Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Fluorine reacts with chlorine dioxide, $\text{ClO}_2$, in the equation shown below. $\text{F}_2(g) + 2\text{ClO}_2(g) \rightarrow 2\text{FClO}_2(g)$ The reaction is first order in the concentration of $\text{F}_2$ and also first order in the concentration of $\text{ClO}_2$. No catalyst is present.
(a(i))[2]

Propose a two-step mechanism for this reaction. step 1 $\rightarrow$ step 2 $\rightarrow$

(a(ii))[1]

State which step is the rate-determining step in this mechanism. Justify your answer.

(b(i))[1]

Complete the rate equation for this reaction, and state the overall order of the reaction. rate $=$ overall order of reaction $=$

(b(ii))[1]

Apply your rate equation in (i) to find the rate of the reaction when the concentrations of $\text{F}_2$ and $\text{ClO}_2$ are each $2.00 \times 10^{-3}\,\text{mol dm}^{-3}$. rate $=$ $\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots$ $\text{mol dm}^{-3}\,\text{s}^{-1}$

(c(i))[2]

In another set of conditions, with a large excess of $\text{ClO}_2$, the rate equation is shown here. rate $= k_1[\text{F}_2]$ The half-life, $t_{\frac{1}{2}}$, for the concentration of $\text{F}_2$ is $4.00\,\text{s}$ under these conditions. Calculate the numerical value of $k_1$, stating its units. Give your answer to three significant figures.

(c(ii))[1]

An experiment is carried out under these conditions with an initial concentration of $\text{F}_2$ of $0.00200\,\text{mol dm}^{-3}$. Draw a graph on the grid in Fig. 1.1 to show how the concentration of $\text{F}_2$ varies over the first $12\,\text{s}$ of the reaction.

(c(iii))[1]

Use your graph in Fig. 1.1 to determine the rate of the reaction when the concentration of $\text{F}_2$ is $0.00100\,\text{mol dm}^{-3}$. Show the method you use on the graph. rate $=$ $\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots$ $\text{mol dm}^{-3}\,\text{s}^{-1}$

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: A valid answer gives initiation and propagation steps that combine to the overall equation: $\mathrm{F_2 + ClO_2 \to FClO_2 + F}$ and $\mathrm{ClO_2 + F \to FClO_2}$ (or any equivalent alternative)

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