Cambridge A Level Chemistry 9701 — 2021 May/June Paper 4 · Variant 2

*2165639790* CHEMISTRY 9701/42 Paper 4 A Level Structured Questions May/June 2021  2 hours You must answer on the question paper. You will need: Data booklet INSTRUCTIONS ● Answer all questions. ● Use a black or dark blue pen. You may use an HB pencil for any diagrams or graphs. ● Write your name, centre number and candidate number in the boxes at the top of the page. ● Write your answer to each question in the space provided. ● Do not use an erasable pen or correction fluid. ● Do not write on any bar codes. ● You may use a calculator. ● You should show all your working, use appropriate units and use an appropriate number of significant figures. INFORMATION ● The total mark for this paper is 100. ● The number of marks for each question or part question is shown in brackets [ ]. IB21 06_9701_42/3RP © UCLES 2021 [Turn over This document has 24 pages. Any blank pages are indicated. Cambridge International AS & A Level

2 9701/42/M/J/21 © UCLES 2021 Answer all the questions in the spaces provided. 1 (a) An aqueous solution of chromium(III) contains the green [Cr(H2O)6]3+ complex ion. (i) Complete the electronic configuration of an isolated, gaseous Cr3+ ion. 1s2 ��������������������������������������������������������������������������������������������������������������������������������� [1] (ii) Define the term complex ion. … … [1] (b) [Cr(H2O)6]3+(aq) shows some similar chemical properties to [Co(H2O)6]2+(aq). Samples of [Cr(H2O)6]3+ are reacted separately with either NaOH(aq), H2O2(aq), or excess NH3(aq). Use this information and the Data Booklet to suggest the formula of the chromium species formed. State the type of reaction taking place in each case. reagent added to [Cr(H2O)6]3+(aq) formula of chromium species formed type of reaction NaOH(aq) H2O2(aq) an excess of NH3(aq)  [5] (c) [Cr(H2O)6]2+ and [Cr2(O2CCH3)4(H2O)2] are both complexes of chromium(II) and have different colours. Explain why the colours of these complexes are different. … … … … [2]

3 9701/42/M/J/21 © UCLES 2021 [Turn over (d) The structure of [Cr2(O2CCH3)4(H2O)2] is shown. Ethanoate ions act as ligands in this complex. The ethanoate ligand, CH3CO2 –, is shown as O O. Cr O O O H2O O O O O OH2 O Cr (i) Water and ethanoate ions behave as different types of ligand in this complex. Suggest an explanation for this statement. … … [1] (ii) Deduce the coordination number of Cr and the geometry around each Cr atom in this structure. coordination number … geometry around Cr atom ���������������������������������������������������������������������������������������������������  [1] (iii) State the type of bond between the two atoms in the Cr–Cr bond. … [1] (e) The [Cr2(O2CCH3)4(H2O)2] complex reacts with aqueous acid to form Cr2+(aq) ions. Cr2+(aq) ions react with O2(aq) under acidic conditions. Cr3+(aq) ions are formed. Use the Data Booklet to answer the following questions. (i) Construct an ionic equation for the reaction of Cr2+(aq) with O2(aq) under acidic conditions. … [2] (ii) Calculate for the reaction in (e)(i).  = … V [1]  [Total: 15]

4 9701/42/M/J/21 © UCLES 2021 2 (a) State and explain the trend observed in the thermal stability of the Group 2 nitrates. … … … … … [3] (b) (i) Lead(II) nitrate, Pb(NO3)2, decomposes on heating in a similar manner to the Group 2 nitrates. Write an equation for the decomposition of lead(II) nitrate. … [1] (ii) Suggest how the ease of decomposition of Pb(NO3)2 would compare to that of Ba(NO3)2. Explain your answer. You may find it useful to refer to the Data Booklet. … … [1] (c) (i) Barium ethanedioate, BaC2O4, decomposes on heating to produce barium oxide and a mixture of two different gases. Construct an equation for the decomposition of barium ethanedioate. … [1]

