ACJC H2 CHEM P3 QP Prelim

1 © ACJC 2017 9729/03/Prelim/2017 [Turn over ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/03 Higher 2 Paper 3 18 August 2017 2 hours Additional Materials: Writing Paper Data Booklet Cover Page READ THESE INSTRUCTIONS FIRST Write your index number and name, form class and tutorial class on all the work you hand in. Write in dark blue or black pen. You may use a pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions in Section A. Answer either question 4 or 5 in Section B. Start each question on a new sheet of writing paper. A Data Booklet is provided. The use of an approved calculator is expected, where appropriate. You are reminded of the need for good English and clear presentation in your answers. At the end of the examination, fasten all your work securely behind a cover sheet. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 17 printed pages, including this cover page. 9729/03/Prelim/17 ANGLO-CHINESE JUNIOR COLLEGE

2 © ACJC 2017 9729/03/Prelim/2017 [Turn over Section A – Answer ALL questions. Begin each question on a fresh piece of writing paper. 1 This question is about Period 3 and Group 2 elements. (a) Phosphorus sulfide, P 4S3, is used in smal l amounts in the tip of a matchstick. On striking a match stick, this compound burns to form sulfur dioxide and phosphorus pentoxide. (i) Write the equation for this reaction. [1] (ii) The melting points of the two oxides formed in (a)(i) differ significantly from that of silicon(IV) oxide (SiO 2). Account for this difference in terms of structure and bonding of each oxide. [2] (iii) The solid oxide formed in (a)(i) dissolves in water to give an acidic solution. Write the equation for this reactio n and give an approximate pH of the solution formed. [2] (b) A Group 2 nitrate exists as a hydrate M(NO3)2.xH2O. On heating, 1.80 g of this hydrate lost 0.55 g in the form of steam, as it was converted into the anhydrous nitrate, M(NO3)2. Subsequent heating to constant mass produced a white residue and mixture of two gases. The gases produced were passed through aqueous sodium hydroxide. The remaining gas occupied 95 cm 3 at 101 kPa and 30 C. (i) Write a n equation, with state symbols, representing the thermal decomposition of Group 2 nitrates, M(NO3)2. [1] (ii) Assuming that the gas behaves ideally, calculate the amount of remaining gas formed. [1] (iii) Hence, calculate the value of x and deduce the identity of metal M. [3] (c) Calcium oxide is the key ingredient for the process of making cement. When 1.50 g calcium is burned i n air, calcium oxide is formed together with a red brown solid. The red brown solid has the following composition by mass: Ca, 81.1%; N, 18.9%. Adding water to the red brown solid produces calcium hydroxide and 19.2 cm3 of ammonia gas at room temperature and pressure. (i) Deduce the formula of the red brown solid. [2] (ii) Write the chemical equation for the reaction between the red brown solid with water. [1] (iii) Write the chemical equation for the reaction of calcium with O2 and hence calculate the mass of CaO formed when 1.5 g of calcium is burnt in O2. [2]

3 © ACJC 2017 9729/03/Prelim/2017 [Turn over 1 (d) A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third Periods of the Periodic Table. For instance, lithium shows similar chemical properties to magnesium. (i) Write the equation for the thermal decomposition of lithium carbonate. [1] (ii) Explain why this is unlike that of the other Group 1 carbonates. [1] (e) Lithium nitride, Li 3N, is a red solid. It is the only stable Group 1 nitride. The nitrides of all the Group 2 elements are known. Li3N is currently investigated as a storage medium for hydrogen gas. State the role of hydrogen. [1] (f) A diagonal relationship also exists between boron and silicon. B2O3 is acidic, like SiO2 but unlike the oxides of the other Group 13 elements. (i) Write a bal anced chemical equation, with state symbols, that illustrates the acidic nature of SiO2. [1] (ii) Write a balanced chemical equation, with state symbols, that illustrates the acidic nature of B2O3, given that it forms a similar anion as in (i). [1] [Total: 20]

4 © ACJC 2017 9729/03/Prelim/2017 [Turn over 2 (a) Divalent metal cations like tin and cadmium form insoluble precipitates with NaOH (aq). The Ksp of Sn(OH)2 is 5.45 x 10-27 mol3 dm-9. (i) If the concentration of Cd(OH)2 in a saturated solution is 1.217 × 10-5 mol dm-3, determine the solubility product of Cd(OH)2. [1] (ii) Calculate the molar solubility of Sn(OH)2 in a solution of pH 8. [2] (iii) A certain solution has 0.002 mol dm-3 each of Cd2+ and Sn2+. Calculate the pH range over which the two cations can be effectively separated. [2] (b) Propanone, also known as “acetone”, is one of the most important solvents in organic chemistry – it can be used to dissolve many things from fats and waxes to airplane glue and nail polish. It decomposes to H and ketene (H2C=C=O). At 600 °C, the decomposition rate constant is 8.7 x 10-3 s-1. (i) Suggest the identity of H. [1] (ii) Determine the half-life of the reaction at 600 °C. [1] (iii) How much time is required for 75% of a sample of propanone to decompose at 600 °C? [1] (iv) The Arrhenius equation links different parameters ( its activation energy, the absolute temperature, and its rate constant at that temperature) of a reaction together. k = Ae−Ea RT Given that the half-life of the reaction at 500 °C is 8700 s and using your answer to (ii), calculate the activation energy (including its units) of this reaction. [2]

5 © ACJC 2017 9729/03/Prelim/2017 [Turn over 2 (c) Under appropriate conditions, butane can be made to isomerise reversibly to 2-methylpropane at 298K. butane 2-methylpropane So / J K-1 mol-1 310 295 ΔHfo / kJ mol-1 -127.2 -135.6 x (i) Calculate the ΔGo for the forward isomerisation reaction. [2] (ii) Given that ΔGo = -RT ln Kp, calculate the equilibrium constant, Kp, for the isomerisation reaction. [1] (iii) Write the Kp expression for the isomerisation equilibrium. [1] (iv) Determine the mole fracti