RJC_Chemistry H2 P2 with Ans Prelims

RAFFLES JUNIOR COLLEGE PRELIMINARY EXAMINATION 2008 HIGHER 2 CANDIDATE NAME C L A S S I N D E X N U M B E R CHEMISTRY 9746/02 Paper 2 Structured 15 September 2008 1 hour 30 minutes Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided on the top of this page. Write in dark blue or black pen in the space provided. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good Eng lish and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Question 1 2 3 4 5 6 Total Marks /10 /10 /10 /10 /10 /10 /60 This question booklet consists of 16 printed pages. ©RJC 2008 [Turn over

2 Answer all the questions in the spaces provided. 1 (a) Household bleach contains sodium chlorate( I), NaC lO. When bleach solution is acidified with dilute hydr ochloric acid, chlorine is produced by the following reaction: ClO–(aq) + 2H+(aq) + Cl–(aq) Æ Cl2(aq) + H2O(l) Chlorine oxidises iodide to iodine, and the amount of iodine produced can be estimated using a standard sodium thiosulphate solution. A 25.0 cm 3 sample of a household bleach is diluted to 250 cm 3. To a 25.0 cm 3 portion of this diluted solution, dilute hydrochloric acid is added. This is followed by the addition of excess potassium iodide solution. The resultant mixture is then titrated against 0.200 mol dm –3 sodium thiosulphate solution. The volume required is 18.50 cm 3. What is the concentration of sodium chlorate( I) in the household bleach? [ 3 ] ©RJC 2008 [Turn over

3 (b) The rate of reaction between peroxodisulphate ions, S2O8 2–, and iodide ions, I–, can be increased by the addition of Fe3+(aq). (i) With the aid of the Data Booklet, explain how Fe 3+ catalyses the reaction between S2O8 2– and I–. (ii) With the aid of a suitable diagram , explain why the rate of the reaction can also be increased by heating. [ 6 ] ©RJC 2008 [Turn over

4 (c) Certain gas–phase or liquid–phase r eactions which are catalysed by solid catalysts often ex hibit zero order kinetics with respect to the reactants at high concentrations of reactants. Suggest a reason why this might be so.