5 9701/42/M/J/21 © UCLES 2021 [Turn over (ii) An impure sample of BaC2O4, of mass 0.500 g, is added to 50.0 cm3 of 0.0200 mol dm–3 acidified MnO4 –(aq), an excess. A redox reaction takes place and all the BaC2O4 reacts. The resulting solution, containing unreacted acidified MnO4 –, is titrated with 0.0500 mol dm–3 Fe2+(aq). The end-point is reached when 30.40 cm3 of 0.0500 mol dm–3 Fe2+(aq) has been added. C2O4 2– 2CO2 + 2e– MnO4 – + 8H+ + 5e– Mn2+ + 4H2O Fe2+ Fe3+ + e– Calculate the percentage by mass of BaC2O4 in the 0.500 g impure sample. Show your working. [Mr: BaC2O4, 225.3]  percentage by mass of BaC2O4 = … [4] (d) Barium hydroxide, Ba(OH)2, is completely dissociated in aqueous solution. Calculate the pH of 0.120 mol dm–3 Ba(OH)2(aq) at 298 K.  pH = … [2]  [Total: 12]

6 9701/42/M/J/21 © UCLES 2021 3 (a) (i) Define the term standard electrode potential. … … … [2] Three redox systems, A, B and C, are shown. The ligand 1,2-diaminoethane, H2NCH2CH2NH2, is represented by en. A [Ru(H2O)6]3+ + e– [Ru(H2O)6]2+ B [Ru(NH3)6]3+ + e– [Ru(NH3)6]2+ C [Ru(en)3]3+ + e– [Ru(en)3]2+ Two electrochemical cells are set up to compare the standard electrode potentials, E o, of three half-cells. The diagrams show the relative potential of each electrode. [Ru(NH3)6]3+ [Ru(NH3)6]2+ salt bridge V [Ru(en)3]3+ [Ru(en)3]2+ + – Pt Pt [Ru(H2O)6]3+ [Ru(H2O)6]2+ salt bridge V [Ru(en)3]3+ [Ru(en)3]2+ + – Pt Pt (ii) Use this information to complete the table by adding the labels A, B and C to deduce the order of E o for the three half-cells. E o redox system most negative least negative  [1]

7 9701/42/M/J/21 © UCLES 2021 [Turn over (iii) The complex [Ru(en)3]3+ shows stereoisomerism. The ligand en is bidentate. Draw three-dimensional diagrams to show the two isomers of [Ru(en)3]3+. Represent the ligand en by using N N . Name the type of stereoisomerism. isomer 1 Ru isomer 2 Ru type of stereoisomerism …  [3] (b) (i) An electrochemical cell consists of a Br2/Br – half-cell and a Ag+/Ag half-cell, under standard conditions. Use the Data Booklet to calculate the . Deduce the direction of electron flow in the wire through the voltmeter between these two half-cells. = … V direction of electron flow from … to ����������������������������������������� [1] (ii) Water is added to the Ag+/Ag half-cell in (b)(i). Suggest the effect of this addition on the Ecell. Place a tick ( ) in the appropriate box. less positive no change more positive Explain your answer. … … … [2]

8 9701/42/M/J/21 © UCLES 2021 (c) Silver bromide, AgBr, dissolves in an aqueous solution of S2O3 2– ions to form the complex ion [Ag(S2O3)2]3–. The S2O3 2– ions act as monodentate ligands. equilibrium 1 AgBr(s) + 2S2O3 2–(aq) [Ag(S2O3)2]3–(aq) + Br –(aq) (i) Define the term ligand. … … [1] (ii) Write an expression for the equilibrium constant, Kc, for equilibrium 1. Kc =  [1] (iii) Some additional data are given about the dissolution of AgBr in S2O3 2–(aq). equilibrium constant numerical value solubility product, Ksp, of AgBr 5.4 × 10–13 stability constant, Kstab, of [Ag(S2O3)2]3– 2.9 × 1013 Use your answer to (c)(ii) and these data to calculate Kc for equilibrium 1. Include the units for Kc.  Kc = … units … [2] (d) The numerical values for the stability constants, Kstab, of two other silver(I) complexes are given. silver(I) complex numerical value of Kstab [Ag(CN)2]– 5.3 × 1018 [Ag(NH3)2]+ 1.6 × 107 An aqueous solution containing Ag+ is added to a solution containing equal concentrations of CN–(aq), NH3(aq) and S2O3 2–(aq). The mixture is left to reach equilibrium. Deduce the relative concentrations of [Ag(CN)2]–, [Ag(NH3)2]+ and [Ag(S2O3)2]3– present in the resulting mixture. Explain your answer. … > … > … highest concentration lowest concentration … … [2]  [Total: 15]

