CIE Nov 2014 v6 Paper 3 Q2

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In Question 1 iodide ions were oxidised by iodate(V) ions. Iodide ions can also be oxidised by peroxodisulfate ions, S2O8 2, and you are to investigate the rate of this reaction. PMT 2I + S2O8 2I2 + 2SO4 2 The rate of this reaction can be measured by adding thiosulfate ions, S2O3 2, and starch solution to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide according to the reaction below. I2 + 2S2O3 22I + S4O6 2 After all the thiosulfate has reacted, the iodine turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes for the mixture to turn blue-black. The rate of the reaction can be represented by rate = reaction time (a) Method FB 4 is aqueous potassium iodide, KI. FB 5 is 0.020 mol dm3 potassium peroxodisulfate, K2S2O8. FB 6 is 0.010 mol dm3 sodium thiosulfate, Na2S2O3. starch indicator Read through the method before you start any practical work and prepare a suitable table for your results in the space at the top of page 5. Empty and wash thoroughly the burette that you used in Question 1. Experiment 1Use the measuring cylinder to add 20 cm3 of FB 4 into a 100 cm3 beaker. Use the 10 cm3 pipette to add 10.0 cm3 of FB 6 to the beaker. Add 10 drops of starch indicator to the beaker. Fill a burette with FB 5. Run 20.00 cm3 of FB 5 into a second 100 cm3 beaker. Add the contents of the first beaker to the second beaker and start timing immediately. Stir the mixture once and place the beaker on a white tile. Stop timing as soon as the solution goes blue-black. Record this reaction time to the nearest second. Wash out both beakers and shake dry. Experiment 2Use the measuring cylinder to add 20 cm3 of FB 4 into a 100 cm3 beaker. Use the 10 cm3 pipette to add 10.0 cm3 of FB 6 to the beaker. Add 10 drops of starch indicator to the beaker. Fill the second burette with distilled water. Run 10.00 cm3 of FB 5 into the second 100 cm3 beaker. Run 10.00 cm3 of distilled water into this second beaker containing FB 5. Add the contents of the first beaker to the second beaker and start timing immediately. Stir the mixture once and place the beaker on a white tile. Stop timing as soon as the solution goes blue-black. Record this reaction time to the nearest second. Wash out both beakers and shake dry.UCLES 2014 9701/36/O/N/14<br /> Record your results in the space below. You should show the volume of FB 5, the volume of water and the reaction time for each experiment. PMT (b) Carry out three further experiments to investigate how the reaction time changes with different volumes of peroxodisulfate. Remember that the combined volume of peroxodisulfate solution, FB 5, and distilled water must always be 20.00 cm3. Do not use a volume of FB 5 that is less than 4.00 cm3. Record the volume of FB 5, the volume of water and the reaction time for each experiment. [3] II III IV [4] (c) Use your results from (a) and (b) to complete the table below. You should show the volume of FB 5, the reaction time and the volume of FB 5reaction time.UCLES 2014 9701/36/O/N/14 [Turn over [2]<br /> PMT (d) A student who had carried out these experiments concluded that the rate of reaction was directly proportional to the volume of FB 5. Using your values from (c), explain whether your results agree with this conclusion.[1] (e) Another student thought that the experiment could be made more accurate by giving longer reaction times. To do this he repeated Experiment 1 with the same volumes but using 0.100 mol dm3 sodium thiosulfate instead of FB 6. He found that the reaction never turned blue-black. Explain why.[1] (f) Describe how you could modify this experiment to investigate the effect of the concentration of iodide ions, FB 4, on the rate of the reaction.[2] [Total: 13]UCLES 2014 9701/36/O/N/14<br />

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