RI Prelim P4 QP

© Raffles Institution 2018 9729/04/A/18 [Turn Over CANDIDATE NAME ( ) CLASS 18S0 RAFFLES INSTITUTION 2018 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY Paper 4 Practical 9729/04 29 August 2018 2 hours 30 minutes Do NOT turn over the Question Booklet until you are told to do so. READ THESE INSTRUCTIONS FIRST Write your name and class on the space provided when instructed to do so. Give details of the practical shift and laboratory, where appropriate, in the space provided. 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. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 17 and 18. 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. Shift Laboratory Bench Number For Examiner’s Use Question Marks 1 / 14 2 / 25 3 / 16 Total / 55 This document consists of 17 printed pages and 1 blank page.

- 2 - © Raffles Institution 2018 9729/04/A/18 [Turn Over Answer all the questions in the spaces provided. 1 Determination of the concentration of CuSO 4 in FA 2 You are provided with the following solutions: FA 1 0.0500 mol dm–3 aqueous sodium thiosulfate, Na2S2O3 FA 2 aqueous CuSO 4 solution of unknown concentration FA 3 100 g dm−3 solution of sodium iodide, NaI Starch solution Copper(II) ions react with iodide ions according to the following equation. 2Cu2+(aq) + 4I⁻(aq) ⎯→ 2CuI(s) + I2(aq) The iodine produced in this reaction may be titrated against FA 1 using starch solution as indicator. Thiosulfate ions react with iodine according to the following equation: 2S2O32–(aq) + I2(aq) ⎯→ S4O62–(aq) + 2I–(aq) Estimating the required dilution factor for FA 2 (a) The solution of FA 2 provided is too concentrated to be used directly for reaction with FA 3 to produce iodine for titration against FA 1. A preliminary experiment was carried out to estimate a dilution factor for the FA 2 solution, so that 25 cm 3 of this diluted solution would require approximately 25 cm 3 of FA 1 for complete reaction. It was found that 30 drops of FA 1 were needed to react completely with the iodine produced from the reaction between 5 drops of FA 2 and excess FA 3. Use the information above to calculate the volume of FA 2 needed to make 250 cm 3 of the diluted CuSO4 solution, FA 4. [1] Preparation of FA 4 by dilution of FA 2 (b) You will now follow the instructions given below to prepare FA 4, a diluted solution of FA 2. 1. Using a burette, measure between 40.00 and 42.00 cm 3 of FA 2 into a 250 cm 3 graduated flask. 2. Record in the space below, your burette readings and the volume of FA 2 transferred to the flask. [2] 3. Fill the graduated flask to the 250 cm 3 mark with deionised water, stopper and mix thoroughly by inverting the flask a number of times. The diluted CuSO4 solution you have prepared is FA 4.

- 3 - © Raffles Institution 2018 9729/04/A/18 [Turn Over Titration 1. Fill the second burette with FA 1. 2. Pipette 25.0 cm 3 of FA 4 into a conical flask. 3. Use the measuring cylinder provided to add 10 cm 3 of FA 3 to the conical flask. As described earlier, the Cu 2+ ions in FA 4 react with I− ions from FA 3 to produce I2, which can be titrated with FA 1 . The reaction also produces a cream precipitate of CuI. 4. Titrate the iodine in the conical flask with FA 1 until the brown colour of the iodine becomes pale brown. 5. Add about 10 drops of starch solution to the flask and continue adding FA 1 until the blue-black colour just disappears. 6. Repeat the titration as many times as you think necessary to obtain consistent results. Record your titration results in the space below. Results [5] (c) Indicate clearly which results you have used, and show with calculation the volume of FA 1 used. ……….....… cm 3 of FA 4 produced an amount of iodine which required ……….....… cm 3 of FA 1 for complete reaction. [1]

- 4 - © Raffles Institution 2018 9729/04/A/18 [Turn Over (d) Calculate the number of moles of S 2O32− in the volume of FA 1 recorded in (c), and hence determine the molar concentration of Cu2+ ions in FA 4. number of moles of S 2O32− = ……………………..…… concentration of Cu2+ in FA 4 = ………………………………. [2] (e) Using the volume of FA 2 recorded in (b), calculate the molar concentration of CuSO 4 in FA 2. concentration of CuSO 4 in FA 2 = ……………………………….[1] (f) A student repeated the experiment using 50 g dm −3 sodium iodide solution instead. Predict, using calculations, if the titre volumes obtained would differ when the new solution of iodide ions is used. (A r: Na = 23.0; I = 126.9) [2] [Total: 14]

- 5 - © Raffles Institution 2018 9729/04/A/18 [Turn Over 2 Determination of the concentration of NaC lO and a value for an enthalpy change of reaction, ΔHrxn You are provided with the following solutions: FA 5 is an aqueous solution containing sodium chlorate( I), NaClO. FA 6 is an aqueous solution of 0.100 mol dm −3 sodium thiosulfate in aqueous sodium hydroxide. You are to perform a series of experiments using different volumes of FA 5 and FA 6 which together give a total volume of 40.0 cm3. The temperature change, ΔT, for each experiment will be determined and you will plot a graph of ΔT against the volume of FA 5 used. You will then use data from your graph to determine the concentration of sodium chlorate( I) in FA 5. You will also use data from your graph to determine a value for the enthalpy change of reaction, ΔH1, for the reaction between aqueous sodium chlorate(I) and sodium thiosulfate. 4NaClO(aq) + Na2S2O3(aq) + 2NaOH(aq) ⎯→ 4NaCl(aq) + 2Na2SO4(aq) + H2O(l) --- Reaction 1 (a) Procedure Follow the steps below to determine the temperature changes when different volumes of FA 5 and FA 6 are reacted together. 1. Use a 50 cm 3 measuring cylinder to transfer 10.0 cm 3 of FA 5 into a Styrofoam cup. Place this cup containing FA 5 inside a second Styrofoam cup which is placed in a 250 cm3 glass beaker to prevent it from tipping over. 2. Place a thermometer into the cup containing FA 5. Stir gently, measure and record the temperature of the FA 5 solution, T1. 3. Using another 50 cm 3 measuring cylinder, transfer 30.0 cm 3 of FA 6 into the cup containing FA 5. Use the thermometer to stir the mixture gently. Measure and record the highest temperature, T2, of the mixture. 4. Empty, wash and carefully dry the Styroform cup. 5. Repeat steps 1 to 4 using 20.0 cm 3, 30.0 cm 3 and 35.0 cm 3 of FA 5, each time using the appropriate volume of FA 6 so that the total volume of the mixture is 40.0 cm3. 6. Record all measurements of volume, temperature and temperature change, ΔT, in an appropriate format in the spaces provided on the next page. The data for these four experiments should be recorded together with the two or three additional experiments described in (b)(ii). ΔH1

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