CIE Nov 2017 v3 Paper 5 Q1

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PMT Iodide ions, I, and persulfate ions, S2O8 2, react according to the following equation. 2I(aq) + S2O8 2(aq) I2(aq) + 2SO4 2(aq) The rate of reaction between these ions can be determined from the time it takes for a certain amount of iodine, I2(aq), to be produced.starch indicator. A mixture of solutions is prepared, containing known volumes ofaqueous ammonium persulfate, (NH4)2S2O8(aq),aqueous sodium thiosulfate, Na2S2O3(aq),A known volume of aqueous potassium iodide, KI(aq), is added to this mixture and a timer is started. After the reactants are mixed, they react slowly to produce iodine, I2(aq). Any iodine initially produced is removed by a reaction with thiosulfate ions. I2(aq) + 2S2O3 2(aq) 2I(aq) + S4O6 2(aq) Iodine, I2(aq), is continuously removed until all of the thiosulfate ions have been used up. After that time any I2(aq) that is produced turns the starch indicator blue. The time of the first appearance of the blue colour is recorded. This procedure is repeated with different volumes of reactants, keeping the total volume of the reaction mixture constant by adding the required volume of distilled water. You are to plan a series of experiments to determine the effect of changing the concentration of iodide ions on the rate of reaction. You are provided with the following materials. solid ammonium persulfate, (NH4)2S2O8(s) 0.20 mol dm3 aqueous KI, a source of I(aq) 0.0050 mol dm3 aqueous Na2S2O3, a source of S2O3 starch indicator 2(aq) (a) (i) Calculate the mass of (NH4)2S2O8(s) that would be required to prepare 250 cm3 of a standard solution of concentration 1.00 mol dm3. [Ar values: N, 14.0; H, 1.0; S, 32.1; O, 16.0] mass of (NH4)2S2O8(s) =g [1]UCLES 2017 9701/53/O/N/17<br /> PMT (ii) Describe how, after weighing the mass calculated in (i), you would prepare this standard solution for use in your experiment. Give the name and capacity, in cm3, of any apparatus used.[2] (iii) Explain how the use of starch solution improves the accuracy of the experiment.[1] (b) A student planned five experiments to investigate the effect of iodide concentration, [I], on the rate of reaction. The table shows the volumes used in experiment 1. Complete the table for experiments 2 to 5. volume of 1.00 mol dm3 (NH4)2S2O8(aq) / cm3 volume of 0.20 mol dm3 KI(aq) / cm3 25.0 10.0 volume of water / cm3 0.0 experiment volume of 0.0050 mol dm3 Na2S2O3(aq) / cm3 5.0 5.0 5.0 5.0 5.0 volume of starch solution / cm3 1.0 1.0 1.0 1.0 1.0 [3]UCLES 2017 9701/53/O/N/17 [Turn over<br /> PMT (c) In a different experiment, a student mixed the following solutions and measured the time taken for the reaction.10.0 cm3 of 1.00 mol dm3 (NH4)2S2O8(aq)5.0 cm3 of 0.0050 mol dm3 Na2S2O3(aq)5.0 cm3 of 0.20 mol dm3 KI(aq)1.0 cm3 of starch indicator (i) The time taken for the blue colour to appear was 134 seconds (to the nearest second). Calculate the rate of production of moles of I2, in mol dm3 s1. rate of production of moles of I2 =mol dm3 s1 [3] (ii) What should the student have done to make sure that the results were reliable?[1] (iii) The 5.0 cm3 of 0.0050 mol dm3 Na2S2O3(aq) was measured using a 50 cm3 burette which had graduations every 0.1 cm3. Calculate the maximum percentage error in the measured volume of this solution. percentage error =% [1] (d) A second student tried to perform the same experiment but found that the reaction mixture turned blue immediately after KI(aq) was added. State what error the student had made.[1]UCLES 2017 9701/53/O/N/17<br /> PMT (e) The following information gives some of the hazards associated with the chemicals used in the procedure. Ammonium persulfate Solid is oxidising and hazardous to the environment. Contact with combustible material may cause fire. It is classified as health hazard, is harmful if swallowed and is irritating to eyes, respiratory system and skin. Solutions equal to or more concentrated than 0.2 mol dm3 should be labelled health hazard and hazardous to the environment. Solutions equal to or more concentrated than 0.05 mol dm3 but less concentrated than 0.2 mol dm3 should be labelled health hazard. Potassium iodide Sodium thiosulfate All solutions are low hazard. All solutions are low hazard. Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer.[1] [Total: 14]UCLES 2017 9701/53/O/N/17 [Turn over<br /> BLANK PAGE PMTUCLES 2017 9701/53/O/N/17<br />

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