CIE Nov 2015 v6 Paper 3 Q1

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PMT You will investigate the rate of reaction between iron(III) ions, Fe3+, and iodide ions, I. 2Fe3+(aq) + 2I(aq)2Fe2+(aq) + I2(aq) The iodine, I2, produced can be reacted immediately with thiosulfate ions, S2O3 2. I2(aq) + 2S2O3 2(aq)2I(aq) + S4O6 2(aq) When all the thiosulfate has been used, the iodine produced will turn starch indicator blue-black. The rate of the reaction can therefore be measured bynding the time for the blue-black colour to appear. FB 1 is aqueous iron(III) chloride, FeCl 3. FB 2 is aqueous potassium iodide, KI. FB 3 is 0.0060 mol dm3 sodium thiosulfate, Na2S2O3. starch indicator You are advised to read the instructions before starting any practical work and draw a table for your results in the space on page 3. (a) Method Experiment 110 cm3 of FB 2 20 cm3 of FB 3 10 cm3 of starch indicator Fill a burette with FB 1. Run 20.00 cm3 of FB 1 into a 100 cm3 beaker. Use the measuring cylinder to place the following in a second 100 cm3 beaker.Add the contents of the second beaker to the first beaker and start timing. Stir the mixture once and place the beaker on the white tile. The mixture turns brown and then yellow before turning a blue-black colour. Stop timing when this blue-black colour appears. Record in your table the volume of FB 1 used, the volume of distilled water used and the time to the nearest second for the blue-black colour to appear. Wash both beakers. For each of Experiments 2-6 you should complete your results table to show the volume of FB 1 used, the volume of distilled water used and the time taken to the nearest second for the blue-black colour to appear. Experiment 2Fill the other burette with distilled water. Run 10.00 cm3 of FB 1 into a 100 cm3 beaker. Run 10.00 cm3 of distilled water into the same beaker. Use the measuring cylinder to place the following in a second 100 cm3 beaker.Add the contents of the second beaker to the first beaker and start timing. Stir the mixture once and place the beaker on the white tile. Stop timing when a blue-black colour appears. Wash both beakers. 10 cm3 of FB 2 20 cm3 of FB 3 10 cm3 of starch indicatorUCLES 2015 9701/36/O/N/15<br /> Experiments 3-6 Carry out four further experiments to investigate the effect of changing the concentration of Fe3+(aq) by altering the volume of aqueous FeCl 3, FB 1, used. You should not use a volume of FB 1 that is less than 6.00 cm3 and the total volume of the reaction mixture must always be 60 cm3. PMT II III IV VI VII VIII [8]UCLES 2015 9701/36/O/N/15 [Turn over<br /> PMT (b) Calculations The rate of reaction can be found by calculating the change in concentration of Fe3+(aq) that occurred when enough iodine was produced to change the colour of the indicator to blue-black. Use your data and the equations on page 2 to carry out the following calculations. Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Calculate the number of moles of thiosulfate ions, S2O3 2 used in each experiment in (a). (ii) Calculate the number of moles of iodine, I2, that react with the number of moles of S2O3 in (i). 2 moles S2O3 2 =mol moles I2 =mol Calculate the number of moles of iron(III) ions, Fe3+, that were used to produce the number of moles of iodine in (ii). (iii) moles Fe3+ =mol (iv) When the moles of Fe3+ that you calculated in (iii) reacted, a change in the concentration of moles of Fe3+ occurred. Calculate this change in concentration. change in concentration of Fe3+(aq) =mol dm3 (v) The following formula can be used as a measure of the rate of reaction. rate of reaction = change in concentration of Fe3+(aq) reaction time106 Complete the table to show the volume of FB 1, the reaction time and the rate in Experiments 1-6. You should include units. If you were unable to calculate a value for the change in concentration of Fe3+(aq) in (iv), you should assume it is 2.50103 mol dm3. (Note: this is not the correct value.) Experiment 1 2 3 4 5 6UCLES 2015 9701/36/O/N/15 [5]<br /> (c) On the grid, plot the rate (y-axis) against the volume of FB 1 (x-axis). Draw a line of bestt through the points. You should identify any points you consider anomalous. PMT [4]UCLES 2015 9701/36/O/N/15 [Turn over<br /> PMT (d) Using your graph, what conclusion can you reach about the effect of changing the concentration of FeCl 3 on the rate of the reaction between Fe3+(aq) and I(aq)?[2] (e) A student wanted to investigate how changing the concentration of I would affect the rate of reaction. Explain how this investigation could be carried out.[2] (f) 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. The student suggested modi cations to the method as used in (a). In each case, state what the effect would be on the reaction time in Experiment 1 and explain how these changes would affect the possible errors in the measurements. Suggested modi cation 1 The reaction was carried out using the same volumes of all reagents but with the concentrations of FB 1 and FB 2 being double their original values.Suggested modi cation 2 The reaction was carried out using half the volume of all reagents.[4]UCLES 2015 9701/36/O/N/15<br /> (g) (i) Which of the experiments you carried out in (a) had the greatest percentage error in the reaction time?(ii) Calculate this percentage error. Assume that the error in measuring the reaction time is 0.5 s. PMT percentage error =% [2] [Total: 27]UCLES 2015 9701/36/O/N/15 [Turn over<br />

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