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

3 9729/02/CJC JC2 Preliminary Examination 2019 [Turn over (iv) By considering the reaction between surface barium carbonate and acid rain, explain how the erosion process can be slowed down. [1] (b) Group 2 carbonates undergo decomposition in the same way at different temperatures. (i) Predict and explain the order of decomposition temperatures for the thr ee carbonates, MgCO3, CaCO3 and BaCO3. [3] The graph given below shows the change in mass when 2.00 g of each CaCO 3 and BaCO3 were heated separately at a temperature T oC. (ii) From the shapes of the graphs, identify CaCO3 and BaCO 3 in t he spaces provided below. Graph A: _______________ Graph B: ________________ [1] BaCO3 + H2SO4  BaSO4 + CO2 + H2O or BaCO3 + H2SO4  BaSO4 + H2CO3 BaCO3 can react with SO 2 / H 2SO4 in the air to form a layer of barium sulfate, BaSO 4. BaSO4 is the least soluble and hence, slow down the erosion process. Decomposition temperature: MgCO3 < CaCO3 < BaCO3 Going down Group 2, size of cation increases from Mg2+ to Ba2+. Hence, charge density and polarising power of cation decreases from Mg2+ to Ba2+. Therefore, the ability of cation to distort the CO32− electron cloud, weakening and break the C−O bond decreases from Mg2+ to Ba2+ Hence less energy is required to decompose MgCO3 and the decomposition temperature is the lowest. 2.00 1.00 0 time mass/g Y(IO3)2 0 Graph A Graph B 1.12 g BaCO3 CaCO3

4 9729/02/CJC JC2 Preliminary Examination 2019 (iii) Sketch on the same axes, a graph that would be obtained by heating 2.00 g of magnesium carbonate, MgCO3, at the same temperature, T oC. [1] [Total: 11] Shorter time to decompose + smaller residual mass CaCO3  CaO + CO2 Amount of CaCO3 = 2.00 / 100.1 = 0.0200 mol CaCO3 ≡ CaO Mass of CaO = 0.0200 x 56.1 = 1.12 g Amount MgCO3 = 2.00 / 84.3 = 0.0237 mol MgCO3 ≡ MgO Mass of MgO = 0.0237 x 40.3 = 0.955 g MgCO3 is less thermally stable than CaCO3. Hence, MgCO3 decompose completely faster. CaCO3  CaO + CO2 Amount of CaCO3 = 2.00 / 100.1 = 2.00 x 10−2 mol CaCO3 ≡ CaO Mass of CaO = 2.00 x 10−2 x 56.1 = 1.12 g 2.00 1.00 0 time mass/g Y(IO3)2 0 Graph A Graph B 1.12 g

5 9729/02/CJC JC2 Preliminary Examination 2019 [Turn over 2 Hydrogen peroxide is a common bleaching agent used to whiten wood pulp during the manufacture of paper. Transition metal ions such as Fe 3+(aq), that are naturally present in wood pulp, would catalyse the decomposition of hydrogen peroxide, and hence reduce the bleaching efficiency. The wood pulp is therefore washed with a chelating agent such as EDTA to remove the transition metal ions before hydrogen peroxide is added. [EDTA = (–O2CCH2)2NCH2CH2N(CH2CO2–)2] (a) (i) Any Fe3+(aq) in the wood pulp would form a polydentate complex with EDTA. Give the formula of the complex formed. You may use the abbreviation ‘EDTA’ in your answer. ................................ ................................ ................................ ...................... [1] (ii) State the coordination number of the complex in (a)(i). ................................ ................................ ................................ ...................... [1] (b) (i) H2O and NH3 are simple molecules. Explain why, at room temperature, H 2O is a liquid while NH3 is a gas. ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ .......................... ................................ ................................ ................................ ...................... [2] Alternative answer: H2O i s a liquid due to the presence of stronger intermolecular hydrogen bonding in H2O than NH3. O is more electronegative than N hence H−O bond is more polar than H−N bond, thus a greater dipole moment of the H−O bond that makes the intermolecular hydrogen bonding stronger . More energy is thus required to overcome the hydrogen bonds in H2O. [Fe(EDTA)] – 6 H2O is a liquid due to the presence of more extensive intermolecular hydrogen bonding in H2O than NH3. On average, H2O can form two hydrogen bonds per molecule while NH 3 can only form one hydrogen bond per molecule . More energy is thus required to overcome the hydrogen bonds in H2O.

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