VJC 2022 H2 Chem Prelim P4 qp

This document consists of 14 printed pages. © VJC 2022 9729/04/PRELIM/22 [Turn over CANDIDATE NAME CT GROUP VICTORIA JUNIOR COLLEGE PRELIMINARY EXAMINATION Higher 2 ……………………………………………….………….. …………………………….. CHEMISTRY 9729/04 Paper 4 Practical Candidates answer on the Question Paper. 29 Aug 2022 2 hours 30 minutes Additional Materials: As listed in the instructions below READ THESE INSTRUCTIONS FIRST Write your name and CT group on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes 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 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 13 and 14. 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 For Examiner’s Use 1 / 9 2 / 12 3 / 11 4 / 15 5 / 8 Total / 55

2 © VJC 2022 9729/04/PRELIM/22 Answer all the questions in the spaces provided. 1 In acidic solutions, iron(III) ions are reduced by iodide ions to form iron(II) ions. The iodide ions are oxidised to iodine. 2Fe3+(aq) + 2I– (aq) → 2Fe2+(aq) + I2(aq) The rate of this reaction can be investigated by using starch indicator, which turns blue-black in the presence of iodine. Sodium thiosulfate is added to the reaction mixture to react with iodine as it is formed. The blue-black colour is seen when all the thiosulfate has reacted. I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) You will investigate how the rate of reaction is affected by changing the concentration of the iodide ions. FA 1 is 0.050 mol dm‒3 potassium iodide, KI. FA 2 is 0.050 mol dm‒3 acidified iron(III) chloride, FeCl3. FA 3 is 0.0050 mol dm‒3 sodium thiosulfate, Na2S2O3. FA 4 is starch indicator. (a) Experiment 1 1. Use the burette labelled FA 1 to transfer 10.00 cm3 of FA 1 into a 250 cm3 conical flask. 2. Use a 25 cm3 measuring cylinder to transfer 10 cm3 of deionised water into the conical flask. 3. Use the burette labelled FA 3 to transfer 20.00 cm 3 of FA 3 and a 50 cm 3 measuring cylinder to transfer 10 cm3 of FA 4 to the same conical flask. 4. Use a 10 cm3 measuring cylinder to measure 10 cm3 of FA 2. 5. Add this FA 2 into the same conical flask and start timing immediately. 6. Swirl the mixture once and place the conical flask on the white tile. 7. Stop timing as soon as the solution turns intense blue-black. 8. Record the time taken, t, to the nearest second. 9. Wash out the conical flask and stand it upside down in a beaker to drain for use again. Experiment 2 10. Repeat Experiment 1 but use 20.00 cm 3 of FA 1 instead of 10.00 cm 3 in step 1 and use 0.0 cm3 of deionised water instead of 10.0 cm3 in step 2. Experiments 3 to 5 11. Carry out three further experiments to investigate how the reaction time changes with different volumes of potassium iodide, FA 1. The combined volume of FA 1 and deionised water must always be 20 cm3. Do not use a volume of FA 1 that is less than 6 cm3. In the spa ce on the next page , prepare a table to record the following for each of the five experiments. • V, Volume of FA 1 used, • Volume of water used, • t, Reaction time, to the nearest second, • Vt • V2t Keep solutions FA 1, FA 3 and FA 4 for use in Question 2.

3 © VJC 2022 9729/04/PRELIM/22 [Turn over Results [4] (b) V is the volume of FA 1 used in cm3 and t is the time taken in seconds. On your table above, compute the values of Vt and V2t for each experiment, to 2 significant figures. [1] (c) The rate of the reaction in this investigation can be calculated using the following formula: rate = 1 / reaction time. Explain why this is so. …………………………………………………………………………………………………………….. …………………………………………………………………………………………………………….. ………………………………………………………………………………………………..……….. [1] (d) Deduce the order of reaction with respect to iodide ions. Explain your answer by referring to your calculated values of Vt and V2t. ………………………………………………………………………………………………..…………… ………………………………………………………………………………………………..…………… …………………………………………………………………………………………………………….. ………………………………………………………………………………………………..……….. [2]

4 © VJC 2022 9729/04/PRELIM/22 (e) Another student investigated the effect of concentration of iron(III) ions on the rate of this reaction. The student carried out another experiment, Experiment 6, and the rate is compared to that of Experiment 1. Suggest the volumes the student could use for Experiment 6. reagent volume / cm3 FA 1 FA 2 FA 3 FA 4 deionised water [1] [Total: 9] 2 In this experiment you will determine the formula of the ion, IOx–, To do this, you will first react IOx– ions with an excess of iodide ions, I– to form iodine, I2. The equation for this reaction is: IOx– + yI– + zH+ → 1 2 y+ I2 + 2 z H2O where x, y and z are all integers. The amount of iodine produced will then be determined by titration with thiosulfate ions. In addition to FA 1, FA 3 and FA 4 used in question 1, you are also provided with the following. FA 5 is a solution containing 0.00600 mol dm–3 IOx– ions. FA 6 is 1.0 mol dm–3 sulfuric acid, H2SO4. (a) Dilution of FA 5 1. Pipette 25.0 cm3 of FA 5 into the 250 cm3 volumetric flask. 2. Make the solution up to the mark using deionised water. 3. Shake the flask thoroughly. 4. Label this diluted solution of IOx– as FA 7. Titration 1. Pipette 25.0 cm3 of FA 7 into a conical flask. 2. Using the burette labelled FA 1, transfer 10.00 cm3 of FA 1 into the same conical flask. 3. Using a measuring cylinder, transfer 10 cm3 of FA 6 into the same conical flask. 4. Titrate the mixture in the conical flask against FA 3 in the burette labelled FA 3 until the solution turns yellow. 5. Add 10 drops of FA 4 into the conical flask. 6. Continue the titration until the blue-black colour just disappears. 7. Carry out as many titrations as you deem necessary to obtain consistent results. 8. In the space provided on the next page, prepare a table to record all your burette readings and the volume of FA 3 used in each titration.

5 © VJC 2022 9729/04/PRELIM/22 [Turn over Results [5] (b) (i) From your titration results, obtain a suitable volume of FA 3 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 3 = ………………… cm3 [1] (ii) Calculate the amount of iodine formed when 25.0 cm 3 of FA 7 reacts with 10.00 cm 3 of FA 1. Show your working. amount of iodine = ………………… mol [1] (iii) Calculate the amount of IOx– in 25.0 cm3 of FA 7. Show your working. amount of IOx– = ………………… mol [1] (iv) Using your answer in (b)(ii) and (b)(iii), determine the value of y. Show your working. y = …………………[1] (v) Using your answer in (b)(iv), determine the value of z. Hence, determine the value of x. z = ……………… and x = …………………[1]

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