SAJC_2008 H2 Prelim Paper3

ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations CHEMISTRY Higher 2 9746/03 Paper 3 Free Response 9 September 2008 2 hours Candidates answer on separate paper. Additional Materials: Data Booklet, Graph Paper, Answer Paper READ THESE INSTRUCTIONS FIRST Write your name and civics 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 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 examination, fasten all your work securely together. This document consists of 10 printed pages including this page.

Answer any four questions. 1(a) The compound K 3CrO4 is a green solid. When mixed with dilute H 2SO4, it disproportionates to yield Cr 3+(aq) and Cr 2O7 2-(aq) ions. The mixture produced was mixed with excess K I and the iodine liberated required 32.00 cm 3 of 0.30 mol dm -3 Na2S2O3 for reaction. Write equations for the following changes: (i) CrO 4 3- to Cr3+ (ii) CrO 4 3- to Cr2O7 2- (iii) the overall equation for the disproportionation of CrO 4 3- in acid. (b) 1 mole of Cr 2O7 2- reacts with excess K I to liberate 3 moles of iodine. Hence, by using the equation in (a)(iii), calculate the mass of K 3CrO4 used in the disproportionation reaction. [6] (c) Chromium plating can be carried out on an object in an acidified solution of 2.0 mol dm -3 sodium dichromate (V I). The process is carried out on an object of surface area of 0.5 m2 and the current used is 40 A. Calculate the time needed to plate the object to a thickness of 1.0 x 10-5 m. [Density of chromium is 7.3 g cm -3.] [4] (d) Explain briefly why it would not be possible to electroplate the object with aluminium from an aqueous solution of aluminium nitrate. [1] (e) Explain why Cr 3+(aq) ion is coloured. [3] (f) (i) A cyclic di-ester can be synthesized from reacting a dicarboxylic acid with a diol. Hence, describe how 2, 3-dichlorobutane can be converted to compound A in 3 steps. Include in your answer, the reagents and conditions for each step, including the structures of the intermediate compounds formed. C C C C O O O O CH2 H H H3C H3C Compound A (ii) Name the isomerism exhibited by but-2-ene and draw the isomers. (iii) A by-product in the reaction between ethylamine, CH 3CH2NH2 and chloromethane, CH 3Cl, has the formula C 5H14NCl and it gives an immediate precipitate with aqueous silver nitrate. Suggest a structure for this compound and write an equation for its formation. [6] [Total : 20] 2

2(a) Carbon dioxide is o