9 9701/42/M/J/21 © UCLES 2021 [Turn over 4 (a) (i) Define the term lattice energy. … … … [2] (ii) Use the following data to calculate a value for the enthalpy change of solution of copper(II) chloride, CuCl 2(s). You might find it helpful to construct an energy cycle. enthalpy change of hydration of Cl – = –378 kJ mol–1 enthalpy change of hydration of Cu2+ = –2099 kJ mol–1 lattice energy of CuCl 2(s) = –2824 kJ mol–1  enthalpy change of solution of CuCl 2(s) = … kJ mol–1 [2] (iii) The enthalpy change of hydration of Ca2+ is –1579 kJ mol–1. Use the Data Booklet to suggest why there is a big difference in the values of ∆Hhyd for Ca2+ and Cu2+. … … … [2] (b) (i) Identify the substances formed at the anode and at the cathode during the electrolysis of saturated CaCl 2(aq). at the anode … at the cathode …  [1] (ii) Calcium can be produced by the electrolysis of molten calcium chloride, CaCl 2(l). Calculate the mass, in g, of Ca formed when a current of 0.75 A passes through CaCl 2(l) for 60 minutes. [Ar: Ca, 40.1]  mass of Ca = … g [2]

10 9701/42/M/J/21 © UCLES 2021 (c) (i) Explain what is meant by the term entropy of a system. … … [1] (ii) Place one tick ( ) in each row of the table to show the sign of each entropy change, ∆S. process ∆S is negative ∆S is zero ∆S is positive NaCl dissolving in water water solidifying to ice  [1] (iii) The evaporation of one mole of water has a standard Gibbs free energy change, ∆G o, of +8.6 kJ at 25 °C. Sketch a graph on the axes to show how ∆G o changes for this process between 25 °C and 150 °C at 101 kPa. positive negative 0 50 75 100 125 150 25 G o temperature / C  [2] (d) The reaction between A and B is feasible at low temperatures but is not feasible at high temperatures. A + B C + D Deduce the signs of ∆H and ∆S for this reaction and explain why the feasibility changes with temperature. sign of ∆H = … sign of ∆S = … … … … … [2]  [Total: 15]

11 9701/42/M/J/21 © UCLES 2021 [Turn over 5 (a) Describe and explain the relative basicities of phenylamine, ethylamine and 4-nitrophenylamine. … > … > … most basic least basic … … … … … [4] (b) The dye R can be synthesised from 4-nitrophenylamine in two steps. O2N O2N OH N N NH2 4-nitrophenylamine step 1 step 2 Q R (i) Deduce and draw the structure of the organic salt Q in the box. [1] (ii) Suggest reagents and conditions for step 1 and 2 in (b). step 1 … step 2 …  [2]

12 9701/42/M/J/21 © UCLES 2021 (c) Compound G can be synthesised from methylbenzene in three steps. E step 1 F G step 2 step 3 CH3 CO2H NO2 Br (i) Give the systematic name of compound G. … [1] (ii) Deduce the identities of E and F and draw their structures in the boxes. [2] (iii) Suggest reagents and conditions for each of steps 1 to 3 in (c). step 1 … step 2 … step 3 …  [3]  [Total: 13]

