VJC Prelim_P3_QP and Ans

1 ©VJC2018 9729/03/PRELIM/18 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/03 Paper 3 Free Response Candidates answer on separate paper. 17 September 2018 2 hours Additional Materials: Cover Page 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. You must start the answer to each question on a fresh piece of writing paper. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 11 printed pages and 1 blank page.

2 ©VJC2018 9729/03/PRELIM/18 [Turn over Section A Answer all the questions in this section. 1 (a) All the elements in the third period of the Periodic Table, sodium to sulfur, form chlorides by direct combination with chlorine. Aluminium chloride may be produced by passing a stream of chlorine over heated aluminium metal in a long hard–glass tube. (i) With the aid of equations, explain t he following observations when different amounts of water were added to solid aluminium chloride. (I) When a limited amount of water was added, a white solid was formed together with steamy fumes. (II) When excess water was added, a solution of pH 3 was obtained. [2] For (I): A lCl 3 reacts with a limited amount of water to give A l(OH)3(s) and HCl(g): AlCl3(s) + 3H2O(l)→ Al(OH)3(s) + 3HCl(g) For (II): AlCl3 undergoes hydrolysis as A l3+ has high charge density / strong polarising power. Polarisation of H2O molecules favours the loss of H+, and hence, acidity of the solution increases (pH ≈ 3). AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl–(aq) [Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(H2O)5OH]2+(aq) + H3O+(aq) (ii) Both aluminium chloride and copper( I) complex solutions are colourless whereas a solution of copper( II) sulfate appears blue. Explain these observations. [3] AlCl3 solution