The rate of the reaction H2(g) + I2(g) ⇌ 2HI(g) is studied.

1. (a)A small amount of H2(g) is mixed with a large excess of I₂(g) at a temperature of 400 K and the reaction is monitored. The graph obtained is shown. [Figure 1.1]
2. (a)(i)Suggest why a large excess of I2(g) is used in this experiment.[1]
3. (a)(ii)The reaction is first order with respect to H2(g). Use data from the graph to confirm this statement.[2]
4. (b)Three separate experiments were carried out at 400K with different starting concentrations of H2(g) and I2(g). The results are shown in the table. experiment [H2(g)]/moldm³ [I2(g)]/mol dm-3 rate of reaction /mol dm-3 s-1 1 1.0 x 10-2 1.0 x 10-2 2.0 × 10-17 2 1.0 x 10-1 1.0 × 10-1 2.0 × 10-15 3 5.0 × 10-1 5.0 × 10-1 5.0 × 10-14
5. (b)(i)Use the data, and the order of reaction with respect to H2(g) given in (a)(ii), to deduce the order of reaction with respect to I2(g). Explain your answer, giving data in support of your explanation.[3]
6. (b)(ii)Use information from (a)(ii) and your answer to (b)(i) to write the rate equation for the forward reaction. rate =[1]
7. (b)(iii)Use your rate equation and data from experiment 1 to calculate the value of the rate constant, k, for the forward reaction at 400K. Include units for k. k = units =[2]
8. (c)At 400K the rate constant for the forward reaction is approximately 1000 times greater than the rate constant for the backward reaction. The overall orders of the forward and backward reactions are the same. forward reaction H2(g) + I2(g) → 2HI(g) backward reaction 2HI(g) → H2(g) + I2(g)
9. (c)(i)Use this information to explain what will happen if equal concentrations of HI(g), H2(g) and I2(g) are mixed at 400K. You should comment on: • the relative initial rates of the forward and backward reactions • the position of the equilibrium reached.[1]
10. (c)(ii)At 700 K the rate constant for the forward reaction is approximately 50 times greater than the rate constant for the backward reaction. Use this information and the information in (c)(i) to deduce the signs of the ΔH values of the forward and backward reactions. Explain your answer.[2]

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