13 9701/42/M/J/21 © UCLES 2021 [Turn over 6 (a) There are four possible structural isomers of C8H10 that contain a benzene ring. Draw the skeletal formulae of the four structural isomers in the appropriate boxes. The number of peaks observed in the carbon‑13 (13C) NMR spectrum of each compound is given. isomer 1 three peaks in 13C NMR isomer 2 four peaks in 13C NMR isomer 3 five peaks in 13C NMR isomer 4 six peaks in 13C NMR  [4]

14 9701/42/M/J/21 © UCLES 2021 (b) A three-step synthesis of X (C10H10O) from benzene is suggested as shown. Cl (CH2)3CO2H Al Cl 3 Al Cl 3 step 1 step 2 step 3 W X (C10H10O) O Cl (i) Step 1 is the alkylation of benzene by electrophilic substitution. Use R–Cl to represent Cl (CH2)3CO2H. Write an equation for the formation of an electrophile from R–Cl and Al Cl 3. … [1] (ii) Deduce and draw the structures of W and X in the boxes. [2] (iii) Suggest the reagents and conditions for step 2. … [1]

15 9701/42/M/J/21 © UCLES 2021 [Turn over (iv) Complete the mechanism for the reaction of benzene with the electrophile formed in (b)(i). Include all relevant charges and curly arrows showing the movement of electron pairs. Draw the structure of the intermediate. R intermediate  [3]  [Total: 11]

16 9701/42/M/J/21 © UCLES 2021 7 (a) In aqueous solution, chlorine dioxide, Cl O2, reacts with hydroxide ions as shown. 2Cl O2 + 2OH– → Cl O3 – + Cl O2 – + H2O A series of experiments is carried out using different concentrations of Cl O2 and OH–. The table shows the results obtained. experiment [Cl O2] / mol dm–3 [OH–] / mol dm–3 initial rate / mol dm–3 min–1 1 0.020 0.030 7.20 × 10–4 2 0.020 0.120 2.88 × 10–3 3 0.050 0.030 4.50 × 10–3 (i) Explain the term order of reaction. … … [1] (ii) Use the data in the table to determine the order of reaction with respect to each reactant, Cl O2 and OH–. Explain your reasoning. … … … … … … [2] (iii) Use your answer to (a)(ii) to construct the rate equation for this reaction. rate = … [1] (iv) Use your rate equation and the data from experiment 1 to calculate the rate constant, k, for this reaction. Include the units of k.  k = … units … [2]

17 9701/42/M/J/21 © UCLES 2021 [Turn over Question 7 continues on the next page.

18 9701/42/M/J/21 © UCLES 2021 (b) The decomposition of benzenediazonium ions, C6H5N2 +, using a large excess of water, is a first‑order reaction. The graph shows the results obtained. 0.025 0.020 0.015 0.010 0.005 0 0 100 200 300 400 time / s 500 600 [C6H5N2 +] / mol dm–3 (i) Draw the structure of the organic product formed in this reaction.  [1] (ii) Use the graph to determine the rate of reaction at 100 s. Show your working.  rate = … mol dm–3 s–1 [1]

19 9701/42/M/J/21 © UCLES 2021 [Turn over (c) Sketch a concentration–time graph for a zero-order reaction. Use your graph to suggest how successive half-lives for a zero-order reaction vary as the concentration of a reactant decreases. Indicate this by placing a tick ( ) in the appropriate box in the table. concentration time successive half-lives decrease no change in successive half-lives successive half-lives increase  [1]  [Total: 9]

20 9701/42/M/J/21 © UCLES 2021 8 (a) State and explain the relative rate of hydrolysis of acyl chlorides, alkyl chlorides and aryl chlorides. … > … > … fastest slowest … … … … … [3]

21 9701/42/M/J/21 © UCLES 2021 [Turn over (b) The drug remifentanil is shown. O O remifentanil O O O N N Remifentanil is completely hydrolysed under acidic conditions. Three different organic compounds are formed. Draw the structures for these organic compounds in the boxes.  [3]

22 9701/42/M/J/21 © UCLES 2021 (c) Compound Y, C5H10O2, reacts with Na2CO3(aq) to evolve bubbles of gas. The proton (1H) NMR spectrum of compound Y in D2O is shown. / ppm 2 1 0 (i) Use this information to suggest a structure for Y.  [1] (ii) Use the Data Booklet, the proton (1H) NMR spectrum and your answer to (c)(i) to complete the table. chemical shift (δ) environment of proton splitting pattern number of 1H atoms responsible for the peak 0.95 1.90 2.20  [3]  [Total: 10]

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24 9701/42/M/J/21 © UCLES 2021 BLANK PAGE Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. Cambridge Assessment International Education is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of the University of Cambridge Local Examinations Syndicate (UCLES), which itself is a department of the University of Cambridge.

