June 2023: AS Paper 2, Depth in chemistry

Mark scheme

Examiner's comment

Most candidates scored both marks. Some lost marks for minor slips such as not giving their final answer to 2 decimal places or making calculator errors. A few didn't know how to attempt the calculation or calculated the average mass.

Notes from the mark scheme and report

• 2 DP required (47.92)
• 1 mark ECF from incorrect sum if answer between 46 and 50 to 2 DP

Skills: M0.2 Use ratios, fractions and percentages; M1.2 Find arithmetic means

Mark scheme

Examiner's comment

Again, most scored this mark. Common errors included using the mass number for number of electrons, no 4s but 3d4 instead, 4d rather than 3d, 4p2 instead of 3d2 or filling up d orbital 3d10.

Notes from the mark scheme and report

• 3d2 4s2 (ALLOW 4s before 3d)
• DO NOT ALLOW [Ar] shorthand

Mark scheme

Examiner's comment

Most candidates gained this mark. Common errors included giving 24 or 22 for number of electrons, or 48 or 24 for neutrons. A few used the relative atomic mass of 47.9 from the periodic table so gave 24.7 for number of neutrons.

Notes from the mark scheme and report

• 22 protons, 26 neutrons, 20 electrons: all three required
• Fractional numbers of subatomic particles are not possible. Candidates should be aware that the relative atomic mass is the weighted average of the masses of all of an element’s isotopes, and the mass number of an isotope must be used to determine the number of protons and neutrons in the nucleus.

Mark scheme

Examiner's comment

Very few candidates gave the correct answer for this question. The most common errors included: titanium oxide, titanium(IV) dioxide, titanium oxide(IV), titanium(II) oxide. A few also attempted to give names like those for organic compounds: 1,1-titanium dioxide or the reverse 1,1-dioxytitanium. How Science Works It is important in Chemistry to have clear communication by use of systematic and unambiguous nomenclature. This includes the use of Roman numerals to indicate the magnitude of the oxidation number when an element, such as Ti, may have different oxidation numbers in different compounds. See specification statement 2.1.5(c) and HSW8.

Notes from the mark scheme and report

• titanium(IV) oxide
• DO NOT ALLOW titanium dioxide

Mark scheme

Examiner's comment

Candidates found this calculation quite challenging, with less than a quarter achieving full marks. The most common errors are highlighted on the mark scheme. Many that struggled were often given credit for the x4 ratio mark but only if it was possible to see this in the working. Many gave multiple, often contradictory attempts at the calculation. It was not always clear how the final answer had been obtained. Clear working enables us to follow the logic and give ECF where appropriate. Many divided 1000 g by the molar mass for TiCl4 and then found 72% of this. It was important here to read the question carefully to ensure complete understanding. This candidate achieved 3 out of the 4 possible marks. The steps in their calculation are logical and it is easy to follow their working and therefore spot the error in their calculation. They have divided by 4 rather than multiplying. It also shows the calculation can be performed in a different order to that on the mark scheme. All intermediate values are used in calculations as calculator values without rounding to ensure an accurate answer.

Notes from the mark scheme and report

• 3 SF AND kg required (2.67)
• common errors: 1.92 (yield ignored), 1.38 (yield inverted), 0.673 (Mr of TiCl4 used for Ti)
• ALLOW ECF throughout

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Mark scheme

Examiner's comment

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

Responses not about observations were very common, e.g. more vigorous reaction, Ba is more reactive. Some described Ba as being less reactive. Many responses did not include the idea of rate (for example, ‘more bubbles’) or were not comparative (for example, ‘vigorous bubbling’).

Notes from the mark scheme and report

• must be an observation with idea of rate (more vigorous fizzing, dissolves faster)
• IGNORE 'more vigorous reaction', 'more bubbles', exothermic

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

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: PAG4 Qualitative analysis of ions

Mark scheme

Examiner's comment

About half the candidates gained all 4 marks. Candidates often find it tricky to recognise the type of structure even when given some details about physical properties. Often giant was omitted especially for Al as metallic bonding. Some used ‘small’ in place of ‘simple’. Common errors included MgS as metallic and Si as simple covalent with PCl3 as giant covalent. Many added unnecessary detail such as filling in the greyed-out boxes for conductivity or adding lattice to each box.

