HCI Prelim P2_QP

This document consists of 19 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 17S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9729/02 12 September 2018 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 / 19 2 / 11 3 / 11 4 / 11 5 / 15 6 /8 Deductions Total / 75 Calculator Model:

2 © Hwa Chong Institution 2018 9729 / 02 / C2 Prelim 2018 1 (a) Describe and explain the trend in thermal stability of Group 17 hydrides, HF to H I, by referring to relevant bond energies from the Data Booklet. ..………………………..………………………………………………………………………… ..………………………..………………………………………………………………………… ..………………………..………………………………………………………………………… ..………………………..………………………………………………………………………[2] (b) The very low acid strength of HF was frequently explained by the same reasoning in (a). However, the dissociation of HF in aqueous solution HF(aq) → H +(aq) + F–(aq) ΔHdissociation involves a cycle of steps. Hence, a whole series of enthalpy changes should be considered, one of which is the enthalpy change of hydration. (i) Explain, with the aid of an equation, what is meant by standard enthalpy change of hydration for F – ion. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………[2]

3 © Hwa Chong Institution 2018 9729 / 02 / C2 Prelim 2018 (ii) Fig. 1.1 shows an incomplete energy level diagram to represent the cycle of steps involved in the dissociation of HF. H +(g) + e– + F(g) energy /kJ mol –1 HF(g) HF(aq) ΔHdissociation Fig. 1.1 Complete Fig. 1.1 by incorporating the following data and relevant data from the Data Booklet. Draw arrows and label each level with the appropriate formulae. electron affinity for F(g) = –328 kJ mol –1 enthalpy change of hydration for H+(g) = –1091 kJ mol –1 enthalpy change of hydration for F–(g) = –515 kJ mol –1 [3] (iii) Use your completed energy level diagram to calculate ΔHdissociation for HF. [1] +48

4 © Hwa Chong Institution 2018