecular formulae

Mark scheme

Notes from the mark scheme and report

• side bonds required on both sides of repeat unit
• n needed on both sides of equation
• DO NOT ALLOW molecular formula for repeat unit
• NO ECF from incorrect repeat unit

Mark scheme

Notes from the mark scheme and report

• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• A = Ba(OH)2, B = NH3
• barium nitride must be Ba3N2
• state symbols required
• no ECF from any incorrect formula

Mark scheme

Notes from the mark scheme and report

• one Cl substituted and HCl formed
• ALLOW molecular formulae

Mark scheme

Notes from the mark scheme and report

• Cl• abstracts H to give HCl
• organic radical + Cl2 regenerates Cl•
• 1 mark if both correct but dots omitted

Mark scheme

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Mark scheme

Notes from the mark scheme and report

• state symbols needed
• entropy decreases because solid BaSO4 formed
• entropy mark dependent on BaSO4(s) as product

Mark scheme

Notes from the mark scheme and report

• ½I2(s) → I(g) with state symbols
• DO NOT ALLOW I2(s) → 2I(g) for equation mark
• entropy increases as gas more disordered

Mark scheme

Notes from the mark scheme and report

• 1 mark only if balanced but H+/H2O not cancelled on both sides
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• C5H10O + 7O2 → 5CO2 + 5H2O
• ALLOW any correctly balanced unsaturated C5 alcohol
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• DO NOT ALLOW CH3CH2CH2COH for the aldehyde
• distil for aldehyde AND reflux for carboxylic acid, conditions linked to product
• wrong primary alcohol max 4

Skills: PAG5 Synthesis of an organic liquid

Mark scheme

Notes from the mark scheme and report

• 2NaNO2 + 6Na → 4Na2O + N2
• ALLOW multiples
• IGNORE state symbols

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• thioester C(=O)–S functional group must be fully displayed
• H2O as second product

Mark scheme

Notes from the mark scheme and report

• cyclic product (1,3-dithiane ring with two CH3 on the same carbon) + H2O
• balanced equation

Mark scheme

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols

Mark scheme

Notes from the mark scheme and report

• ALLOW multiples
• IGNORE state symbols
• IGNORE explanation in terms of electronegativity

Mark scheme

Notes from the mark scheme and report

• state symbols required
• IGNORE spectator ions shown on both sides

Mark scheme

Notes from the mark scheme and report

• starts from Sr+(g) not Sr(g)
• state symbols required
• IGNORE states on the electron

Mark scheme

Notes from the mark scheme and report

• Al2S3 + 6H2O → 2Al(OH)3 + 3H2S
• IGNORE state symbols
• ALLOW multiples

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Notes from the mark scheme and report

• NO• + O3 → NO2• + O2 then NO2• + O → NO• + O2
• one mark only if dots missing or extra

Mark scheme

Notes from the mark scheme and report

• butanone structure
• ALLOW molecular formulae in equation