23MIPrelimQP (H2 Chem Paper 2)

Class Adm No Candidate Name: This question paper consists of 17 printed pages and 1 blank page. 2023 Preliminary Examination Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 13 September 2023 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission 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. Do not use staples, paper clips, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question 1 2 3 4 Total Marks 20 14 20 21 75

2 1 (a) The mercury cell is a non-rechargeable electrochemical battery making use of the reaction between mercuric oxide, HgO, and Zn electrodes. The reaction is performed in an alkaline electrolyte. The main reaction of the battery is: Zn + HgO → ZnO + Hg equation 1.1 The ion-electron equation for the reaction of HgO is: [R]: HgO + H2O + 2e– → Hg + 2OH– EHgO/Hg = +0.10 V As mercury is toxic, the sale of mercury cells is now banned in many countries. For Examiners’ Use (i) Due to the alkaline conditions, the oxidation of Zn forms Zn(OH) 42– as the product. It then undergoes another (non-redox) chemical reaction to form ZnO. Write an ion-electron equation for the oxidation of Zn to Zn(OH)42–. [O]: …………………………………………………………………………………………… [1] (ii) A voltmeter is placed across a button -sized mercury cell to measure the cell potential of equation 1.1. An enlarged schematic of the button -sized mercury cell is shown in Fig 1.1. Fig 1.1 Determine the charges of the metal cap/can and identify which is the anode/cathode. charge anode or cathode metal cap metal can [2] metal cap (top) metal can (bottom) HgO Zn membrane (with alkaline electrolyte)

3 [Turn over (iii) Hence, label the flow of electrons in the external circuit of Fig 1.1. [1] (iv) Suggest the function of the membrane. ………………………………………………………………………………………………… [1] (v) The cell potential of equation 1.1 is found to be +1.35 V. Calculate the reduction potential of the Zn(OH)42–/Zn half-cell. [1] (vi) Deduce how the cell potential of equation 1.1 will change if the [Zn(OH)42–] is increased. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2]

4 (b) Mercury is typically unreactive and does not react with most acids. However, it dissolves in ‘aqua regia’, which is a mixture of nitric acid and hydrochloric acid. When nitric acid and hydrochloric acid are mixed together, nitrosyl chloride and chlorine gas are formed: HNO3 + 3HCl → NOCl + Cl2 + 2H2O (i) Suggest the shape about the central N atom of NOCl. shape: ……………………………………… [1] (ii) Explain, with reference to the Valence Shell Electron Pair Repulsion theory, the shape of the NOCl molecule. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2]

5 [Turn over (iii) At 500 K, NOCl readily dissociates into NO and Cl2: 2NOCl(g) ⇌ 2NO(g) + Cl2(g) equation 1.2 0.05 mol of NOCl and 0.05 mol Cl2 were introduced into a 1 dm3 reaction vessel at 500 K and left to equilibrate. The total amount of gases at equilibrium was found to be 0.122 mol, and the total pressure of gases was 5.00 atm. Calculate the value of Kp for equation 1.2 at 500 K. [4]

6 (c) Fig 1.2 shows the graph of pV/RT against varying pressures, p, for 1.0 mol of O2. Fig 1.2 (i) State the two main assumptions of the kinetic theory of ideal gases. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (ii) Explain the shape of the graph for the ‘ideal gas’ in Fig 1.2. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] (iii) Sketch the graph for NOCl on Fig 1.2, explaining your reasoning. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] [Total: 20] pV/RT 1.0 p O2 ideal gas

7 [Turn over 2 Kinetics studies, especially for organic reactions, provide strong evidence of their hypothesised mechanisms. In one study, the kinetics of a nucleophilic substitution reaction involving C6H5CH2CH2Cl and CN– was investigated. A series of 4 experiments was conducted using different concentrations of C6H5CH2CH2Cl while keeping the concentration of CN– constant at 2.00 mol dm-3. Fig 2.1 shows the results: Fig 2.1 For Examiners’ Use (a) (i) The order of reaction with respect to C6H5CH2CH2Cl is 1. Explain how the graph of Fig 2.1 shows this. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (ii) Briefly explain why the concentration of CN– has to be kept constant. …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] initial rate / mol dm-3 s-1 [C6H5CH2CH2Cl] / mol dm-3 3.0 × 10 -5 0.040 0 x x x x

8 (iii) The rate equation for the reaction was found to be: rate = k[C6H5CH2CH2Cl][CN–] Determine a value for the rate constant, k, stating its units clearly. [2] (iv) Explain how the rate of reaction is expected to change when the reaction is carried out at a higher temperature. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] (v) On the axes provided, sketch how you would expect the concentration -time graph of C6H5CH2CH2Cl to look like for the reaction. [2] [C6H5CH2CH2Cl] / mol dm-3 time

9 [Turn over (b) (i) Outline the mechanism for the reaction between C6H5CH2CH2Cl and CN–. Show clearly any relevant charges, lone pairs of electrons, and movement of electrons. [3] (ii) When C6H5CHClCH3 was used in place of C6H5CH2CH2Cl, the rate equation was found as: rate = k[C6H5CHClCH3] Explain this observation. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] [Total: 14]

10 3 (a) Table 3.1 shows the reduction potentials, atomic radii , and ionic radii for three Group 2 elements. element Eo [M2+/M] / V atomic radii of M / nm ionic radii of M2+ / nm Mg –2.38 0.160 0.065 Ca –2.87 0.197 0.099 Sr –2.89 0.215 0.113 Table 3.1 For Examiners’ Use (i) Rank and give an explanation for the relative reactivity of the Group 2 elements in Table 3.1 as reducing agents. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………