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 t