2019 HCI Prelim H2 Chem P2 QP

This document consists of 16 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 18S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9729/02 18 September 2019 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 / 17 2 / 13 3 / 22 4 / 23 s.f. units Total / 75 Calculator Model:

2 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 1 (a) Butanone can be converted to compound C via the following series of reactions. (i) Suggest the reagents and conditions for step 1. ……………………………………………………………………………………………... [1] (ii) In the boxes below, draw the structures of compounds A, B and C. A B C [3]

3 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 (b) 2-iodobutane is converted to its corresponding alcohol by heating with aqueous sodium hydroxide. ∆H < 0 The rate equation is: rate = k[2-iodobutane][NaOH] (i) Describe the mechanism of this reaction. In your answer you should show all charges and lone pairs and show the movement of electrons by curly arrows. [3] (ii) Draw a fully labelled reaction pathway diagram for the reaction between 2-iodobutane and sodium hydroxide. [2] (c) Describe and explain how the volatilities of the halogens vary from chlorine to iodine. ....………………………..………………………………………………………………………...... ....………………………..………………………………………………………………………...... ..………………………..…………………………………………………………..……………...... ..………………………..…………………………………………………………..……………...... ..………………………..……………………………………………………….……………….. [2]

4 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 (d) Some hydrogen halides are unstable to heat. (i) Write an equation for the reaction undergone on heating a hydrogen halide. ……………………………………………………………………………………………... [1] (ii) Describe and explain how the thermal stabilities of the hydrogen halides (HC l, HBr and HI) vary down the group. ....………………………..……………………………………………………………………. ....………………………..……………………………………………………………………. ..………………………..……………………………………..……………………..……….. ..………………………..…………………………………………………………..……….... ..………………………..……………………………………………………….………… [2] (e) When sodium thiosulfate, Na2S2O3, is reacted with bromine, NaHSO4 is formed. However, when Na2S2O3 reacts with iodine, Na2S4O6 is formed instead. Use the following data, and data from the Data Booklet, to suggest an explanation for this difference. S4O62− + 2e− ⇌ 2S2O32− Eo = +0.09 V 4SO2 + 4H+ + 6e− ⇌ S4O62− + 2H2O Eo = +0.51 V SO42− + 4H+ + 2e− ⇌ SO2 + 2H2O Eo = +0.17 V ....………………………..………………………………………………………...………………… ..………………………..……………………………………..………………………..…..……….. ....………………………..………………………………………………………...………………… ..………………………..……………………………………..………………………..…..……….. ..………………………..……………………………………………………………..…..……….... ....………………………..………………………………………………………...………………… ..………………………..……………………………………..………………………..…..……….. ..………………………..……………………………………………………………..…..……….... ..………………………..……………………………………..………………………..…..……….. ..………………………..……………………………………………………………..…..……….... ..………………………..…………………………………………………………..….………… [3] [Total: 17]

5 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 2 (a) 4-Morpholineethanesulfonic acid (MES) is commonly used to prepare buffer solutions. In aqueous solution, MES exists as a dipolar ion which dissociates to give H+ ions: FA 1 is a buffer solution prepared from MES with a pH of 5.5. The ratio of MES and its conjugate base is 4:1. (i) Determine the pKa of MES. [1] (ii) State the mole ratio of MES to its conjugate base if the pH = pKa of MES. [1] A student titrated 10.0 cm 3 of FA 1 with aqueous sodium hydroxide, and found that 20.0 cm3 of aqueous NaOH was required for complete neutralisation. (iii) What volume of aqueous NaOH should be added to 10.0 cm 3 of FA 1 such that the pH of the resulting solution becomes equal to pKa of MES? [2] (iv) Write an equation to show why the pH at equivalence point is more than 7. [1]

6 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 (v) The pH of the solution at equivalence point is 9.4. Suggest a suitable indicator for the titration of FA 1 with aqueous NaOH, giving a reason for your choice. Indicators Working range Congo red 3.0 – 5.0 Cresolphthalein 8.2 – 9.8 Indigo carmine 11.4 – 13.0 ..………………………..…………………………………………………………………….… ..………………………..………………………………………………………………….…… ..………………………..…………………………………………………………………….… ..………………………..…………………………………………………………………….[2] (b) Zymase, an enzyme that occurs naturally in yeasts, catalyses the fermentation of sugar into ethanol and carbon dioxide. sugar  ethanol + carbon dioxide Fig. 2.1 shows the results of an investigation of the initial rate of fermentation of sugar by zymase. In the experiments, the initial concentration of sugar was varied but that of zymase was kept constant. (i) Explain fully the shape of the graph. ..………………………..…………………………………………………………………….… ..………………………..………………………………………………………………….…… ..………………………..…………………………………………………………………….… ..………………………..…………………………………………………………………….[2] (ii) Sketch on Fig. 2.1 the shape of the line you would expect when the amount of zymase added was increased. [1]

7 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 (iii) Three other experiments were carried out to measure the initial rates of fermentation at different concentrations of sugar and zymase. The results are shown in Table 2.2. Table 2.2 Experiment [sugar] /mol dm–3 [zymase] /mol dm–3 Relative initial rate 1 0.20 0.10 1 2 0.40 0.10 2 3 0.10 0.20 1 Deduce the orders of reaction with respect to sugar and zymase, showing how you arrive at your answers. [3] [Total:13]

8 © Hwa Chong Institution 2019 9729 / 02 / C2 Prelim 2019 3 This question is about alkynes, which are hydrocarbons that contain a carbon -carbon triple bond. (a) In the past, ethyne gas, C2H2, was prepared from calcium carbide, CaC2. Calcium carbide is a crystalline solid with a melting point of approximately 2160 °C. (i) Draw the dot-and-cross diagram of CaC2. [1] (ii) When excess water with a drop of universal indicator i s added to some calcium carbide, ethyne is produced and the solution changes from green before the reaction, to blue after the reaction is complete. Write an equation, with state symbols, for this reaction. ..………………………..…………………………………………………………………….[1] (b) (i) Draw the structure of ethyne, showing correct geometry, and clearly indicate the bond angle in your drawing. State the type of hybridisation of the carbon atoms in ethy