November 2021: Paper 3, Unified chemistry

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(a): Candidates were assessed on their understanding of partial pressure.

Notes from the mark scheme and report

• n(CO2) = 2.5 and n(O2) = 3.75
• mole fraction 0.4 × 50.0 = 20 atm
• ALLOW ECF from incorrect total moles only

Skills: 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

Q1(b): Candidates were required to apply their understanding of Kc to a novel context requiring some mathematical awareness.

Notes from the mark scheme and report

• square brackets required in Kc
• common errors: ÷2 instead of √ (2.00 × 10–4), inverse Kc expression (2.5)

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

Q1(c) Intermolecular forces and electronegativity. Q1(c): Candidates were required to use electronegativity and boiling point data to explain the difference in boiling points of CH4, NH3 and HF. In Q1(c), candidates were expected to use data on electronegativity to help explain differences in boiling points of three simple molecular compounds, CH4, NH3 and HF.

Notes from the mark scheme and report

• boiling point increases with electronegativity difference
• HF and NH3 hydrogen bonding, CH4 London forces
• HF has stronger hydrogen bonding than NH3
• IGNORE permanent dipole interactions
• IGNORE strength of covalent bonds

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

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. Q1(f): Candidates were required to apply their knowledge of nucleophilic substitution of haloalkanes to outline a similar mechanism for an unfamiliar nucleophile.

Notes from the mark scheme and report

• dipole on C–X and curly arrow from C–X bond to X
• curly arrow from lone pair or – charge on O of CH3COO– to C
• products methyl ethanoate AND X–
• halogen must be consistent
• ALLOW SN1 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. Q2(a): Candidates were asked to draw a labelled diagram for a familiar practical set up.

Notes from the mark scheme and report

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

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: PAG1 Moles determination

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: M0.2 Use ratios, fractions and percentages

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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

Notes from the mark scheme and report

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

Skills: PAG9 Rates of reaction – continuous monitoring

Mark scheme, with the levels and indicative points

Examiner's comment

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

Q4(c)(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

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

Notes from the mark scheme and report

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

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

Mark scheme

Examiner's comment

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(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. Q5(a)(ii): As with earlier quantitative questions, candidates are very comfortable in tackling stock mole- based calculations, here involving the ideal gas equation.

Notes from the mark scheme and report

• V = 16.0 × 10–3 m3 and T = 290 K
• n(N2) = 0.796
• × 2/3 for n(NaN3)
• 34.5 g to 3 SF
• common errors: 2/3 omitted (51.7/51.8) 4 marks, inverted gas equation (54.4) 4 marks

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

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. Q5(b)(i): Most candidates found this pH calculation easy and most obtained a pH of 2.75 for both marks.

Notes from the mark scheme and report

• pH = 2.75 to 2 DP
• common errors: no square root (pH 5.50) 1 mark, square root of only one term (3.20, 5.05) 1 mark

Skills: M0.4 Use calculators to find and use power, exponential and logarithmic functions; M2.3 Substitute numerical values into algebraic equations using appropriate units for physical quantities; M2.5 Use logarithms in relation to quantities that range over several orders of magnitude

Mark scheme

Examiner's comment

Q5(b)(ii): This unfamiliar acid–base pair question was answered comparatively well.

Notes from the mark scheme and report

• HN3 + H2O ⇌ N3– + H3O+
• labels A1 B2 B1 A2 (or A2 B1 B2 A1)
• 1 mark for one correct acid–base pair with correct labels

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, with the levels and indicative points

Examiner's comment

Q2(c), Q2(d): Predicting the effect of modifications to an experiment Q5(c): Interpreting information in a Level of Response question provided in an unfamiliar contexts • Had difficulty when writing equations for unfamiliar reactions, e.g. Q5(c) is a Level of Response question which is commented on after this section Level of response questions Q4(b): Candidates were required to analyse the graphical results of a rates investigation to determine the reaction order, rate and rate constant. Q5(c): Candidates were supplied with information about two reactions, supported by numerical data and an IR spectrum.

Notes from the mark scheme and report

• E = Cu (M = 3.81/0.06 = 63.5), F = H2O, G = N2 (M = 0.560/0.02 = 28)
• 3CuO + 2NH3 → 3Cu + 3H2O + N2
• I has C=O (~1700) and N–H (3200–3400) so CH3CONH2
• H = CH3COCl, J = NH4Cl
• CH3COCl + 2NH3 → CH3CONH2 + NH4Cl

Skills: M0.2 Use ratios, fractions and percentages; M3.1 Translate information between graphical, numerical and algebraic forms