This document consists of 16 printed pages. © UCLES 2021 [Turn over Cambridge International AS & A Level CHEMISTRY 9701/42 Paper 4 A Level Structured Questions May/June 2021 MARK SCHEME Maximum Mark: 100 Published This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge International will not enter into discussions about these mark schemes. Cambridge International is publishing the mark schemes for the May/June 2021 series for most Cambridge IGCSE™, Cambridge International A and AS Level components and some Cambridge O Level components.

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 2 of 16 Generic Marking Principles These general marking principles must be applied by all examiners when marking candidate answers. They should be applied alongside the specific content of the mark scheme or generic level descriptors for a question. Each question paper and mark scheme will also comply with these marking principles. GENERIC MARKING PRINCIPLE 1: Marks must be awarded in line with: • the specific content of the mark scheme or the generic level descriptors for the question • the specific skills defined in the mark scheme or in the generic level descriptors for the question • the standard of response required by a candidate as exemplified by the standardisation scripts. GENERIC MARKING PRINCIPLE 2: Marks awarded are always whole marks (not half marks, or other fractions). GENERIC MARKING PRINCIPLE 3: Marks must be awarded positively: • marks are awarded for correct/valid answers, as defined in the mark scheme. However, credit is given for valid answers which go beyond the scope of the syllabus and mark scheme, referring to your Team Leader as appropriate • marks are awarded when candidates clearly demonstrate what they know and can do • marks are not deducted for errors • marks are not deducted for omissions • answers should only be judged on the quality of spelling, punctuation and grammar when these features are specifically assessed by the question as indicated by the mark scheme. The meaning, however, should be unambiguous. GENERIC MARKING PRINCIPLE 4: Rules must be applied consistently, e.g. in situations where candidates have not followed instructions or in the application of generic level descriptors.

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 3 of 16 GENERIC MARKING PRINCIPLE 5: Marks should be awarded using the full range of marks defined in the mark scheme for the question (however; the use of the full mark range may be limited according to the quality of the candidate responses seen). GENERIC MARKING PRINCIPLE 6: Marks awarded are based solely on the requirements as defined in the mark scheme. Marks should not be awarded with grade thresholds or grade descriptors in mind. Science-Specific Marking Principles 1 Examiners should consider the context and scientific use of any keywords when awarding marks. Although keywords may be present, marks should not be awarded if the keywords are used incorrectly. 2 The examiner should not choose between contradictory statements given in the same question part, and credit should not be awarded for any correct statement that is contradicted within the same question part. Wrong science that is irrelevant to the question should be ignored. 3 Although spellings do not have to be correct, spellings of syllabus terms must allow for clear and unambiguous separation from other syllabus terms with which they may be confused (e.g. ethane / ethene, glucagon / glycogen, refraction / reflection). 4 The error carried forward (ecf) principle should be applied, where appropriate. If an incorrect answer is subsequently used in a scientifically correct way, the candidate should be awarded these subsequent marking points. Further guidance will be included in the mark scheme where necessary and any exceptions to this general principle will be noted. 5 ‘List rule’ guidance For questions that require n responses (e.g. State two reasons …): • The response should be read as continuous prose, even when numbered answer spaces are provided. • Any response marked ignore in the mark scheme should not count towards n. • Incorrect responses should not be awarded credit but will still count towards n. • Read the entire response to check for any responses that contradict those that would otherwise be credited. Credit should not be awarded for any responses that are contradicted within the rest of the response. Where two responses contradict one another, this should be treated as a single incorrect response. • Non-contradictory responses after the first n responses may be ignored even if they include incorrect science.

