VJC H2 Chem 2012 Prelim P3 Soln

 VJC 2012 9647/03/PRELIM/12 [Turn over VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Candidates answer on separate paper. 17 September 2012 2 hours Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen on both sides of the paper. 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 any four questions. A Data Booklet is provided. You are reminded of the need for good English and clear explanation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 11 printed pages.

 VJC 2012 9647/03/PRELIM/12 2 Answer any 4 questions. 1 (a) The elements of Group VII are all oxidising agents , but some are more oxidising than others. Describe and explain, with the aid of equations, t he reactions of the elements with the reducing agent, sodium thiosulfate. [3] Oxidising power of X 2 decreases down the group. For Cl2, Br 2 4X 2 + S 2O3 2– + 5H 2O → 8X – + 2SO 4 2- O.S. of S increases by 4 units from +2 to +6 For I2, I 2 + 2S 2O3 2– → 2I – + S 4O6 2– O.S. of S increases by 0.5 units from +2 to +2½ [3.5, max 3] (b) Potassium chlorate(V), KC lO3, is one of the products formed during the electrolysis of concentrated aqueous KC l solution. (i) State, with the aid of balanced equations, how KC lO3 is produced in the electrolysis of concentrated aqueous KC l solution. Electrolysis of KC l solution gives KOH, H 2 and C l2 KC l + H 2O → KOH + ½H 2 + ½C l2 C l2 undergoes disproportionation and reacts with KOH under hot conditions to give KC lO3. 3C l2 + 6KOH → KC lO3 + 5KC l + 3H 2O (ii) The standard electrode potentials, E/ring2, and standard Gibbs free energy changes, ∆ G/ring2, of different chlorine-containing species are tabu lated below. Half-equation E/ring2/ring2 /ring2/ring2 / V ∆∆ ∆∆ G/ring2/ring2 /ring2/ring2 / kJ mol −− −− 1 2C lO3 − + 12H + + 10e − ⇌ Cl2 + 6H 2O +1.47 − 1420 Cl2 + 2e − ⇌ 2C l− +1.36 − 262 Write a half-equation for the conversion of C lO 3 − to C l− . Using your knowledge of Hess’ Law for ∆ G/ring2 , calculate ∆ G/ring2 for this conversion.

 VJC 2012 9647/03/PRELIM/12 [Turn over 3 ∆ G/ring2 and E/ring2 are related by the following equation: ∆ G/ring2 = − zFE /ring2 where ∆ G/ring2 is the standard Gibbs free energy chang