Notes from the mark scheme and report

• giant ionic, giant metallic, giant covalent, simple molecular (ALLOW simple covalent)

Mark scheme

Examiner's comment

Candidate explanations often lacked clarity even if the correct structure had been identified in (i). Most gained at least 1 mark, usually for recognising that for MgS to have a higher melting point that it must contain stronger bonds than in PCl3. Responses highlighted a range of misconceptions including the presence of intermolecular forces in ionic/metallic substances, oppositely charged atoms in ionic compounds, and thinking London forces are between atoms. Most were able to gain the conductivity mark, but some compared to PCl3 rather than Si as asked in the question. Some described ‘mobile ions’ in Al or that Si has ‘no electrons’. The use of ‘free electrons’ was seen in many responses, and we would encourage the use of ‘delocalised electrons’ for a more accurate description of metallic bonding.

Notes from the mark scheme and report

• answer must be comparative
• MgS ionic bonds vs PCl3 intermolecular forces
• Al delocalised/mobile electrons, Si none
• IGNORE 'free electrons'
• OCR have produced a KS4-KS5 transition guide for bonding and structure to support teaching of these tricky concepts.
• A bonding delivery guide is also available.

Mark scheme, with the levels and indicative points

Examiner's comment

Very few candidates managed to access the higher level on this question. They found the calculation very challenging. In addition to a typical mole calculation, they needed to use the tonnes conversion (given on the data sheet), look at the ratio of I2 to KI and use a mass concentration. Many used the mass concentration as a molar concentration. Even if a balanced equation was given, it didn’t guarantee the correct ratio in calculations and a common error was to find moles of I2 using the relative atomic mass of iodine (126.9). As with previous calculations on this paper, it was often challenging to follow the logic in the steps of the calculation. It is important to encourage candidates to label each step of their calculation, showing them as independent steps and only using = when terms are equal. Explanations of relative reactivities of chlorine and iodine often lacked detail or had extra irrelevant information, such as reference to electronegativity, ionisation energy, bond enthalpy or intermolecular forces. Some candidates gave responses for the general trend in reactivity in the group rather than comparing Cl and I. Confusion around nuclear charge and nuclear attraction was also often seen. A key skill for longer answer questions is to plan out answers to avoid contradictions. Candidates should also be encouraged to re-read their answers to check that what they have written makes sense. This exemplar demonstrates a L3 6 mark response. A comprehensive explanation of relative reactivities of chlorine and iodine is given. The steps in the calculation are logical with each step labelled, making it easy to follow. A balanced equation is also given.

Notes from the mark scheme and report

• comparison required throughout
• IGNORE 'nuclear charge' for nuclear attraction
• 2:1 ratio I–:I2
• n(I2) = 1 × 10^6/253.8
• acceptable errors: 126.9 used for I2 (1.74 × 10^7) or missing × 2 (4.36 × 10^6), not both for L3

Skills: M0.0 Recognise and make use of appropriate units in calculations; M0.1 Recognise and use expressions in decimal and ordinary form; M0.2 Use ratios, fractions and percentages; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities

Mark scheme

Examiner's comment

Most were able to explain the term disproportionation. Some missed the mark by not stating an element or chlorine. A very common error was giving final oxidation numbers of Cl as +2 and/or -2, rather than per atom. The link between oxidation number and species was not always clearly indicated or changes not specified as oxidation/reduction (or given as the wrong way round). It is vital to set out answer s clearly showing oxidation numbers, species and stating if oxidised or reduced. It is not enough to write on the equation given in the question as it often challenging to read these numbers, or they contradict the main answer. Some attempted to show that Ca has been disproportionated.

Notes from the mark scheme and report

• oxidation AND reduction of the same element
• 0 to +1 in Ca(OCl)2 and 0 to –1 in CaCl2
• IGNORE 'species'/'reactant' for element
• 1 mark for three correct oxidation numbers without oxidation/reduction stated

Mark scheme

Examiner's comment

Just under half the candidates gained all 3 marks for this mechanism. Marks were often lost for incorrect positioning or a missing arrow for the breaking of the C-Br bond. Dipoles were often added to the hydroxide ions. Some also showed the lone pair for OH- going to the C-Br bond and not to the C+. A range of spellings of nucleophilic were seen, including: nucleophilic, nucleophilic, nucleophilic, and nucleophilic. It is important to promote good literacy in science, including meanings and spellings of technical language.

Notes from the mark scheme and report

• arrow from lone pair or negative charge on O of OH– to C of C–Br
• dipole on C–Br AND arrow from bond to Br
• no double-headed or half arrows
• ALLOW SN1
• name nucleophilic substitution

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: PAG7 Qualitative analysis of organic functional groups

Mark scheme

Examiner's comment

Approximately half of candidates scored both marks. Many lost marks for saying ‘energy required’ or ‘energy needed’ which demonstrated a lack of understanding that for an exothermic reaction this would be the activation energy not the enthalpy change. Some also had 1 mole of substance but this was reacted with 1 mole of oxygen or formed 1 mole of products.

