AJC H2 Chem 2012 Prelim P3 QP

ANDERSON JUNIOR COLLEGE Preliminary Examinations 2012 CHEMISTRY 9647/03 Higher 2 17 September 2012 Paper 3 Free Response Questions 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet Writing paper Graph paper READ THESE INSTRUCTIONS FIRST Write your name, PDG and register number on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Start each question on a fresh sheet of paper. A Data Booklet is provided. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. This document consists of 12 printed pages. AJC Prelim 2012 9647/03/H2 [Turn over

2 AJC Prelim 2012 9647/03/H2 1 (a) Sodium dichromate(VI), Na 2Cr2O7, is used widely in organ ic synthesis as well as in the process of leather tanning. The principal ore of chromium called chromite, FeCr 2O4, is used in the manufacture of sodium dichromate(VI) via a 2–stage process. In the first stage of the process, the ore is fused with soda ash (sodium carbonate) and air at a temperature of 1000 °C. The following equation represents this stage. 4FeCr2O4 + aNa2CO3 + bO2 cFe2O3 + dNa2CrO4 + eCO2 (i) Complete the balancing of the above equation by deducing the values for a, b, c, d and e. In the second stage, the products are extracted with hot water. Orange crystals of sodium dichromate(VI) separate out during the evaporation stage of the process. (ii) Suggest a reagent that can be added to the hot water extract to produce the dichromate(VI) solution in the second stage of the process. [2] (b) Draw a fully labeled diagram to show how the standard electrode potential of the Cr2O72–(aq)/Cr3+(aq) system could be measured by a standard hydrogen electrode. Show the direction of electron flow in the circuit. [3] (c) When zinc metal is added to an acidifi ed solution of aqueous chromium (III) solution, the solution turns from green to blue. When the excess zinc is filtered off and the blue solution is left to stand for several hours in the absence of air, it slowly turned back to green and a colourless gas is evolved. Explain these observations, quoting appropriate Eo values from the Data Booklet. [3] (d) A chromium–containing ion, CrOxn–, disproportionates in acid solution to produce a mixture of chromium(III) ions and dichromate(VI) ions. A solution containing 7.5 x 10 –3 mol of CrO xn– ions was acidified and the disproportionation occurred. The resulting solution contained 2.5 x 10–3 mol of dichromate(VI) ions. Using the information given, calculate the oxidation number of chromium in th