HCI Prelim C2_P2_QP

This document consists of 19 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 16S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9729/02 11 September 2017 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 / 12 2 / 14 3 / 16 4 / 12 5 / 13 6 /8 Deductions Total / 75Calculator Model:

2   Hwa Chong Institution 2017 9729 / 02 / C2 Prelim 2017 1 (a) Atmospheric hydrogen and iodine, each 0.10 mol, are placed in a 2 dm 3 evacuated flask at 400 °C. After 30 minutes, the following equilibrium was established and the amount of H I was found to be 0.12 mol. H2 (g) + I2 (g) 2H I (g) (i) Write the expression for Kc and calculate its value at 400 °C. [2] (ii) At 40 minutes, the temperature of the system was raised to 600 °C and equilibrium was re-established at 60 minutes. Given that the K c at 600 °C was 0.36, show that the equilibrium concentrations of H 2, I2 and HI in the equilibrium mixture at 60 min are 0.0385 mol dm¯ 3, 0.0385 mol dm¯ 3 and 0.0231 mol dm¯3 respectively. [2]

3   Hwa Chong Institution 2017 9729 / 02 / C2 Prelim 2017 (iii) On the grid below, sketch the concentration versus time graphs for I2 and HI respectively under the conditions as described in (a)(i) and (a)(ii) from 0 to 70 minutes. Label the graphs and indicate significant values on the axes. [4] (iv) To find the amount of H I present at equilibrium at 30 minutes, the flask can be rapidly cooled and the H I is dissolved in water. The solution obtained can be titrated against NaOH(aq). Explain why the flask has to be rapidly cooled. ………………………………………………………………………………………………………. ………………………………………………………………………………………………………. [1]

4   Hwa Chong Institution 2017 9729 / 02 / C2 Prelim 2017 (b) When I2 combines with I¯, it forms the I3¯ ion which is responsible for the characteristic brown colour of aqueous I2. Draw a dot-and-cross diagram to show the bonding in I3¯. [1] (c) Explain, in terms of structure and bonding, why I 2 has a higher boiling point than HI. ………………………………………………………………