2021 EJC Prelims Paper 2 QP

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2021 General Certificate of Education Advanced Level Higher 2 CANDIDATE NAME CIVICS GROUP 2 0 – REGISTRATION NUMBER CHEMISTRY Paper 2 Structured Questions 9729/02 16 September 2021 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not use paper clips, highlighters, glue or correction fluid. Write your name, civics group, registration number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Answer all questions on the Question Paper The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. For Examiner’s Use Paper 2 1 / 15 2 / 12 3 / 13 4 / 15 5 / 20 Total / 75 This document consists of 19 printed pages and 1 blank page.

2 © EJC 9729/02/J2PE/21 For Examiner’s Use 1 Nitrosyl chloride, NOCl decomposes at temperatures above 100 oC to form nitrogen monoxide and chlorine as shown in the equation below. 2NOCl(g) → 2NO(g) + Cl2(g) (a) A sample of NOCl was allowed to decompose at 127 ºC and 147 atm. Assuming that NOCl behaves ideally, show that its initial concentration is 4.48 mol dm−3. [2] (b) A student pointed out that the assumption in (a) that NOCl behaves ideally is not valid. Suggest and explain two possible reasons why this may be so. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ ................................ .. ................................ ................................ ................................ .............................. [2]

3 © EJC 9729/02/J2PE/21 [Turn Over For Examiner’s Use (c) The change in the concentration of NOC l was monitored as the decomposition reaction occurred over time. The data obtained is shown in Fig. 1.1. Fig. 1.1 (i) Using Fig. 1.1, show that the reaction is second order with respect to NOCl. [3] (ii) Given that the rate equation is   2 rate NOCk= l , calculate the rate constant of the reaction, stating its units clearly. [2]

4 © EJC 9729/02/J2PE/21 For Examiner’s Use (iii) A second experiment was conducted under the same conditions, but using half the initial concentration of NOCl. On the axes provided below, sketch the graph that will be obtained using data from the second experiment, relative to that provided for the first experiment. [1] (d) (i) Define half-life. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [1] (ii) The rate equation for the second-order decomposition of NOC l can also be represented in the following manner:    0 11 NOC NOCt kt−=ll where  NOC tl = concentration of NOCl at time, t,  0NOCl = original concentration of NOCl, and k = rate constant Using this equation, derive an expression for the half-life of the reaction. [1] rate first experiment [NOCl]

5 © EJC 9729/02/J2PE/21 [Turn Over For Examiner’s Use (iii) Hence, state and explain whether the half-life of a second-order reaction will increase or decrease as the reaction proceeds. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [1] (e) The reverse of the decomposition reaction can also occur such that NOCl is formed from NO and Cl2. Given that a possible mechanism for the reaction is as shown below, suggest the rate equation for the reaction. Step 1: fast 22NO NO N O+ Step 2: slow 2 2 2C N O 2NOC+ ⎯⎯⎯ →ll [2] [Total: 15]

6 © EJC 9729/02/J2PE/21 For Examiner’s Use 2 During the 2000s, researchers looked into creating high-capacity rechargeable batteries for use in energy storage in renewable energy generation. The zinc-cerium battery shown in Fig. 2.1 is one example of such a rechargeable battery involving redox flow. Fig. 2.1 The half-cells are made up of the following: half-cell electrolyte electrode Zn2+/Zn Zn(CH3SO3)2(aq), CH3SO3H(aq) carbon-based polymer (similar to graphite) with fine Zn metal particles on surface Ce4+/Ce3+ Ce(CH3SO3)3(aq), Ce(CH3SO3)4(aq), CH3SO3H(aq) carbon-based polymer (similar to graphite) The two electrolytes , consisting of methanesulfonic acid, CH3SO3H, and the respective salts, are separated by a proton-selective membrane. (a) During the discharge process, the Zn2+/Zn half-cell undergoes oxidation. (i) Identify the polarity of the electrodes for the half -cells during the discharge process. Zn2+/Zn: ................................ . Ce4+/Ce3+: ................................ [1] (ii) Draw on Fig. 2.1 the direction of electron flow in the external circuit during the discharge process. [1]

7 © EJC 9729/02/J2PE/21 [Turn Over For Examiner’s Use (iii) Write the overall equation for the reaction during the discharge process. ................................ ................................ ................................ ....................... [1] (iv) Given that E ( ) .43Ce Ce 1 44 V++ =+ , with the aid of the Data Booklet, calculate the cellE for the reaction during the discharge process. [1] (v) Hence, calculate the standard Gibbs free energy in kJ mol–1. [1] (b) Methanesulfonic acid dissociates fully in water. During the discharging process, the Zn2+/Zn half-cell becomes more positive than the Ce4+/Ce3+ half-cell over time. (i) Explain how the proton exchange membrane maintains electrical neutrality. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ....................... [1] When a high concentration of methanesulfonic acid was used, it was found that cerium(III) methanesulfonate, Ce(CH3SO3)3, precipitates out in the electrolyte. (ii) Suggest the effect on cellE when a higher concentration of methanesulfonic acid was used as the electrolyte. Explain your answer. ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... ................................ ................................ ................................ ........................... .................