2019 MI Prelim H2 Chem P2 ANS

Class Adm No Candidate Name: This question paper consists of 11 printed pages and 1 blank page. 2019 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 16 Sept 2019 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 5 Total Marks 7 23 16 14 15 75

2 1 (a) Zinc-air batteries worked by oxidising zinc with oxygen from the air. At the cathode, oxygen converts to hydroxide ions. At the anode, zinc reacts with the hydroxide ions to form zincate, [Zn(OH)4]2-. (i) Construct an ion-electron equation for the reaction that take s place at each electrode under alkaline conditions. [2] Cathode: O2 + 2H2O + 4e- → 4OH- Anode: Zn + 4OH- → [Zn(OH)4]2- + 2e- (ii) At 298 K, the standard electrode potential of the [Zn(OH)4]2-(aq)|Zn(s) half-cell is -1.25 V. Calculate the cell voltage using relevant data from the Data Booklet. [1] Ecell = 0.40 – (-1.25) = +1.65 V (iii) Using relevant data from the Data Booklet, deduce if oxygen gas can be replaced with chlorine gas. [2] Ecell = 1.36 – (-1.25) = +2.61 V Since the E cell value is more positive when oxygen gas is replaced with chlorine gas, the reaction is feasible. (b) Another type of battery known as the nickel-cadmium battery is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. The electrode reaction equations for the discharging process under alkaline conditions is given below. Cathode: Cd + 2OH- → Cd(OH)2 + 2e- Anode: 2NiO(OH) + 2H2O + 2e- → 2Ni(OH)2 + 2OH- (i) Construct the overall equation for the reaction that takes place during charging. [1] 2Ni(OH)2 + Cd(OH)2 → 2NiO(OH) + Cd + 2H2O (ii) Suggest a disadvantage of using nickel-cadmium battery. [1] Cadmium is a toxic heavy metal which can result in pollution if discarded in landfill or incin