ACJC H2 CHEM P3 Ans 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, P4S3, 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. P4S3 + 8O2  3SO2 + P4O10 [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. The two above -mentioned oxides are simple covalent molecules while SiO2 has a giant covalent structure. Overcoming we ak IMF between P4O10 and SO2 molecules versus strong covalent bonds between Si and O atoms. SiO2 has the highest melting point. [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. P4O10 + 6H2O  4H3PO4 pH 2 or 3 [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. M(NO3)2 (s)  MO (s) + 2NO2 (g) + ½ O2 (g) [1] (ii) Assuming that the gas behaves ideally, c alculate the amount of remaining gas formed. 3.81 × 10−3 [1] (iii) Hence, calculate the value of x and deduce the identity of metal M. No. of moles of nitrate present = (ii) × 2 = 7.621 × 10−3 No. of moles of water = 0.55 / 18.0 = 3.056 × 10−2 [3]

3 © ACJC 2017 9729/03/Prelim/2017 [Turn over x = 3.056 ×10−2 7.621 ×10−3 = 4 Ar of M = 40.0 Calcium (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. Mass percentage / Ar Simplest ratio Ca 2.0275 3 N 1.35 2 Ca3N2 [2] (ii) Write the chemical equation for the reaction between the red brown solid with water. Ca3N2 + 6H2O  3Ca(OH)2 + 2NH3 [1] (iii) Write the chemical equation for the reaction of calcium with O 2 and hence calculate the mass of CaO formed when 1.5 g of calcium is bu rnt in O2. 2Ca (s) + O2 (g)  2CaO (s) n(calcium) = 0.0374 n(calcium oxide) = 0.0374 m(calcium oxide) = 0.0374 x (40 + 16) = 2.10 g [2]

4 © 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. Li2CO3 (s)  CO2 (g) + Li2O (s) [1] (ii) Explain why this is unlike that of the other Group 1 carbonates. The charge densities (or polarising power) of Li+ and Mg2+ are similar. or The electronegativities of lithium and magnesium are similar. [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. Electrophile / Proton donor / Acid / Bronsted acid / Lewis acid [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. SiO2 (s) + 2NaOH (l) / (aq)  Na2SiO3 (l) / (aq) + H2O (l) [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). B2O3 (s) + 6NaOH (l) / (aq)  2Na3BO3 (l) / (aq) + 3H2O (l) [1] [Total: 20]

5 © 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. 4(1.217 × 10−5)3 = 7.21 × 10−15 mol3 dm-9 [1] (ii) Calculate the molar solubility of Sn(OH)2 in a solution of pH 8. [OH-] = 10-6 mol dm-3 No. of moles of Sn(OH)2 that can dissolve in 1 dm3 water = 5.45 × 10−27 10−12 = 5.45 × 10−15 mol [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. 5.45 × 10−27 0.00