Iodine monochloride, ICl, is a yellow-brown gas. It reacts with hydrogen gas under certain conditions as shown. 2ICl(g) + H2(g) → 2HCl(g) + I2(g) Experiments are performed using different starting concentrations of ICl and H2. The initial rate of each reaction is measured. The following results are obtained. experiment | [ICl]/mol dm⁻³ | [H2]/mol dm⁻³ | relative rate of reaction ---|---|---|--- 1 | 4.00 × 10⁻³ | 4.00 × 10⁻³ | 1.00 2 | 4.00 × 10⁻³ | 7.00 × 10⁻³ | 1.75 3 | 4.00 × 10⁻³ | 1.00 × 10⁻² | 2.50 4 | 5.00 × 10⁻³ | 8.00 × 10⁻³ | 2.50 5 | 7.00 × 10⁻³ | 8.00 × 10⁻³ | 3.50

1. (a)Identify a change, taking place in the reaction mixture, that would enable measurements of the rate of this reaction to be made.[1]
2. (b)Use the data in the table to show that the reaction is first order with respect to H2(g).[1]
3. (c)Use the data in the table to show that the reaction is first order with respect to ICl(g).[1]
4. (d)Complete the rate equation for the reaction between ICl(g) and H2(g).[1]
5. (e)Use experiment 3 to calculate a numerical value for the rate constant, k.[1]
6. (f)The reaction 2ICl(g) + H2(g) → 2HCl(g) + I2(g) is first order with respect to ICl(g) and first order with respect to H2(g). Suggest a mechanism for this reaction. You should assume • the mechanism has two steps, • the first step is much slower than the second step.[2]
7. (g)(i)Describe two ways of using these results to show the reaction is first order with respect to H2(g) concentration.[3]
8. (g)(ii)Explain the reason for using a large excess of ICl(g).[1]
9. (h)A chemical reaction may be speeded up by the presence of a catalyst. Explain why a catalyst increases the rate of a chemical reaction.[1]