Iodide ions, I⁻, and persulfate ions, S₂O₈²⁻, react according to the following equation. 2I⁻(aq) + S₂O₈²⁻(aq) → I₂(aq) + 2SO₄²⁻(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. • A mixture of solutions is prepared, containing known volumes of ◦ aqueous ammonium persulfate, (NH₄)₂S₂O₈(aq), ◦ aqueous sodium thiosulfate, Na₂S₂O₃(aq), ◦ starch indicator. • 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) + 2S₂O₃²⁻(aq) → 2I⁻(aq) + S₄O₆²⁻(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 dm⁻³ aqueous KI, a source of I⁻(aq) 0.0050 mol dm⁻³ aqueous Na₂S₂O₃, a source of S₂O₃²⁻(aq) starch indicator

1. (a)(i)Calculate the mass of (NH₄)₂S₂O₈(s) that would be required to prepare 250cm³ of a standard solution of concentration 1.00 mol dm⁻³. [Aᵣ values: N, 14.0; H, 1.0; S, 32.1; O, 16.0] mass of (NH₄)₂S₂O₈(s) = ..................................... g [1][1]
2. (a)(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]
3. (a)(iii)Explain how the use of starch solution improves the accuracy of the experiment.[1]
4. (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 dm⁻³ (NH₄)₂S₂O₈(aq) /cm³ volume of 0.20 mol dm⁻³ KI(aq) /cm³ volume of water /cm³ volume of 0.0050 mol dm⁻³ Na₂S₂O₃(aq) /cm³ volume of starch solution /cm³ experiment 1 25.0 10.0 0.0 5.0 1.0 2 5.0 1.0 3 5.0 1.0 4 5.0 1.0 5 5.0 1.0 [3][3]
5. (c)(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 dm⁻³ s⁻¹. rate of production of moles of I₂ = .............................................................. mol dm⁻³ s⁻¹ [3][3]
6. (c)(ii)What should the student have done to make sure that the results were reliable?[1]
7. (c)(iii)The 5.0 cm³ of 0.0050 mol dm⁻³ 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][1]
8. (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]
9. (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 dm⁻³ should be labelled health hazard and hazardous to the environment. Solutions equal to or more concentrated than 0.05 mol dm⁻³ but less concentrated than 0.2 mol dm⁻³ should be labelled health hazard. Potassium iodide All solutions are low hazard. Sodium thiosulfate 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]