The reaction between iodide ions, I⁻(aq), and aqueous hydrogen peroxide, H2O2(aq), takes place in acidic conditions. reaction 1 2I⁻(aq) + H2O2(aq) + 2H⁺(aq) → I2(aq) + 2H2O(l) A student carries out a series of experiments to investigate the order of reaction with respect to the concentration of I⁻(aq) ions. The student does this by measuring the time taken for a fixed amount of iodine to form. A known amount of aqueous thiosulfate ions, S2O3²⁻(aq), in the reaction mixture react with I2(aq) formed in reaction 1. reaction 2 I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq) After all of the S2O3²⁻(aq) ions have reacted in reaction 2, any further I2(aq) formed is detected using starch indicator. The following materials are used: • 50 cm³ beaker containing the correct mass of solid potassium iodide crystals needed to make 250.0 cm³ of 0.100moldm⁻³ KI(aq) • 0.100 mol dm⁻³ Na2S2O3(aq) • 0.100 mol dm⁻³ H2O2(aq) • 0.200 mol dm⁻³ H2SO4(aq) • starch indicator • distilled water.

1. (a)A second student looked at the equation for reaction 1 and stated the order with respect to the concentration of I⁻(aq) ions must be second order because the balanced equation contains 2I⁻(aq). Suggest why a balanced equation alone cannot be used to determine the order of a reaction.[1]
2. (b)Describe how the student makes 250.0 cm³ of 0.100 moldm⁻³ KI(aq) starting from the sample of solid potassium iodide in the 50 cm³ beaker. Give the name and size of any key apparatus used. Describe how the student ensures the volume is exactly 250.0cm³. You may wish to write your answer using a series of numbered steps.[3]
3. (c)The student carries out Experiment 1 using the following steps. step 1 Add 25 cm³ of 0.200 moldm⁻³ H2SO4(aq) to a conical flask. step 2 Add 20.00 cm³ of distilled water to the conical flask from a burette. step 3 Add 5.00 cm³ of 0.100moldm⁻³ KI(aq) to the conical flask from a burette. step 4 Add 5.00 cm³ of 0.100 moldm⁻³ Na2S2O3(aq) to the conical flask from a burette. step 5 Add 2cm³ of starch indicator to the conical flask. step 6 Use a burette to add 10.00 cm³ of 0.100 moldm⁻³ H2O2(aq) to a small beaker. step 7 Add the contents of the beaker to the conical flask and start a timer immediately. Stop the timer when the starch indicates the presence of I2(aq). The student carries out a further six experiments by repeating steps 1 to 7, using the volumes shown in Table 2.1.
4. (c)(i)State how the student could improve the reliability of the experiment.[1]
5. (c)(ii)State the independent variable in Experiments 1 to 7.[1]
6. (c)(iii)In Experiment 5, the starch indicator changed colour immediately on adding H2O2(aq). The student realised an error had been made. Suggest which step was missed in Experiment 5.[1]
7. (d)The rate equation is represented as rate = k[I⁻]ⁿ. • [I⁻] is proportional to the volume of KI(aq) • n is the order of reaction with respect to I⁻ • rate is proportional to 1/t • log (rate) = log k + nlog [I⁻]
8. (d)(i)Use the results from the student's experiments in (c) to complete Table 2.2. V is the volume of KI(aq) and t is the time taken for the colour to change. Give all values to three significant figures. The results for Experiment 5 should not be used.[2]
9. (d)(ii)Plot a graph on the grid to show the relationship between log V and log (1/t). Use a cross (x) to plot each data point. Draw a line of best fit.[2]
10. (d)(iii)A timing error caused the most anomalous point on the graph. Circle this point and explain the error in timing which led to this point.[1]
11. (d)(iv)Use your graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be selected from your line of best fit. Give the gradient to two decimal places. coordinates 1 .......................... coordinates 2 .......................... gradient = ..........................[2]
12. (d)(v)The total percentage error from measurements is determined to be 5.25%. The true order of reaction is 1. Use this and your gradient from (d)(iv) to determine whether the error in the experiment could be accounted for by error from measurements or is caused by other factors. Show any working.[1]

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