CIE Nov 2016 v3 Paper 3 Q2

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PMT In Question 1, hydrogen peroxide was oxidised to form oxygen. Hydrogen peroxide can also be reduced to form water. This occurs, for example, when hydrogen peroxide, H2O2, reacts with iodide ions, I(aq), to form iodine, I2(aq). H2O2(aq) + 2H+(aq) + 2I(aq)I2(aq) + 2H2O(l) The rate of this reaction can be measured by adding acidified hydrogen peroxide, H2O2, to a mixture of iodide ions, I , thiosulfate ions, S2O3 2, and starch indicator. 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. In this experiment you will measure two reaction times and use one of these to calculate the concentration of the thiosulfate ions in FA 6. FA 3 is 1.0 mol dm3 sulfuric acid, H2SO4. FA 4 is the diluted hydrogen peroxide, H2O2, you prepared in 1(a). FA 5 is 1.0 mol dm3 potassium iodide, KI. FA 6 is aqueous sodium thiosulfate, Na2S2O3. starch indicator Read through the instructions carefully before starting any practical work. (a) MethodUse the measuring cylinder to transfer 20 cm3 of FA 3 into the 100 cm3 beaker. Use the measuring cylinder to add 10 cm3 of FA 4 into the 100 cm3 beaker. Rinse the measuring cylinder with distilled water and drain. Use the measuring cylinder to transfer 20 cm3 of FA 5 into the 250 cm3 beaker. Use the measuring cylinder to transfer 20 cm3 of FA 6 into the 250 cm3 beaker. Use the measuring cylinder to transfer 10 cm3 of starch indicator into the 250 cm3 beaker. Add the contents of the 100 cm3 beaker to the 250 cm3 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. Rinse both beakers and drain. Keep all FA solutions and starch indicator for use in (c). reaction time =s [2]UCLES 2016 9701/33/O/N/16<br /> PMT (b) Calculations Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) The expression for the rate of reaction is shown. rate = concentration of iodine at the reaction time reaction time The concentration of iodine at the reaction time is the concentration of iodine that would have been present at the time the blue-black colour appeared if no thiosulfate ions had been added. In this experiment, you should assume the rate = 2.61105 mol dm3 s1. Use this value for the rate to calculate the concentration of iodine that would have been present at the reaction time if no thiosulfate ions had been added. concentration of I2 =mol dm3 (ii) Use your answer to (i) and the total reaction volume, to calculate the number of moles of iodine that would have been present at the reaction time if no thiosulfate ions had been added. (If you were unable to calculate an answer to (i), you should use the value of 1.32103 mol dm3. This may not be the correct value.) (iii) Calculate the number of moles of thiosulfate ions that reacted with the moles of I2, calculated in (ii). I2(aq) + 2S2O3 2(aq)2I (aq) + S4O6 2(aq) moles of I2 =mol (iv) Use your answer to (iii) to calculate the concentration of thiosulfate ions in FA 6. moles of S2O3 2 =mol concentration of S2O3 2 in FA 6 =mol dm3 [4]UCLES 2016 9701/33/O/N/16 [Turn over<br /> (c) Repeat the experiment in (a) using the following quantities of each reagent. PMTadd to the 100 cm3 beaker 20 cm3 of FA 3 10 cm3 of FA 4 add to the 250 cm3 beaker 20 cm3 of FA 5 10 cm3 of FA 6 10 cm3 of starch indicator 10 cm3 of distilled water Add the contents of the 100 cm3 beaker to the 250 cm3 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. reaction time =s [1] (d) (i) Explain the relationship between the value of the reaction time in (a) and the value of the reaction time in (c).(ii) A student states that the error in the total volume of the reaction mixture in (a) is the same as the error in the total volume of the reaction mixture in (c). State whether or not you agree with the student and explain your answer.[4] [Total: 11]UCLES 2016 9701/33/O/N/16<br />

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