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 4 of 16 6 Calculation specific guidance Correct answers to calculations should be given full credit even if there is no working or incorrect working, unless the question states ‘show your working’. For questions in which the number of significant figures required is not stated, credit should be awarded for correct answers when rounded by the examiner to the number of significant figures given in the mark scheme. This may not apply to measured values. For answers given in standard form (e.g. a × 10n) in which the convention of restricting the value of the coefficient (a) to a value between 1 and 10 is not followed, credit may still be awarded if the answer can be converted to the answer given in the mark scheme. Unless a separate mark is given for a unit, a missing or incorrect unit will normally mean that the final calculation mark is not awarded. Exceptions to this general principle will be noted in the mark scheme. 7 Guidance for chemical equations Multiples / fractions of coefficients used in chemical equations are acceptable unless stated otherwise in the mark scheme. State symbols given in an equation should be ignored unless asked for in the question or stated otherwise in the mark scheme.

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 5 of 16 Question Answer Marks 1(a)(i) [1s2] 2s22p63s23p63d3 1 1(a)(ii) (a molecule or ion formed by a central) metal atom/ion surrounded by / bonded to one or more ligands 1 1(b) [Cr(H2O)6]3+(aq) formula of chromium species formed type of reaction + NaOH(aq) Cr(OH)3 or Cr(OH)3(H2O)3 precipitation + H2O2(aq) Cr2O72– / CrO42– redox / oxidation + excess NH3(aq) Cr(NH3)63+ ligand substitution chromium species: one mark for each correct species type of reaction: two correct for one mark and three correct for two marks 5 1(c) M1: ΔE is different OR energy gap between d-orbitals is different M2: different frequency / wavelength is absorbed OR different energy / light in visible region is absorbed 2 1(d)(i) ethanoate ions are bidentate whereas as H2O are monodentate OR ethanoate ions form two dative bonds whereas H2O forms one (dative) bond OR ethanoate ions donate two lone pairs whereas H2O donates one (lone) pair 1 1(d)(ii) (coordination number) six AND (geometry around Cr) octahedral 1 1(d)(iii) coordinate / (dative) covalent 1 1(e)(i) 4Cr2+ + O2 + 4H+  4Cr3+ + 2H2O OR 2Cr2+ + O2 + 2H+  2Cr3+ + H2O2 M1: correct species M2: balancing 2 1(e)(ii) Eocell = 1.23 – (–0.41) = (+)1.64 V OR Eocell = 0.68 – (–0.41) = (+)1.09 V value linked to (e)(i) 1

