CIE Jun 2014 v1 Paper 3 Q2

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An acidi ed solution of hydrogen peroxide is able to oxidise iodide ions, I(aq), to iodine, I2(aq). PMT H2O2(aq) + 2H+(aq) + 2I(aq)I2(aq) + 2H2O(l) In this experiment, you will investigate how the rate of this reaction depends on the concentration of the hydrogen peroxide and on the concentration of the iodide ions. The rate of this reaction can be measured by adding thiosulfate ions, S2O3 2, and starch indicator to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide ions. I2(aq) + 2S2O3 2(aq)2I(aq) + S4O6 2(aq) When all the thiosulfate has reacted, the iodine then turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes the reaction mixture to turn blue-black. FA 2 is aqueous hydrogen peroxide, H2O2. FA 3 is 1.0 mol dm3 sulfuric acid, H2SO4. FA 4 is 0.50 mol dm3 potassium iodide, KI. FA 5 is 0.025 mol dm3 sodium thiosulfate, Na2S2O3. starch indicator Read through the instructions carefully and prepare a table for your results before starting any practical work. (a) Method Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. Experiment 110 cm3 of FA 320 cm3 of FA 410 cm3 of FA 510 cm3 of distilled waterAdd 10 drops of starch indicator to the beaker. Rinse the measuring cylinder with water and shake dry. Use the measuring cylinder to measure 10 cm3 of FA 2. Add the contents of the measuring cylinder to the beaker and start timing immediately. Stir the mixture once and place the beaker on a white tile. Stop timing as soon as the solution turns blue-black. Record this reaction time to the nearest second. Wash out the beaker and the measuring cylinder with water and shake dry. Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. Experiment 210 cm3 of FA 3 20 cm3 of FA 4 10 cm3 of FA 5 Add 10 drops of starch indicator to the beaker. Rinse the measuring cylinder with water and shake dry. Use the measuring cylinder to measure 20 cm3 of FA 2.UCLES 2014 9701/31/M/J/14<br /> Add the contents of the measuring cylinder to the beaker and start timing immediately. Stir the mixture once and place the beaker on a white tile. Stop timing as soon as the solution turns blue-black. Record this reaction time to the nearest second. Wash out the beaker and the measuring cylinder with water and shake dry. PMT Use the measuring cylinder to transfer the following volumes into the same 100 cm3 beaker. Experiment 310 cm3 of FA 310 cm3 of FA 410 cm3 of FA 510 cm3 of distilled waterAdd 10 drops of starch indicator to the beaker. Rinse the measuring cylinder with water and shake dry. Use the measuring cylinder to measure 20 cm3 of FA 2. Add the contents of the measuring cylinder to the beaker and start timing immediately. Stir the mixture once and place the beaker on a white tile. Stop timing as soon as the solution turns blue-black. Record this reaction time to the nearest second. Record all your results in a single table. You should include the volume of hydrogen peroxide, the volume of potassium iodide, the volume of distilled water and the reaction time. You should also include the rate of reaction which is given by the following expression. rate of reaction = reaction time II III IV VI [6] (b) From your results, what can you conclude about how the rate of reaction is affected by, (i) the concentration of hydrogen peroxide,(ii) the concentration of potassium iodide?[1]UCLES 2014 9701/31/M/J/14 [Turn over<br /> PMT (c) It is not possible to draw a reliable conclusion about the effect of changing the concentration of a reagent on the rate of reaction from only two experiments. Suggest three experiments that could be carried out in addition to Experiments 2 and 3 to investigate the effect of changing the concentration of FA 4, potassium iodide. In each case give the volumes of each solution / liquid that would be used. Do not carry out these experiments. [2] (d) A student was unable to complete all three experiments on the same day but had to return to the investigation the following morning. The student noticed that the results were signi cantly different. Give two possible reasons for these differences and explain how the student could have overcome these problems.[2] (e) There are many sources of error in these experiments. In the following, assume that the only error involves measuring the reaction time. (i) If the error in each recorded time is 1 s, calculate the percentage error in the time recorded in Experiment 1. percentage error =% (ii) Another student decided to repeat Experiment 1 but used 0.050 mol dm3 sodium thiosulfate in place of FA 5. How would this change the percentage error in the recorded time? Explain your answer.[2] [Total: 13]UCLES 2014 9701/31/M/J/14<br />

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