TJC_H2_CHEM_P3_ANS(1) Prelim

2015 TJC H2 Chemistry Preliminary Exam [Turn Over 1 CHEMISTRY (H2) 9647/03 Paper 3 Free Response 16th September 2015 2 hours Candidates answer on separate paper. Additional materials: Answer paper Graph Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, Civics Group, Centre number and Index number in the spaces provided on the cover page and on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a HB pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer any four questions. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. 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 20 printed pages.

2015 TJC H2 Chemistry Preliminary Exam [Turn Over 2 1 (a) The halogens and their compounds are useful laboratory reagents and have many applications. (i) Describe the trend in the colour of the halogens. • Increasing intensity or darker down the group. • Cl2 (g) yellowy-green; Br2 (l) reddish-brown; I2 (s) black (ii) Explain, in terms of structure and bonding, the trend in the volatility of the halogens.  The halogens have simple molecular structure and weak van der Waals forces of attraction between molecules.  Down the group, Mr of X2 increases, giving a larger electron-cloud, which can be more easily distorted, resulting in stronger van der Waals forces hence requiring more energy to overcome. The volatility decreases from Cl2 to I2. (iii) Halogens are oxidising agents. By quoting appropriate values from the Data Booklet , explain the reactions of the halogens with sodium thiosulfate. Write balanced equations for the reactions. [6] ½Cl2 + e Cl- E = +1.36 V ½Br2 + e Br- E = +1.07 V ½I2 + e I- E = +0.54 V  (With quoting of Data) Oxidising power of the halogens decreases down the group as can be seen from the less positive E value, hence iodine can only oxidise thiosulfate to tetrathionate. [•Bonus mark] 4Cl2 + S2O3 2- + 5H2O  8Cl- + 2SO4 2- + 10H+ 4Br2 + S2O3 2- + 5H2O  8Br- + 2SO4 2- + 10H+  I2 + 2S2O3 2-  S4O6 2- + 2I- (b) Hydrocarbons react with halogens under different conditions to give a wide range of organic compounds. The Wohl-Ziegler bromination is shown below. In this reaction, a bromine atom is incorporated into a molecule at the position next to a carbon-carbon double bond. The reagent used for the