Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Iodine monochloride, $\text{ICl}$, is a yellow-brown gas. Under suitable conditions it reacts with hydrogen gas, as shown below. $2\text{ICl}(\text{g}) + \text{H}_2(\text{g}) \rightarrow 2\text{HCl}(\text{g}) + \text{I}_2(\text{g})$ A set of experiments uses different initial concentrations of $\text{ICl}$ and $\text{H}_2$. The initial rate for each reaction is then measured.
(a)[1]

Identify a change occurring in the reaction mixture that would make it possible to measure the rate of this reaction.

(b)[1]

Use the data in the table to show that the reaction is first order with respect to $\text{H}_2(\text{g})$.

(c)[1]

Use the data in the table to show that the reaction is first order with respect to $\text{ICl}(\text{g})$.

(d)[1]

Complete the rate equation for the reaction between $\text{ICl}(\text{g})$ and $\text{H}_2(\text{g})$.

(e)[1]

Use experiment 3 to work out a numerical value for the rate constant, $k$. $k = \ldots$

(f)[2]

The reaction $2\text{ICl}(g) + \text{H}_2(g) \rightarrow 2\text{HCl}(g) + \text{I}_2(g)$ is first order with respect to $\text{ICl}(g)$ and first order with respect to $\text{H}_2(g)$. Suggest a mechanism for this reaction. You should assume: • there are two steps in the mechanism, • the first step is much slower than the second step. first step $\ldots \rightarrow \ldots$ second step $\ldots \rightarrow \ldots$

(g(i))[3]

Describe two methods that use these results to show the reaction is first order with respect to concentration of $\text{H}_2(g)$.

(g(ii))[1]

Explain why a large excess of $\text{ICl}(g)$ is used.

(h)[1]

A chemical reaction may be speeded up by the presence of a catalyst. Explain why a catalyst increases the rate of a chemical reaction.

Worked solution & mark scheme

This 12-mark question has a full step-by-step worked solution and mark scheme. One marking point: colorimetry / colour change / monitor absorbance

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