JPJC Prelim P3 QP

¤ Jurong Pioneer Junior College [Turn Over NAME CLASS 20S JURONG PIONEER JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION 2021 CHEMISTRY 9729/03 Higher 2 Paper 3 Free Response Questions 20 September 2021 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 on both sides of the paper. You may use a 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. If additional space is required, you should use the pages at end of this booklet. The question number must be clearly shown. Section A Answer all questions. Section B Answer one question. 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. For Examiner’s Use 1 20 2 20 3 20 4 or 5 20 Penalty (delete accordingly) Lack 3sf in final answer –1 / NA Missing/wrong units in final ans –1 / NA Bond linkages –1 / NA Total 80 This document consists of 27 printed pages and 1 blank page.

2 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAMINATION/2021 Answer all the questions. 1 Period 3 elements react with oxygen and nitrogen in various ways. For ([DPLQHU¶V Use (a) Compounds A, B and C are the solid oxides of three Period 3 elements. x When excess cold water is added separately to A, B and C, only B and C are able to dissolve readily. x The resulting aqueous solution from B reacts with solid sodium carbonate in a mole ratio of 2:3. Effervescence is observed. x The resulting aqueous solution from C reacts with solid ammonium nitrate to give a pungent gas upon heating. x When A is added separately to the resulting aqueous solutions from B and C, it dissolves. State and explain the identities of A, B and C. You are to include the equations for the reactions of the oxides, B and C with water. [6] ...................................................................................................................................... …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. ………………………………………………………………………………………………….

3 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAMINATION/2021 [Turn Over (b) When heated, solid magnesium amide, Mg(NH2)2 decomposes to form magnesium nitride, Mg3N2, and ammonia gas as the only products. For ([DPLQHU¶V Use (i) Construct a balanced equation for the thermal decomposition of Mg(NH2)2. [1] (ii) The variation in the thermal stability of Group 2 amides is the same as that of Group 2 carbonates. Explain whether magnesium amide is more or less thermally stable than barium amide. [2] (iii) A sample of Mg was burnt in air to give 1.00 g of powder which was then added to hot water. The reaction is represented as follows. Mg3N2(s) + 6H2O(l) o 3Mg(OH)2(s) + 2NH3(aq) The ammonia evolved required 12.0 cm3 of 0.50 mol dm–3 hydrochloric acid for complete neutralisation. Calculate the percentage by mass of magnesium nitride in the 1.00 g of powder obtained from the burning of Mg in air. [2] ...................................................................................................................................... …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. ………………………………………………………………………………………………….

4 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAMINATION/2021 (c) The presence of halide ions in natural water sources such as rivers and lakes are usually detected using aqueous ammonia and aqueous silver nitrate. The following equation shows the precipitation of AgCl, Ag+(aq) + Cl–(aq) o AgCl(s) Thermodynamic quantity Value Ksp(AgCl) 1.8 u 10–10 mol2 dm–6 'S ppt (AgCl) –410 J mol–1 K–1 'H hyd (Ag+) –473 kJ mol–1 'H hyd (Cl–) –378 kJ mol–1 Table 1.1 For ([DPLQHU¶V Use (i) 5 cm3 of 0.0100 mol dm–3 of silver nitrate is added to a 30 cm3 sample of river water containing chloride ions. What is the minimum concentration, in mol dm–3, of chloride ions present in the river water when the first trace of precipitate appears? [2] (ii) To another sample of river water, aqueous silver nitrate is added, followed by excess aqueous ammonia. A cream precipitate is formed which is partially soluble in excess. Suggest the identity of the precipitate and thus identify the halide present in this sample of river water. [1] (iii) When a precipitate is formed, 'G ppt is given by the following expression. 'G ppt = RT ln Ksp Using appropriate data in Table 1.1, calculate 'G ppt, in kJ mol–1, and hence 'H ppt for the precipitation of AgCl at room temperature. Ag+(aq) + Cl–(aq) → AgCl(s) [2] (iv) Using your answer in (c)(iii) and data in Table 1.1, calculate the lattice energy for AgCl. [2] (v) The theoretical lattice energy for AgCl is found to be –770 kJ mol–1. State and explain the reason for the difference between the theoretical value and your answer in (c)(iv). [2] ...................................................................................................................................... …………………………………………………………………………………………………. …………………………………………………………………………………………………. ………………………………………………………………………………………………….

5 ¤ Jurong Pioneer Junior College 9729/03/J2 PRELIMINARY EXAMINATION/2021 [Turn Over …………………………………………………………………………………………………. ...................................................................................................................................... …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. …………………………………………………………………………………………………. ………………………………………………………………………………………………… ………………………………………………………………………………………………… …………………………………………………………………………………………