2019 CJC Prelim H2 Chem P2 ANS

9729/02/CJC JC2 Preliminary Examination 2019 CANDIDATE NAME CLASS 2T CHEMISTRY 9729/02 Paper 2 Structured Questions Friday 23 August 2019 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 examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Catholic Junior College JC2 Preliminary Examination Higher 2 WORKED SOLUTIONS

2 9729/02/CJC JC2 Preliminary Examination 2019 Answer all the questions in the spaces provided. 1 The Taj Mahal is an ivory -white marble monument in the Indian city of Agra. The famous building is becoming brown and green from environmental pollution. Acid rain accelerates the erosion of marble monuments. Marble consists primarily of calcium carbonate. (a) You should refer to the following solubility product data to help answer the following questions. Substance CaCO3 CaSO4 BaCO3 BaSO4 Solubility product / mol2 dm-6 1.69 x 10−8 2.03 x 10−3 8.28 x 10−9 8.84 x 10−11 The pollutant SO2 dissolves in rainwater to form sulfuric acid. The sulfuric acid slowly converts calcium carbonate into solid calcium sulfate. (i) Write an equation for the reaction between calcium carbonate and sulfuric acid. [1] (ii) By calculating the solubilities of calcium carbonate and calcium sulfate, deduce why acid rain accelerates the erosion of marble. [3] The lifespan of Taj Mahal can be extended by treatment with an aqueous mixture of barium hydroxide, Ba(OH) 2 and urea, CO(NH 2)2. As this solution soaks into the porous marble, the urea slowly hydrolyses forming ammonia and carbon dioxide. The carbon dioxide released reacts with the barium hydroxide in the mixture forming barium carbonate. (iii) Write an equation for the reaction between barium hydroxide and carbon dioxide. [1] CaCO3 + H2SO4  CaSO4 + H2CO3 Or CaCO3 + H2SO4  CaSO4 + H2O + CO2 Calculate the solubility of CaCO3, x: CaCO3(s) ⇌ Ca2+(aq) + CO32−(aq) Ksp (CaCO3) = [Ca2+][CO32−] 1.69 x 10−8 = x2 x = 1.30 x 10−4 mol dm−3 Calculate the solubility of CaSO4, y: CaSO4(s) ⇌ Ca2+(aq) + SO42−(aq) Ksp (CaSO4) = [Ca2+][SO42−] 2.03 x 10−3 = y2 y = 4.51 x 10−2 mol dm−3 Acid rain contains H2SO4, which converts CaCO3 into CaSO4, which is more soluble in water. Ba(OH)2 + CO2  BaCO3 + H2O

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