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