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 9729 / 02 / C2 Prelim 2018 (c) (i) Explain why two HF molecules can form a hydrogen bond between them. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………[2] (ii) When we measure the distance between a hydrogen nucleus and a fluorine nucleus in solid HF, two H to F ‘bond distances’ are found: 0.092 nm and 0.163 nm. Draw a diagram containing two HF molecules, showing why there are two different ‘bond distances’. Name and label which bond has ‘bond distance’ 0.092 nm, which has ‘bond distance’ 0.163 nm. [2] (d) Nitrogen also forms hydrides, such as hydrogen azide, HN 3, and ammonia, NH3. HN3 has two bond angles: 104.5° and 180°. Suggest a dot-and-cross diagram for HN3. [1]

5 © Hwa Chong Institution 2018 9729 / 02 / C2 Prelim 2018 (e) NH3 is produced from N2 and H2 in the presence of a catalyst. N2(g) + 3H2(g) = 2NH3(g) (i) State and explain how the entropy will change when N2 and H2 react. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………… …………………………...… [2] (ii) Write an expression for K p for the above equilibrium. [1] (iii) In an experiment, N 2 and H2 are placed in a sealed reactor in the molar ratio 1:3. The reactor is maintained at a temperature of 750 K. At equilibrium, 20% of N 2 is converted to NH3, and the total pressure is 20.0 Mpa. Calculate a value for Kp for the equilibrium, stating its units. (1 MPa = 106 Pa) [3] [Total: 19]

6 © Hwa Chong Institution 2018 9729 / 02 / C2 Prelim 2018 2 In addition to carbon and hydrogen, organic co mpounds commonly contain other elements covalently bonded in their molecules. A strategy to identify these other elements in the compounds is to convert them to ionic forms so that these can be detected subsequently by qualitative tests. (a) State the type of reaction that a chloroalkane can be subjected to so as to convert the chlorine present to chloride ions. Give the reagents and conditions for the reaction. type of reaction: ………………………………………………... reagents and conditions: …...…….…………………………………………………………[1] “Sodium fusion” is an approach which uses the above strategy for the detection of other elements, in particular, halogens, nitrogen and/or sulfur, in organic compounds. The procedure is outlined below. 1. A sample of sodium metal is gently heated in a small boiling tube until molten. 2. The organic compound is added to the boiling tube a little at a time. Effervescence of hydrogen is observed immediately if the organic compound contains “active hydrogen”. 3. The boiling tube is heated gently at first, then strongly while the “fusion reaction” between sodium and the organic compound occurs. During this process, other elements in the organic compound are converted to ions such as those in Table 2.1 and ionic sodium compounds are formed. 4. The contents of the hot boiling tube are completely emptied into a beaker of water. The “fusion reaction” is quenched because any excess sodium reacts with the water. 5. The contents of the beaker are filtered. 6. The filtrate is tested for relevant ions qualitatively. Table 2.1 element(s) other than carbon and hydrogen in organic compound ion after “fusion reaction” chlorine chloride, Cl− bromine bromide, Br − both nitrogen and sulfur thiocyanate, SCN − nitrogen in the absence of sulfur cyanide, CN − sulfur in the absence of nitrogen sulfide, S 2− (b) (i) Suggest two different functional groups which may be present in organic compounds containing “active hydrogen”, leading to effervescence in step 2. ……………………………………………………………………………………………[1]

7 © Hwa Chong Institution 2018 9729 / 02 / C2 Prelim 2018 (ii) One reason that sodium is used is that the sodium ion in the filtrate does not interfere with the tests in step 6. Using relevant data from Table 2.2 about sodium, suggest another two reasons why sodium is used for the “fusion reaction”. Table 2.2 density 0.97 g cm −3 melting point 98 C standard redox potential, E(Na+/Na) −2.71 V ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………[2] (c) In step 6, to test for the sulfide ion, the reagent sodium nitroprusside is added to the filtrate. A positive test is indicated by the appearance of a violet colouration owing to the formation of compound A, which contains an octahedral complex ion cont