2023 RI H2 Chem Prelims P4 Questions

© Raffles Institution 2023 9729/04/A/23 [Turn Over CANDIDATE NAME ( ) CLASS 23S0 RAFFLES INSTITUTION 2023 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY Paper 4 Practical 9729/04 23 August 2023 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 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 on the Question Paper. The number of marks is given in brackets [ ] at the end of each question or part question. 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. Shift Laboratory Bench Number For Examiner’s Use Question Marks 1 / 14 2 / 13 3 / 17 4 / 11 Total / 55 This document consists of 21 printed pages and 3 blank pages.

2 © Raffles Institution 2023 9729/04/A/23 [Turn Over Answer all the questions in the spaces provided. 1 Determination of the kinetics of the reaction between iron(III) and iodide ions Iron(III) ions, Fe3+, oxidise iodide ions, I−, to iodine, I2. equation 1 2Fe3+(aq) + 2I−(aq) ⎯→ 2Fe2+(aq) + I2(aq) The rate of this reaction can be measured by adding thiosulfate ions, S2O32−, and some starch indicator to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide ions. equation 2 I2(aq) + 2S2O32−(aq) ⎯→ 2I−(aq) + S4O62−(aq) When all the thiosulfate ions have reacted, the iodine which continues to be produced in equation 1 then turns the starch indicator blue-black. The rate of reaction may be determined by measuring the time taken for the reaction mixture to turn blue-black. FA 1 is aqueous iron(III) chloride, FeCl3. FA 2 is 0.0500 mol dm−3 aqueous potassium iodide, KI. FA 3 is 0.00500 mol dm−3 sodium thiosulfate, Na2S2O3. You will perform a series of four experiments. 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. (a) Experiment 1 1. Using appropriate measuring cylinders, add the following into a 250 cm3 conical flask. • 20 cm3 of FA 2 • 20 cm3 of FA 3 2. Using a measuring cylinder, add 25.0 cm 3 of deionised water into the same conical flask. 3. Add 10 drops of starch indicator into the same conical flask. 4. Measure 5.0 cm3 of FA 1 using a measuring cylinder. 5. Transfer the FA 1 to the 250 cm3 conical flask and start the stopwatch at the same time. 6. Swirl the conical flask once to mix the contents thoroughly before placing it on a white tile. 7. Stop the stopwatch when a blue-black colour first appears. You may ignore any initial colour changes that appear before the intense blue-black colouration. 8. Record the time taken, t, to the nearest 0.1 second for the first appearance of the intense blue-black colour. 9. Discard the reaction mixture. Wash out the conical flask and stand it upside down on a paper towel to drain.

3 © Raffles Institution 2023 9729/04/A/23 [Turn Over Experiments 2, 3 and 4 Repeat Experiment 1 using three other volumes of FA 1, keeping the volumes of FA 2, FA 3 and starch indicator the same. 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 use suitable volumes of FA 1 that are more than 5.0 cm3 to a maximum of 15.0 cm3. You should alternate the use of two 250 cm3 conical flasks. Prepare a table in the space provided below, in which to record: • all volumes, to an appropriate level of precision, except those of FA 2, FA 3 and the starch indicator, • all values of t to the nearest 0.1 second, • all values of rate, to 3 significant figures, which you will calculate in part (c). Results [4]

4 © Raffles Institution 2023 9729/04/A/23 [Turn Over (b) Calculate the amount of iodine produced in each experiment just before the blue-black colour first appeared. Hence, calculate the concentration of this iodine in the reaction mixture (you may ignore the volume of starch indicator used in each experiment). amount of iodine produced = ………………........................ mol concentration of iodine = ………………......................... mol dm-3 [2] (c) The rate of the reaction can be represented by the following formula. rate = concentration of iodine from (b) time taken, t Use your experimental results in (a) to calculate the rate for Experiments 1 to 4. Then, in your table of results in (a), include the value of rate , to 3 significant figures, for each experiment in a separate column. If you were unable to answer (b), you may assume the concentration of iodine is 0.00130 mol dm−3 (this is not the correct value). [1]

5 © Raffles Institution 2023 9729/04/A/23 [Turn Over (d) (i) Plot a graph of rate, on the y -axis, against the volume of FA 1, on the x-axis on the grid in Fig 1.1. Draw a line of best-fit, taking into account all of your plotted points. [3] Fig. 1.1

6 © Raffles Institution 2023 9729/04/A/23 [Turn Over (ii) With reference to your graph in Fig 1.1, show that the reaction is first o rder with respect to Fe3+. …………………………………………………………………………………..…….......... …………………………………………………………………………………..…….......... ………………………………………………………………………………….…………[1] (e) A student suggested that the percentage error in the time taken for the intense blue black colour to appear could be reduced if a higher concentration of sodium thiosulfate was used in place of FA 3. Do you agree with this student? Explain your answer. ……………… …………………………………………………………………..… ….......... ……………… …………………………………………………………………..… ….......... ………………………………………………………………………………………………….. [1] (f) For the reaction between Fe 3+ and I− in equation 1, the order of reaction was experimentally determined to be second order with respect to I−. (i) Write the rate equation for the reaction between Fe3+ and I−. ………………………………………………………………………………….…………[1] A student proposed the following mechanism for this reaction. Step 1: Fe3+(aq) + I−(aq) ⎯→ [FeI]2+(aq) (fast) Step 2: [FeI]2+(aq) + I−(aq) ⎯→ Fe2+(aq) + I2−(aq) (slow) Step 3: 2I2−(aq) ⎯→ I3−(aq) + I−(aq) (fast) (ii) Suggest, with reasoning, if the proposed mechanism is consistent with the observed kinetics data. …………………………………………………………………………………..…….......... …………………………………………………………………………………..…….......... ………………………………………………………………………………….…………[1] [Total: 14]

7 © Raffles Institution 2023 9729/04/A/23 [Turn Over BLANK PA