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 6 of 16 Question Answer Marks 2(a) M1: increases down the group M2: radius / size of cation / M2+ increases OR charge density of cation / M2+ decreases M3: less polarisation / less distortion of anion / NO3– ion OR less weakening of NO bond 3 2(b)(i) Pb(NO3)2  PbO + 2NO2 + ½O2 1 2(b)(ii) lead nitrate / Pb(NO3)2 would decompose more / easier AND as Pb2+ is smaller / Pb2+ has larger charge density (so more polarising) 1 2(c)(i) BaC2O4  BaO + CO2 + CO OR BaC2O4  BaO + 2CO + ½O2 1 2(c)(ii) M1: [a] initial moles MnO4– = 0.0200 × 0.050 = 1.00 × 10–3 [b] moles Fe2+ = 0.050 × 0.0304 = 1.52 × 10–3 M2: [a] moles MnO4– unreacted = 1.52 × 10–3 / 5 = 3.04 × 10–4 [b] moles MnO4– reacted = 1.00 × 10–3 – 3.04 × 10–4 = 6.96 × 10–4 M3: moles C2O42- reacted = 6.96 × 10–4 × 5/2 = 1.74 × 10–3 M4: mass of BaC2O4 = 225.3 × 1.74 × 10–3 = 0.392 g % Purity of BaC2O4 = 100 × 0.392/0.50 = 78.4 4 2(d) M1: [OH–] = 2 × 0.12 = 0.24 (mol dm–3) [H+] = 1 × 10–14/0.24 = 4.17 × 10–14 / pOH = –log(0.24) OR 0.62 M2: pH = –log[H+] = 13.4 OR pH = 14 – 0.6 = 13.4 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 7 of 16 Question Answer Marks 3(a)(i) M1: voltage of an electrode / a half-cell compared to / connected to (standard) hydrogen electrode / half-cell M2: (at concentration of) 1 mol / dm3 AND (pressure of) 1 atm / 101 kPa (or in Pa) AND 298 K / 25°C 2 3(a)(ii) 1 3(a)(iii) OR M1 / M2: two 3D isomers of [Ru(en)3]3+ M3: optical / enantiomerism 3 3(b)(i) Eocell = 1.07 – 0.80 = (+)0.27 V AND direction of electron flow = Ag+ / Ag to Br2/ Br– 1 3(b)(ii) M1: Eocell 3rd box ticked M2: [Ag+] decreases AND so (Ag+ / Ag) equilibrium shifts to the left OR [Ag+] decreases AND E for (Ag+ / Ag) becomes less positive / more negative 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 8 of 16 Question Answer Marks 3(c)(i) (a species) that uses / shares a lone pair of electrons to form a coordinate bond to a metal atom / ion 1 3(c)(ii) Kc = [Ag(S2O3)23–] [Br–] / [S2O32–]2 1 3(c)(iii) M1: Kc = Kstab × Ksp = 15.7 M2: 1 OR none / no units 2 3(d) M1: highest [Ag(CN)2]– [Ag(S2O3)2]3– [Ag(NH3)2]+ lowest M2: Kstab [Ag(CN)2]– is highest / [Ag(CN)2]– is the most stable OR higher Kstab forms the more stable complex 2 Question Answer Marks 4(a)(i) M1: energy change when 1 mole of a ionic compound is formed M2: from its gaseous ions under standard conditions 2 4(a)(ii) ΔHsol = (–2099) + (2× – 378) – (–2824) ΔHsol = –31 kJ mol–1 M1: use of ×2 as only multiplier M2: correct signs and evaluation 2 4(a)(iii) M1: Cu2+ is smaller OR Cu2+ has a higher charge density M2: Cu2+ attracts water molecules more / stronger OR (Cu2+) forms stronger ion-dipole forces to water molecules 2 4(b)(i) anode: chlorine / Cl2 cathode: hydrogen / H2 1 4(b)(ii) M1: Q = 0.75 × 60 × 60 = 2700 C AND 96 500 or 193 000 used M2: [a] moles of Ca = 2700 / 193 000 = 0.0140 [b] mass = 0.0140 × 40.1 = 0.56 g 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 9 of 16 Question Answer Marks 4(c)(i) measure / degree of (dis)order / randomness (of a system) OR the number of possible arrangements of the particles and their energy (in a given system) 1 4(c)(ii) ΔS is negative ΔS is zero ΔS is positive solid dissolving in water  water solidifying to ice  1 4(c)(iii) two correct for 1 mark, three correct for two marks: • starting at +8.6 kJ / in positive region close to the y-axis • line passes through x-axis around 100°C • negative gradient straight / curve line through the x-axis (no clear positive inflexions) 2 4(d) M1: ΔH negative / – , ΔS negative / – M2: as temperature increase, ΔG becomes (more) positive / less negative ora OR at low(er) T, (ΔH more negative than TΔS) so ΔG is negative OR at high(er) T, (ΔH less negative than TΔS) so ΔG is positive 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 10 of 16 Question Answer Marks 5(a) M1: ethylamine phenylamine 4-nitrophenylamine most basic least basic M2 / 3 / 4: explanation two correct for one mark, three correct for two marks, four correct for three marks • (basicity linked to) lone pair / p orbital on N AND being able accept / donate to / coordinate to a proton / H+ • ethyl / alkyl group is electron donating / has a positive inductive effect (so lone pair on N is more able to accept a proton) • (phenylamines are less basic than ethylamine as) p orbital / lone pair on N is delocalised (into the ring so less able to accept a proton) • (4-nitrophenylamine is less basic than phenylamine as) nitro / NO2 group is electron withdrawing (so lone pair on N is less able to accept a proton) 4 5(b)(i) 1 5(b)(ii) M1: step 1: HNO2, ⩽ 10°C OR NaNO2, HCl(aq), ⩽ 10°C M2: step 2: NaOH / alkaline AND 1-naphthol / α-naphthol / structure below 2 5(c)(i) 4-bromo-2-nitrobenzoic acid OR 4-bromo-2-nitro(-1-)benzenecarboxylic acid 1

