TJC Prelim P3_Ans

1 2017 TJC H2 Chemistry Preliminary Exam [Turn Over CHEMISTRY 9729/03 Paper 3 Free Response 13th September 2017 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Graph Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and civics group on all the work you hand in. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. 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 and 2 blank pages.

2 2017 TJC H2 Chemistry Preliminary Exam [Turn Over Section A Answer all the questions in this section. 1 Calcium carbonate is found in rocks and is the main component of pearls and the shells of eggs, snails and many marine organisms. It is the active ingredient in agricultural lime and is also medicinally used as a calcium supplement or as an antacid. (a) (i) Describe and explain the trend in thermal stability of the Group 2 carbonates. [3]  Down the group, charge of the cation remains constant but ionic radius increases. Hence charge density of the cation decreases.  (CO32- ion is a large anion so is easily polarised but) Polarising power of cation decreases resulting in less distortion of the electron cloud of CO32- ion, weakening the C-O covalent bond less.  More energy needed to decompose the carbonate, hence down the Group, carbonates become more stable to heat, therefore decomposition temperature increases and thermal stability increases down the group. (ii) Determine the minimum temperature for the spontaneous decompo sition of calcium carbonate, given that ΔH = +178.5 kJ mol-1 and ΔS = +163.2 J K-1 mol-1. [2] ΔG = ΔH – TΔS  ΔH – TΔS < 0 T > ΔH / ΔS T > 178.5 x 1000 / 163.2  T > 1090 K (3 sig fig) (b) Limestone is predominantly calcium carbonate, a slightly soluble salt with a K sp value of 3.3 x 10 -9. This rocky material began