Notes from the mark scheme and report

• complete combustion of 1 mol of substance
• DO NOT ALLOW 'energy required' or 1 mol of atoms
• IGNORE standard conditions

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme, with the levels and indicative points

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

Those that were more confident with the calculation in (ii) were more likely to get the answer here. ECF was occasionally seen from an incorrect mass or enthalpy change depending on method used. Many were able to work out q using 150g of water even if they had incorrectly calculated q in (ii) - such as previously using 1.31g.

Notes from the mark scheme and report

• 0.75 × 1.31 = 0.9825 g (ALLOW 0.98–0.99)
• ALLOW ECF from (b)(ii)

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

Mark scheme

Examiner's comment

Candidates’ working was often unclear and very difficult to follow for this question, making it challenging to find errors and award ECF where appropriate. Despite being given the displayed formula, many were unable to correctly count the bonds in the molecules. Missing some or all of C-C, C-O and 3 x O=O from the bonds broken was often seen. Some only counted 2 x C=O bonds in the products. Some subtracted the bonds broken from bonds made or omitted the enthalpy change from their calculation. Very few candidates gave a negative value for their final bond enthalpy.

Notes from the mark scheme and report

• +805
• +3220 (not divided by 4) scores 3
• common errors: –805 (sign), 718 (C–C missed), 431.5 (3 × O=O missed), 167 (rearrangement)
• 6 O–H made, 1 O–H broken

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

Mark scheme

Examiner's comment

Very few candidates scored both marks here. Many just gave a generic response describing an exothermic reaction i.e. ‘more energy is released in bond forming than needed for bond breaking’ rather than answering the question asked. It was evident from many responses that candidates are unclear that energy is needed for bond breaking and released in bond forming. Some tried to compare specific bonds but often failed to account for them all, comparing presence of C-O and extra C-H in methoxymethane to O-H in ethanol but forgetting the C-C bond, resulting in methoxymethane having stronger bonds/more energy to break bonds. Lots didn't recognise that the same products were formed in both reactions.

Notes from the mark scheme and report

• less energy needed to break bonds in methoxymethane (bonds weaker)
• energy released forming bonds is the same (same bonds formed)
• IGNORE calculations

Mark scheme

Examiner's comment

A significant number of candidates lost the mark for missing -an- in their answer i.e. 3-methylpent-2-ol. Others lost the mark for incorrect spelling of methyl.

Notes from the mark scheme and report

• 3-methylpentan-2-ol
• DO NOT ALLOW 2-methylpentan-3-ol or misspellings such as 3-methylpent-2-ol

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

Many responses made no reference to ‘priority’ and/or discussed alkene C, suggesting that they didn’t read the question fully. Candidates often struggled to find the right language to express themselves, such as reference to ‘functional groups’ or ‘molecules’ rather than priority groups. Lots discussed using Mr to assign priority with only a few stating correctly that it is atomic number that is used for CIP rules. Many, despite stating that priority groups are on the same side, didn't identify these groups so didn't get the second mark.

Notes from the mark scheme and report

• priority groups (CH3CH2 and CH3) on the same side
• IGNORE relative mass/Mr of groups: priority is by atomic number

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

Most candidates were able to gain credit for their structure, with only a few missing out by only drawing one repeat unit, keeping the C=C, having no end bonds or missing/extra Hs. Lots struggled to gain the second mark for the reason for solubility in water as they didn't refer to H-bonding. There were quite a few misconceptions highlighted in the responses to this question. These included the misunderstanding that a H-bond is an an intermolecular force between -OH on alcohol and water, rather than the covalent bond in the molecule. Some thought the -OH would behave as an alkali, even referring to ions, so would ‘fully dissociate’. Some described a reaction with water and breaking apart, perhaps confusing with condensation polymers which can be hydrolysed.

Notes from the mark scheme and report

• two repeat units with end bonds shown (DO NOT ALLOW one repeat unit)
• OH groups form hydrogen bonds with water (DO NOT ALLOW 'it forms hydrogen bonds')
• DO NOT ALLOW –HO connectivity

Mark scheme

Examiner's comment

About a quarter of candidates didn't gain any credit here as they struggled to identify useful processes. Lots referred to cracking or breaking down into smaller chains, possibly thinking about fractional distillation of crude oil and how we make better use of larger fractions. Some identified possible use as a fuel but didn’t say to generate energy/electricity. We also saw reference made to photodegradable/biodegradable polymers which isn't relevant to hydrocarbon polymers. There was evidence of the misunderstanding of the use as ‘feedstock’ with reference being made to animals (livestock or animal feed) or as food to eat.

Notes from the mark scheme and report

• recycling / combustion for energy / organic feedstock
• IGNORE reused