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 11 of 16 Question Answer Marks 5(c)(ii) 2 5(c)(iii) M1: step 1 conc. H2SO4 and conc. HNO3 M2: step 2 Br2 and AlBr3 M3: step 3 hot (alkaline / acidified) MnO4– / KMnO4 3

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 12 of 16 Question Answer Marks 6(a) correct isomers and correct assignment to peaks: mark as •  •  •  •  4 6(b)(i) RCl + AlCl3  R+ + AlCl4– OR Cl(CH2)3COOH + AlCl3  +(CH2)3COOH + AlCl4– 1 6(b)(ii) 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 13 of 16 Question Answer Marks 6(b)(iii) SOCl2 OR PCl5 ALLOW PCl3 AND heat 1 6(b)(iv) M1: arrow to R+ OR arrow to positive carbon of +(CH2)3COOH M2: correct structure of intermediate M3: arrow from C-H bond into the ring AND H+ 3 Question Answer Marks 7(a)(i) the power to which a concentration of a reactant is raised in the rate equation / law 1 7(a)(ii) M1: (using expt 1 and 3) as [ClO2] × 2.5 rate × 6.25 so 2nd order M2: (using expt 1 and 2) as [OH–] × 4 rate × 4 so 1st order 2 7(a)(iii) rate = k[ClO2]2[OH–] 1 7(a)(iv) M1: k = rate / [ClO2]2[OH–] k = 7.20 × 10-4 / (0.02)2(0.03) k = 60 M2: mol–2 dm6 min–1 2

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 14 of 16 Question Answer Marks 7(b)(i) structure of phenol: C6H5OH OR 1 7(b)(ii) tangent drawn correctly AND rate = 0.015 / 260 = 5.8 × 10–5 ALLOW values consistent with tangent drawn at 100 sec 1 7(c) AND half-life decreases (1st box) 1

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 15 of 16 Question Answer Marks 8(a) M1: acyl chlorides > alkyl chlorides > aryl chlorides fastest slowest M2 / 3: Any two from: • acyl chlorides carbon in C-Cl bond is more electron deficient since it is also attached to an oxygen atom OR C-Cl bond is weakest / weakened in acyl chlorides since it is also attached to an oxygen atom / two electronegative atoms • aryl chlorides (no hydrolysis) C-Cl bond is part of delocalised system / partially double bond character (so C-Cl bond is stronger) OR lone pair / p-orbital on Cl delocalises with π ring (so C-Cl bond is stronger) • alkyl chlorides C-Cl bond strengthened by electron donating effect / positive inductive effect of alkyl / R group (as compared to acyl chlorides) OR carbon atom has a smaller δ+ and the C-Cl bond is stronger (than the C-Cl bond in COCl) due to (the carbon) being only attached to one electronegative atom 3 8(b) ALLOW amine salt for the third structure – mono or di ion 3 8(c)(i) 1

9701/42 Cambridge International AS & A Level – Mark Scheme PUBLISHED May/June 2021 © UCLES 2021 Page 16 of 16 Question Answer Marks 8(c)(ii) chemical shift (δ) environment of proton splitting pattern (words required) number of 1H atoms responsible for the peak 0.95 alkane / CH3 doublet 6 1.90 alkane / CH ALLOW alkyne multiplet 1 2.20 R / alkyl / CH2 next to C=O / COOH doublet 2 mark as • •  • •  • •  3