2019 SAJC H2 Chem Prelim P3 ANS

1 NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examination H2 Chemistry (9729) Paper 3 Free Response 18 September 2019 2 hours Additional Materials: Data Booklet, Answer Booklet READ THESE INSTRUCTIONS: Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Marks [60] Section B Answer one question. Marks [20] A Data Booklet is provided. 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 14 printed pages.

2 Section A Answer all the questions in this section. 1 (a) Use of the Data Booklet is relevant to this question. The Hoffmann voltameter is used to electrolyse concentrated iron ( III) chloride solution. The setup of the Hoffmann voltameter is shown below. (i) Describe the observations at the cathode and anode. Explain your answer with the aid of suitable ion-electron equations. [2] Anode (+): 2Cl–  Cl2 + 2e Greenish-yellow gas was evolved at the anode. Cathode (-): Fe3+ + e  Fe2+ The yellow solution turned green (ii) State the difference in the observations at the anode when dilute aqueous iron ( III) chloride solution is us ed instead. Explain your answer using relevant E0 data. [2] Anode (+): 2H2O  O2 + 4H+ + 4e (equation not required) Colourless gas is evolved the E°(O2/H2O) is +1.23V which is more negative (less positive) / preferentially oxidised compared to E°(Cl2/ Cl–) = +1.36V. (b) The electrolysis of a hot aqueous solution of Na X yielded a reddish brown gas, X2, at the anode and hydrogen gas at the cathode. The red litmus paper turned blue when dipped into the solution at the cathode. (i) Suggest the identity of the reddish brown gas X2. [1] Pt electrode Pt electrode + – electrolyte

3 Br2 (ii) Write an ion-electron equation for the reaction at the cathode. [1] Cathode: 2H2O + 2e  H2 + 2OH– Comments:  Many students chose the SHE equation. This is not correct as H+ was not mentioned to be present. (iii) Some of the X2 dissolved in the alkaline medium to form X– and XO3– ions. Write a balanced equation to represent the reaction of X2 in alkaline medium. [1] 3Br2 + 6OH–  5Br– + BrO3– + 3H2O (iv) Describe a chemical test, which does not involve silver nitrate, that could distinguish between sodium chloride and sodium iodide. [2] Add Br2 (aq). For KCl, orange Br2 is not decolourised. For KI, orange solution of Br2 turns brown. 2I– + Br2  I2 + 2Br– (c) Describe and explain how the thermal stability of the hydrogen halides varies down Group 17. [2] Down the group, size of haloge