23MIPrelim Answers (H2 Chem Paper 2)

Class Adm No Candidate Name: This question paper consists of 18 printed pages and 2 blank pages. 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] Zn + 4H2O → Zn(OH)42– Zn + 4H2O → Zn(OH)42– + 4H+ Zn + 4H2O → Zn(OH)42– + 4H+ + 2e– Zn + 4H2O + 4OH– → Zn(OH)42– + 4H+ + 4OH– + 2e– [O]: Zn + 4OH– → Zn(OH)42– + 2e– ; (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. metal cap (top) metal can (bottom) HgO Zn membrane (with alkaline electrolyte)

3 [Turn over charge anode or cathode metal cap metal can [2] charge anode or cathode metal cap negative anode metal can positive cathode charge pair ; electrode pair ; (iii) Hence, label the flow of electrons in the external circuit of Fig 1.1. [1] from cap to can (anticlockwise) (iv) Suggest the function of the membrane. ………………………………………………………………………………………………… [1] It acts as the salt bridge to maintain electrical neutrality. OR To separate the HgO and Zn to allow electron flow via the external circuit. ; (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] Ecell = Ecat – Ean = Ered – Eox +1.35 = EHgO/Hg - EZn(OH)42-/Zn EZn(OH)42-/Zn = +0.10 – (+1.35) = -1.25 V ; (vi) Deduce how the cell potential of equation 1.1 will change if the [Zn(OH)42–] is increased. …………………………………………………………………………………………………… ……………………………………………………………………………………………………

4 …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] Zn(OH)42– + 2e– ⇌ Zn + 4OH– --- Equilibrium 1 A increase in [Zn(OH)42–] shifts position of Equilibrium 1 to the right. ; EZn(OH)42-/Zn will become less negative, leading to Ecell becoming less positive. ; (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] bent ; (ii) Explain, with reference to the Valence Shell Electron Pair Repulsion theory, the shape of the NOCl molecule. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [2] There are 3 electron pairs / 2 bond pairs 1 lone pair around the central atom, N. VSEPR states that to minimise repulsion, ; the electron pairs are arranged as far apart as possible about the central atom (bent for 3 electron pairs). ; (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.

5 [Turn over [4] 2NOCl(g) ⇌ 2NO(g) + Cl2(g) I / mol 0.05 0 0.05 C / mol -2x +2x +x E / mol 0.05-2x 2x 0.05+x ; 0.05-2x + 2x + 0.05+x = 0.122 x = 0.022 mol ; 2NOCl(g) ⇌ 2NO(g) + Cl2(g) E / mol 0.006 0.044 0.072 Kp = (0.044 0.122×5) 2 (0.072 0.122×5) (0.006 0.122×5) 2 ; generic Kp expression = 159 atm (3sf) ;

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] Vol of gas particles are negligible compared to volume of the container. ; Intermolecular forces between gas particles is negligible. ; (ii) Explain the shape of the graph for the ‘ideal gas’ in Fig 1.2. …………………………………………………………………………………………………… …………………………………………………………………………………………………… ………………………………………………………………………………………………… [1] pV = nRT pV/RT = n = constant (1 mol) The ideal gas curve is a horizontal line of constant value (pV/RT = 1) OR Ideal gases always obey the ideal gas equation pV=nRT ; don’t mark for “1 mol” (iii) Sketch the graph for NOCl on Fig 1.2, explaining your reasoning. …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… pV/RT 1.0 p O2 ideal gas

7 [Turn over ………………………………………………………………………………………………… [2] NOCl deviates more from ideality as the permanent dipole–permanent dipole interactions / instantaneous dipole -induced dipole interactions between its molecules are stronger than the instantaneous dipole –induced dipole interactions between O 2 molecules. ; [Total: 20] ideal gas pV/RT 1.0 p O2 NOCl

8 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. …………………