2020 HCI Prelim Paper 2 Question Paper

This document consists of 18 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 19S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9729/02 31 August 2020 2 hours READ THESE INSTRUCTIONS FIRST Write your name, CT group, centre number and index number clearly in the spaces above. Write in dark blue or black pen. 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 in the Question Paper. 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 / 11 2 / 14 3 / 15 4 / 13 5 / 22 Deductions Total / 75

2 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over Answer all the questions in the spaces provided. 1 This question concerns the reactions of Group 17 hydrides. For Examiner's use (a) HCl, HBr and HI exhibit different behaviour on heating. (i) Describe the differences in decomposition behaviour for these three hydrides. HCl: …...…………………………………………….………………………………………… …………………………………………………………………………………………………. HBr: …...…………………………………………….………………………………………… …………………………………………………………………………………………………. HI: …...…………………………………………….………………………………………….. ……………………………………………………………………………………………… [2] (ii) Explain the variation in the thermal stabilities of these hydrides. ……………………………………………………….………………………………………… ……………………………………………………………………………………………….[1] (b) Using HCl and a different base in each case , write equations, with state symbols, to illustrate: (i) the Arrhenius theory of acids and bases, ……………………………………………………………………………………………….[1] (ii) the Lewis theory of acids and bases. ……………………………………………………………………………………………….[1] (c) HF and HCl have different acid strengths when dissolved in water. (i) Given that the pH of 0.100 mol dm ‐3 HF is 2.10, explain, with the aid of relevant calculations, why HF is a weak acid. ……………………………………………………….………………………………………… ……………………………………………………….………………………………………… ……………………………………………………………………………………………….[2] (ii) Hence, calculate a value for the Ka of HF. [1]

3 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over 25.0 cm3 of 0.0500 mol dm‐3 NaOH was placed in a conical flask. 0.100 mol dm‐3 HF was then added from a burette. The changes in pH for the titration was monitored throughout the experiment until 50.0 cm3 of HF(aq) was added from the burette. The experiment was then repeated by replacing HF(aq) with HCl(aq) of the same concentration. For Examiner's use (iii) Describe one way in which the pH titration curves are similar for these two experiments, apart from their initial pH. ……………………………………………………….………………………………………… ……………………………………………………………………………………………….[1] (iv) Describe two ways in which the pH titration curves are different. Briefly explain your answers. 1 …………………………………………………….………………………………………… ……………………………………………………….………………………………………… ……………………………………………………….………………………………………… ……………………………………………………….………………………………………… 2 …………………………………………………….………………………………………… ……………………………………………………….………………………………………… ……………………………………………………….………………………………………… ……………………………………………………………………………………………….[2] [Total: 11]

4 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over 2 (a) The graph in Fig. 2.1 shows the successive ionisation energies for the removal of the first 15 electrons of Fex+. For Examiner's use Fig. 2.1 Deduce the value of x. ………………………………………………………………………………………………………. ………...…………………………………………….………………………………………………. ………………………………………………………………………………………….…………[2] (b) (i) The standard reduction potential for the following electrode system is ‐1.02 V. 2CO2(g) + 2HCO2–(aq) + 6H+(aq) + 6e–  C4H4O62–(aq) + 2H2O(l) methanoate ion tartrate ion Draw a fully labelled diagram of the set -up you would use to measure the above standard reduction potential. Energy/ kJ mol-1 Number of electrons removed

5 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over [3] (ii) Peroxodisulfate(VI) ions, S 2O82–, are capable of oxidising C 4H4O62– ions to CO 2 and HCO2–. Explain why this reaction is expected to be slow. ……………………………………………………….………………………………………… ……………………………………………………………………………………………….[1] For Examiner's use (iii) By considering relevant Eo values from the Data Booklet, show by means of balanced equations, how Fe2+(aq) ions can act as a homogeneous catalyst in the reaction. ………...…………………………………………….………………………………….……… ……………………………………………………………………………………………….… ………...…………………………………………….…………………………………………. ………………………………………………..………………………………………………... ………...…………………………………………….……………………….………………… …………………………………………………………………………….…………………… ………...…………………………………………….……………………….………………… ………………………………………………..……………………….….………….……...[3] (c) Wine contains a mixture of organic acids, such as tartaric acid and malic acid. These acids produce a prickling sensation on the tongue during wine-tasting. malic acid tartaric acid Which of the two acids has a lower pKa1? Explain. ……………………………………………………….……………………………………………… ……………………………………………………….……………………………………………… …………………………………………………………………………………………………….[2]

6 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over (d) Tartaric acid can undergo selective hydrodeoxygenation to form compound K. When K is heated strongly, compound L is formed, according to the scheme in Fig. 2.2. Fig. 2.2 Fig. 2.3 shows the mechanism of conversion of K to L. K L Fig. 2.3 For Examiner's use (i) With reference to Fig. 2.3, suggest why the conversion of K to L can be described as addition-elimination. ………...…………………………………………….……………………….………………… …………………………………………………………………………….…………………… ………...…………………………………………….……………………….………………… ………………………………………………..……………………….….………….……...[2] (ii) L slowly reacts with water, giving K again. Draw the structural formula of product M (C6H10O4) formed if L were to react with ethanol instead of water. [1] + _

7 © Hwa Chong Institution 2020 9729/02/C2Prelim 2020 [Turn over [Total: 14] 3 (a) 0.200 mol PCl5 and 3.00 mol PCl3 were left in a 10.0 dm3 sealed container and allowed to reach equilibrium at 500 K. At equilibrium, there was 0.0500 mol Cl2 gas. PCl5(g)  PCl3(g) + Cl2(g) For Examiner's use (i) Determine the total number of moles of gas at equilibrium. [1] (ii) Calculate the total pressure at equilibrium, giving your answer in Pa. [1] (iii) Hence, calculate the value of the equilibrium constant, Kp, for the reaction. [2] (iv) Explain how the position of equilibrium would be affected if the volume of the container above is decreased to 5.0 dm3. ..………………………..……………….……………………………………………………… ..………………………..……………….………………………………………………