YIJC 2024 Prelim P4 Solutions

1 ©YIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME SUGGESTED ANSWERS CLASS H2 CHEMISTRY 9729/04 Paper 4 Practical Paper 20 August 2024 2 hours 30 minutes Candidates answer on question paper. READ THESE INSTRUCTIONS FIRST This document consists of 24 printed pages and 1 blank page. Write your name and class in the spaces at the top of this page. Give details of the practical shift and laboratory where appropriate, in the boxes 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 23 and 24. 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 2 3 4 Total Shift Laboratory

2 ©YIJC [Turn over Answer all the questions in the spaces provided. 1 Determination of the kinetics of a redox reaction Equation 1 represents the reaction between iron(III) ions and iodide ions. equation 1 2Fe3+(aq) + 2I−(aq) → 2Fe2+(aq) + I2(aq) When starch is added to the reaction mixture, a blue colour is immediately seen due to the formation of an iodine−starch complex. If a small amount of sodium thiosulfate, Na2S2O3, is also present in the reaction mixture, the formation of the blue colour is delayed. The Na2S2O3 reacts with I2 as shown in equation 2. equation 2 I2(aq) + 2S2O32−(aq) → S4O62−(aq) + 2I−(aq) FA 1 is iron(III) chloride, FeCl3. FA 2 is potassium iodide, KI. FA 3 is 0.0060 mol dm−3 sodium thiosulfate, Na2S2O3. Solution S is starch indicator. You will perform a series of five experiments. Then, you will graphically analyse your results to determine the order with respect to the concentration of Fe3+ ions, [Fe3+]. You will add a fixed amount of sodium thiosulfate, FA 3, to each of your experiments. In each experiment, you will need to ensure that the same total volume of reaction mixture is used by adding deionised water as required. The time taken for the blue colour to form allows the reaction rate to be determined. (a) Prepare a table in the space provided on page 4 in which to record, to an appropriate level of precision: • all volumes, except the volume of sodium thiosulfate, potassium iodide and starch, • all values of t.

3 ©YIJC [Turn over Experiment 1 The end-point of the reaction is the first appearance of a blue colour. 1. Fill a burette with FA 1. 2. Transfer 20.00 cm3 of FA 1 into a 250 cm3 beaker labelled “reaction mixture”. 3. Use appropriate measuring cylinder, transfer to a clean 100 cm3 beaker. • 10.0 cm3 of FA 2, • 10.0 cm3 of solution S, • 20.0 cm3 of FA 3. 4. Pour this mixture rapidly into the beaker containing FA 1 and stir the contents using a glass rod. Start the stopwatch during this addition. 5. The mixture turns brown and then yellow before turning a blue colour. Stop the stopwatch when the solution first turns blue. 6. Record the time taken, t, to the nearest second in your table. 7. Carefully wash out the beaker. Stand it upside down on a paper towel to drain. Experiment 2 1. Fill another burette with deionised water. 2. Transfer 6.00 cm3 of FA 1 into another 250 cm3 beaker labelled “reaction mixture”. 3. Transfer 14.00 cm3 of deionised water into the beaker containing FA 1. 4. Repeat points 3 − 7 from experiment 1.

4 ©YIJC [Turn over Experiment 3 to 5 Carry out three further experiments to investigate the effect of changing the concentration of Fe3+(aq) by changing the volume of aqueous FA 1 used. You should not use a volume of FA 1 that is less than 6.00 cm3. In each case, you will need to ensure that the same total volume of reaction mixture is used by adding deionised water as required. You should alternate the use of the two 250 cm3 beakers labelled “reaction mixture”. Record all required volumes and time taken in your table. Results experiment VFA1 / cm3 Vdeionised water / cm3 time/ s 1 20.00 0.00 15 2 6.00 14.00 24 3 10.00 10.00 21 4 14.00 6.00 17 5 17.00 3.00 16

5 ©YIJC [Turn over (b) The rate of reaction can be found by calculating the change in concentration of Fe 3+(aq) that occurred when enough iodine was produced to change the colour of the indicator to blue. (i) Calculate the amount, in moles, of thiosulfate ions, S2O32−, used in each experiment. Amount of S2O32– = 20 1000 × 0.00600 = 1.20 × 10−4 mol (ii) Calculate the amount of iron(III) ions, Fe3+, that were used to produce the amount of iodine that react with the amount of S2O32− in (b)(i). Mole ratio of S2O32– : I2 : Fe3+ is 2 : 1 : 2 Amount of Fe3+ = 1.20 × 10−4 mol (iii) Calculate the change in the concentration of iron(III) ions up to the time of appearance of the blue colour. Change in [Fe2+] = (1.20 × 10−4)  (60  1000) = 2.00 × 10−4 mol dm−3 (iv) The rate of this reaction relative to [Fe3+] can be determine using the following expression. rate = change in concentration of Fe3+(aq) reaction time × 106 Complete the table to show the volume of FA 1 , the reaction time and the rate in Experiments 1 to 5. You should include units. If you were unable to calculate a value for the change in concentration of Fe3+(aq) in (b)(iii), you should assume it is 2.50 × 10−3 mol dm−3. Note that this is not the correct value. experiment VFA1 / cm3 reaction time / s rate / mol dm−3 s−1 1 20.00 15 133 2 6.00 24 83.3 3 10.00 21 95.2 4 14.00 17 118 5 17.00 16 125

6 ©YIJC [Turn over (c) (i) Plot a graph of rate on the y-axis against volume of FA 1 on the x-axis. Draw the best -fit straight line taking into account all of your plotted points.

7 ©YIJC [Turn over (ii) Deduce the order of reaction with respect to [Fe 3+]. Use evidence from your graph in (c)(i) to support your answer. Reaction is first order with respect to [Fe3+]. The straight-line graph shows that rate is proportional to [Fe3+] (or volume of Fe3+). 60 70 80 90 100 110 120 130 140 4 6 8 10 12 14 16 18 20 22 24 26 28 rate / mol dm−3 s−1 VFA1 / cm3

8 ©YIJC [Turn over (d) It was found by carrying out experiments similar to those used in (a), that increasing the concentration of I− increased the rate of the reaction. A student suggested a modification to the method by using the same volumes of all reagents but with the concentrations of FA 1 and FA 2 being double their original values. State what the effect would be on the reaction time in Experiment 1 and explain how this change would affect any possible errors in the measurements. Reaction time is less/shorter (not faster). The measurement will be less accurate since there is a larger percentage error in time. (e) Another student thought that the experiment could be made more accurate by repeating Experiment 1 with the same volumes of all reagents but using 0.060 mol dm−3 sodium thiosulfate instead of FA 3. He found that the reaction mixture never turned blue. Explain why. S2O32− is in excess. All the iodine reacts with S2O32− so no iodine is left to turn starch solution blue.

9 ©YIJC [Turn over 2 Investigation of