2019 CJC Prelim H2 Chem P3 ANS

9729/03/CJC JC2 Preliminary Examination 2019 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/03 Paper 3 Free Response Friday 30 August 2019 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class 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. Section A Answer all questions. Section B Answer one question. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. At the end of examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 8 printed pages. Catholic Junior College JC2 Preliminary Examinations Higher 2 WORKED SOLUTIONS

2 9729/03/CJC JC2 Preliminary Examination 2019 Section A Answer all the questions in this section. 1 (a) Chemical energy can be directly converted to electrical energy in batteries and fuel cells. Fuel cells require a continuous source of fuel and oxygen to sustain the chemical reaction, whereas in batteries, the chemical energy is usually derived from reactions of metals or metal oxides present in the battery. The nickel-cadmium (NiCd) battery is a rechargeable battery used in many devices such as handphones and laptops. A simplified diagram of the NiCd ele ctrochemical cell is as shown: Electrode X is made of a solid nickel compound, NiO(OH), and electrode Y is made of solid cadmium. The electrolyte is aqueous potassium hydroxide. D uring discharge, Ni(OH)2(s) and Cd(OH)2(s) are formed at the respective electrodes. (i) Construct the half-equations at the electrodes of this alkaline NiCd electrochemical cell. Hence, give the balanced overall equation for the reaction that occurs during discharge. [2] Electrode X: NiO(OH) + H2O + e-  Ni(OH)2 + OH- Electrode Y: Cd + 2OH–  Cd(OH)2 + 2e- Overall equation: Cd + 2NiO(OH) + 2H2O Cd(OH)2 + 2Ni(OH)2 (ii) Write a cell representation for the NiCd electrochemical cell, indicating clearly the direction of electron flow in the external circuit. [2] (iii) NiCd batteries can be recharged by applying a current across the two electrodes. How long would it take to recharge a NiCd battery at a current of 2.0 A, if 5.6 g of cadmium was converted to Cd(OH)2? [2] Moles of Cd converted = 5.6 / 112.4 = 0.0498 mol Moles of electrons required = 0.0498 x 2 = 0.0996 mol Cd(s), Cd(OH)2(s) // NiO(OH)(s), Ni(OH)2(s) e– flow

3 9729/03/CJC JC2 Preliminary Examination 2019 [Turn over Total charge required = 0.0996 x 96500 = 9615 C Time required = 9615 / 2.0 = 4807 s (or 80.1 min) (b) Methane gas, CH