CJC Prelim P2 SOL

[Turn over 9729/02/CJC JC2 Preliminary Examination 2017 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/02 Paper 2 Structured Questions 18 August 2017 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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. Answer all questions in the spaces provided on the Question Paper. 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. For Examiner’s Use Paper 1 30 Paper 2 Q 1 12 Q 2 19 Q 3 18 Q 4 9 Q 5 10 Q 6 7 75 Paper 3 Q 1 22 Q 2 20 Q 3 18 Q 4 20 Q 5 20 80 Total 185 Catholic Junior College JC2 Preliminary Examinations Higher 2 Suggested Solutions

2 9729/02/CJC JC2 Preliminary Examination 2017 1 Sedimentary rocks formed at the earth's surface by the accumulation of particulate matter are usually transported to the place of deposition by water, wind, or mass movement of glaciers. Dolostone, a sedimentary rock, consists of a mixture of minerals such as calcium and magnesium carbonates. Table 1.1 below shows some properties of calcium carbonate and magnesium carbonate. Table 1.1 (a) (i) Write an expression for the solubility product, Ksp, of calcium carbonate. ................................ ................................ ................................ ...................... [1] (ii) Calculate the solubility, in mol dm-3, of calcium carbonate in water at 25 °C. [1] (iii) A saturated solution X containing calcium hydroxide and calcium carbonate has a pH of 13 at 25 °C. Given that the Ksp of calcium hydroxide is 5.5 x 10-6 mol3 dm-9 and using your answer in (a)(i), calculate the solubility of calcium carbonate in solution X. [2] Numerical values of Ksp at 25 °C Decomposition temperature / °C calcium carbonate magnesium carbonate 5.0 x 10-9 1.0 x 10-5 900 540 Ksp= [Ca2+] [CO32-] Let solubility of CaCO3 be x [Ca2+] = [CO32-] = x Ksp = [Ca2+][CO32-] x2 = 5.0 x 10-9 x = 7.07 x 10-5 mol dm-3 pOH = 14 - 13 [OH-] = 10-(14-13) = 0.100 mol dm-3 [Ca2+][OH-]2 = 5.5 x 10-6 [Ca2+] = 5.5 x 10-4 mol dm-3 [Ca2+][CO32-] = 5.0 x 10-9 [CO32-] = 5.0 x 10-9 / 5.5 x 10-4 [CO32-] = 9.09 x 10-6 mol dm-3 Since mol CaCO3 dissolved ≡ CO32– in saturated solution Solubility of CaCO3= = 9.09 x 10-6 mol dm-3

3 [Turn over 9729/02/CJC JC2 Preliminary Examination 2017 (iv) Comment on, and explain , the discrepancy in the solubilities of calcium carbonate in water and in solution X. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] (v) When rainwater runs over dolostone, a chemical reaction occurs as shown below: MCO3(s) + CO2(aq) + H2O(l) ⇌ 2HCO3-(aq) + M2+(aq) where M is a group 2 metal I. Suggest the source of CO2 (aq) for the reaction. ................................ ................................ ................................ ...................... [1] II. When a sample of saturated solution of Ca(HCO3)2 is heated, a reaction occurs and a precipitate is formed. Using the equation above, suggest why this happens. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] (b) Calcium carbonate can be decomposed by heating at high temperatures. (i) Write an equation for the thermal decomposition of calcium carbonate. ................................ ................................ ................................ ...................... [1] (ii) Calcium carbonate decomposes at a higher temperature than magnesium carbonate. Explain why this is so. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] [Total: 12] Air containing CO2 that is dissolved in the rainwater. Solubility of calcium carbonate in solution X is lower . Presence of common ion, Ca 2+, shifts po sition of equilibrium (1) to the left by Le Chatelier’s Principle. CaCO3 (s) ⇌ Ca2+ (aq) + CO32- (aq) ---------(1) Boiling causes CO 2 to be released from the solution. Based on Le Chatelier’s Principle, position of equilibrium of above equation shifts left, which results in the precipitation of CaCO3 (s). Ca2+ has a larger ionic size than Mg 2+, thus charge density of Ca2+ is lower, leading to lower polarising power of the cation Ca 2+ compared to Mg2+. As a result, the CO32- ion is polarised (or distorted) to a smaller extent and the C -O bond is weakened to a larger extent . Hence, decomposition of the CaCO 3 occurs with greater difficulty and at higher temperature compared to MgCO3. CaCO3 (s)  CaO (s) + CO2 (g)

4 9729/02/CJC JC2 Preliminary Examination 2017 2 (a) Table 2.1 gives data about some physical properties of the elements calcium and copper. physical property calcium copper relative atomic mass 40.1 63.5 atomic radius (metallic) / nm 0.197 0.128 ionic radius (2+) / nm 0.099 0.069 melting point / °C 839 1085 boiling point / °C 1484 2562 Table 2.1 (i) Explain why the atomic radius of copper is smaller than that of calcium. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] (ii) Explain why copper has a higher melting point than calcium. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] (iii) Apart from the properties given in Table 2.1, state clearly the difference in one other physical property between copper and calcium. ................................ ................................ ................................ .......................... ................................ ................................ ................................ ....................