1.1. A student is asked to investigate the effect of changing the concentration of iodide ions on the iodine clock reaction between hydrogen peroxide and iodide ions.

Hydrogen peroxide reacts with iodide ions to form iodine which then immediately reacts with the limited amount of thiosulfate ion as shown.

H₂O₂ (aq) + 2H⁺ (aq) + 2I^– (aq) → I₂ (aq) + 2H₂O (l)

2S₂O₃^2– (aq) + I₂ (aq) → 2I^– (aq) + S₄O₆^2– (aq)

Explain how the colour change in the indicator is caused.

1.2. The student is given 100 cm³ of 0.10 mol dm^-3 potassium iodide solution.  

Describe how they could produce a minimum of five different concentrations of potassium iodide solution. Your answer should consider possible control variables.

1.3. A second student completes the same experiment. Using their results, the second student plots a rate-concentration graph. 

Describe how the second student calculates the rate for their graph.

1.4. The graph of the second student is shown.

From the graph, the second student concludes that the reaction is first order with respect to iodide ions. 

Evaluate the second student’s conclusion.