ACJC Prelim P2_Answers

3 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over Index No. Name Form Class Tutorial Class Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/02 Higher 2 Paper 2 Structured Questions 16 August 2018 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial class and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions. A Data Booklet is provided. 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. For Examiner's Use Question no. Marks 1 / 14 2 / 6 3 / 9 4 / 13 5 / 12 6 / 9 7 / 12 TOTAL / 75 This document consists of 21 printed pages, including this cover page. 9729/02/Prelim/18 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2018 Department of Chemistry [Turn over

4 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over Answer all the questions in the spaces provided. 1 A mixture contains two white solids XCO3 and YSO4 where X and Y could be either magnesium, zinc or barium. The following investigations were carried out to determine the identity of X and Y. (a) When the mixture of solids was treated with excess dilute hydrochloric acid, a colourless gas was evolved and some, but not all, of the mixture dissolved. The resulting mixture was filtered. To the filtrate, dilute aqueous ammonia was added dropwise till in excess. The white precipitate formed was insoluble in excess aqueous ammonia. Deduce the identity of elements X and Y, showing your reasoning clearly. [3] • X- Mg Y- Ba • BaSO 4 insoluble in dil HCl , • filtrate contains Mg2+ which ppt to form Mg(OH)2 white ppt insol in excess aq ammonia OR • White ppt contains Mg2+ as Zn2+ forms ppt but it is soluble in excess NH3 (b) Group 2 carbonates decompose according to the following equation: ZCO3 (s) ZO (s) + CO2 (g) element magnesium calcium barium enthalpy change for the reaction, ∆Hr/ kJ mol−1 +101 +178 +269 (i) Use the data above to state and explain the trend in the thermal stability of the Group 2 carbonates. [3] • ∆Hr become more endo, thermal stability increases • increasing ionic radii leading to decreasing charge density/ polarising power a