RI Prelim P2 (Qns)

© Raffles Institution 2020 9729/02/S/20 [Turn Over RAFFLES INSTITUTION 2020 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured Questions 15 September 2020 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. Do not write anything in it. You are reminded of the need for good English and clear presentation in your answers. 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 / 9 2 / 16 3 / 16 4 / 15 5 / 19 Total / 75 This document consists of 23 printed pages.

2 © Raffles Institution 2020 9729/02/S/20 For examiner’s use Answer all the questions in the spaces provided. 1 (a) The Cannizzaro reaction involves the conversion of aldehyde into a primary alcohol and a carboxylate ion under alkaline conditions. Benzaldehyde is heated with potassium hydroxide to give phenylmethanol and potassium benzoate, as shown in equation 1. (i) State the oxidation numbers of C(1), C(2) and C(3). oxidation number of C(1) = ……………. oxidation number of C(2) = ……………. oxidation number of C(3) = ……………. [2] (ii) Hence, state the type of reaction for equation 1. ………………………………………………………………………………………………. [1] (b) A student suggested performing the Cannizzaro reac tion in a non -alkaline medium as shown in equation 2. equation 2 2C6H5CHO(l) + H2O(l)  C6H5CH2OH(l) + C6H5COOH(s) Table 1.1 shows some data on the standard enthalpy changes of formation, ∆Hf, and standard enthalpy change of vapourisation, ∆Hvap. Table 1.1 ∆Hf of benzaldehyde, C6H5CHO(l) –87.0 kJ mol–1 ∆Hf of phenylmethanol, C6H5CH2OH(l) –160.7 kJ mol–1 ∆Hf of benzoic acid, C6H5COOH(s) –385.2 kJ mol–1 ∆Hvap of water, H2O(l) +44.0 kJ mol–1

© Raffles Institution 2020 9729/02/S/20 [Turn Over 3 For examiner’s use (i) Using relevant data from the Data Booklet, calculate ∆Hf(H2O(g)). [1] (ii) Using the standard enthalpy changes given in Ta ble 1.1 and your answer in (b)(i), draw an appropriate energy cycle and calculate the standard enthalpy change, ∆Hr, for the reaction shown in equation 2. [3]

4 © Raffles Institution 2020 9729/02/S/20 For examiner’s use (iii) Table 1.2 shows the standard entropy, S, of the species involved in equation 2. Table 1.2 compound standard entropy, S / J K–1 mol–1 benzaldehyde, C6H5CHO(l) 221.2 phenylmethanol, C6H5CH2OH(l) 216.7 benzoic acid, C6H5COOH(s) 167.6 water, H2O(l) 70.0 Using the data in Table 1.2 and your answer to (b)(ii), calculate ∆G at 298 K for the reaction shown in equation 2. Hence, comment on the feasibility of the reaction at 298 K. [2] [Total: 9]

© Raffles Institution 2020 9729/02/S/20 [Turn Over 5 For examiner’s use 2 (a) The concentration of h alide ions in a sample can be determined by back titration via the Volhard method. An excess amount of AgNO3 is first added to a sample containing halide ions to form silver halide precipitate s. The silver halide precipitate s are then filtered off. The amount of remaining Ag+ ions in the filtrate is determined by titration with aqueous KSCN. Ag+(aq) + SCN(aq)  AgSCN(s) Fe(NO3)3 solution is added as an indicator. The end -point is reached when a blood red complex of [FeSCN]2+ is observed. Sample X contains equimolar amounts of C l– and Br– ions. 15.0 cm3 of 0.700 mol dm 3 aqueous AgNO3 was first added to a 25 .0 cm3 of sample X and the resulting mixture was filtered. 10.0 cm3 of the filtrate was then titrated against 0.100 mol dm 3 aqueous KSCN, using Fe(NO3)3 as an indicator. A blood red colouration was observed when 14 .25 cm3 of aqueous KSCN was added. (i) Explain the effect on the titre value if the silver halide precipitates were not filtered away before titration. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ……………………………………………………………………………………..….............. ...……………………………………………………………………………………………..[2] (ii) Calculate the amount of remaining Ag+ ions in 10.0 cm3 of the filtrate. [1]

6 © Raffles Institution 2020 9729/02/S/20 For examiner’s use (iii) Show, by calculation, that the concentration of Cl– ions in sample X is 0.0960 mol dm3. [3] (b) The values of the solubility product, Ksp, of AgCl and AgBr at 25 oC are given in Table 2.1. Table 2.1 compound Ksp (at 25 °C) AgCl 1.77  10–10 AgBr 5.35  10–13 To separate the two halide ions in 25 .0 cm3 of sample X via selective precipitation , just enough solid AgNO3 was added to precipitate the maximum amount of AgBr from the mixture, without precipitating AgCl. (i) Given that the c oncentration of Cl– ions in sample X is 0.0960 mol dm 3, calculate the concentration of Ag+ ions in the solution at the end of the separation. [1]

© Raffles Institution 2020 9729/02/S/20 [Turn Over 7 For examiner’s use (ii) Calculate the concentration of Br – ions remaining in the solution at the end of the separation. [1] (iii) Using your answers in (b)(i) and (b)(ii), calculate the mass of solid AgNO 3 that was added to separate the halide ions. [Mr of AgNO3 = 169.9] [3]

8 © Raffles Institution 2020 9729/02/S/20 For examiner’s use (c) The solubility product, Ksp, of Ag2CrO4 at 25 oC is 1.12  10–12 mol3 dm–9. 0.100 mol of potassium chromate(VI), K2CrO4(s), was dissolved in 500 cm3 of a saturated solution of Ag2CrO4 at 25 °C. (i) Calculate the solubility of Ag2CrO4 in water before the addition of K2CrO4(s). [1] (ii) Calculate the solubility of Ag2CrO4 after the dissolution of K2CrO4(s). [2] (iii) Hence, calculate the mass of Ag2CrO4 precipitated after the dissolution of K2CrO4. [Mr of Ag2CrO4 = 331.8] [2] [Total: 16]

© Raffles Institution 2020 9729/02/S/20 [Turn Over 9 For examiner’s use Question 3 starts on the next page.

10 © Raffles Institution 2020 9729/02/S/20 For examiner’s use 3 Urea is well known for its agricultural and biological importance. Hydrolysis of urea can take place in both alkaline and acidic conditions , as well as in the presence of the enzyme urease. (a) The kinetics of the hydrolysis of urea , CO(NH 2)2, in alkaline medium is studied using different concentrations of CO(NH2)2 and OH− ions as shown in Table 3.1. CO(NH2)2 + 2OH−  2NH3 + CO32– Table 3.1 experiment initial [CO(NH2)2] / mol dm–3 initial [OH−] / mol dm–3 A