VJC 2020 H2 Chem Prelim P3 QP

1  VJC 2020 9729/03/PRELIM/20 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/03 Paper 3 Free Response Candidates answer on the Question Paper. Additional Materials: Data Booklet 21 September 2020 2 hours READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all the questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use Section A 1 / 20 2 / 21 3 / 19 Section B 4 OR 5 / 20 Total / 80 This document consists of 25 printed pages and 1 blank page.

2  VJC 2020 9729/03/PRELIM/20 Section A Answer all the questions in this section. 1 Chlorine is in Group 17 and undergoes many reactions. (a) Sketch and label, separately, the 3pz and 3dx2–y2 orbitals present in chlorine. [2] (b) An element A reacts with chlorine to form ACl2. The electronic configuration of A2+ is [Kr]4d10. Identify the element A. [1] …………………………………………………………………………………………………………….. (c) The following reaction occurs when chlorine dissolves in water. Cl2(g) + H2O(l)  HClO(aq) + HCl(aq) When solutions of chlorine are used for water purification, the pH of the se solutions are kept near to pH 7 by the addition of a base. Chlorine is dissolved in water to produce 1000 cm3 of a solution containing 0.170 mol of HClO and 0.170 mol of HCl. A buffer solution is then prepared by adding 0.200 mol of NaOH(s) to this solution. The NaOH reacts initially with the HCl. The remaining NaOH then reacts with HClO. (i) Write two equations to explain how this buffer is able to maintain a relatively constant pH when a few drops of H+(aq) or OH–(aq) are added to it. [1] (ii) Calculate the pH of the buffer solution. [Ka of HClO is 2.9  10–8 mol dm–3.] [1] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ……………………………………………………………………………………………………….

3  VJC 2020 9729/03/PRELIM/20 [Turn over ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. (d) A sample of an alkane, RH, and limited amount of chlorine are mixed together and irradiated with ultraviolet light. The mechanism involved two propagation steps. Propagation step 1: RH + Cl  R + HCl Propagation step 2: R + Cl2  RCl + Cl Some data for the enthalpy change of reaction and the activation energy of the two propagation steps of an alkane, RH, are given below. Step H / kJ mol1 Ea / kJ mol1 Propagation step 1 +4 17 Propagation step 2 109 4 Use the data provided to sketch the energy profile diagram of the two propagation steps of the chlorination of RH. Indicate all the given energy changes, reactants, intermediates and products on the diagram. [3]

4  VJC 2020 9729/03/PRELIM/20 (e) In the laboratory, there are three bottles labelled B, C and D. Each bottle contains one of the following chemicals, Cl2(aq), NaI(aq) and KBr(aq). Three tests were carried out using the chemicals in the bottles. The results are summarised in the table below. test procedure observation 1 mix chemical in bottle B with chemical in bottle C colourless solution obtained 2 mix chemical in bottle B with chemical in bottle D yellow solution obtained 3 mix chemical in bottle C with chemical in bottle D yellow solution obtained (i) Deduce which bottle contains Cl2(aq). Write equations to support your reasoning. [2] (ii) Suggest a reagent that could be added s eparately after performing test 2 and test 3 to confirm the identities of the chemicals in the bottles. State the observations clearly. [2] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ……………………………………………………………………………………………………….

5  VJC 2020 9729/03/PRELIM/20 [Turn over (f) Solid aluminium chloride sublimes at 180 C. In the vapour phase, an equilibrium is established between aluminium chloride and its dimer. Al2Cl6(g) ⇌ 2AlCl3(g) At 400 ºC and 1 atm, 0.84 g of the equilibrium gaseous mixture occupies 268 cm3. (i) Calculate the average relative molecular mass of the gaseous mixture. [2] (ii) Use your answer in (f)(i) to determine the degree of dissociation, , of Al2Cl6. [1] (iii) Write an expression for the equilibrium constant, Kp, for the dissociation of Al2Cl6. [1] (iv) Hence, calculate the equilibrium constant, Kp, for the dissociation of A l2Cl6 at 400 ºC, including its units. If you were unable to calculate the degree of dissociation of Al2Cl6 in (f)(ii), assume  = 0.456. This is not the correct value of . [2] (v) State and explain the effect of increasing the temperature on the average Mr of the equilibrium mixture. [2] ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. ……………………………………………………………………………………………………….

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