Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

In aqueous solution, acidified iodate(V) ions, $\text{IO}_3^-$, react with iodide ions, as shown: $\text{IO}_3^- + 6\text{H}^+ + 5\text{I}^- \rightarrow 3\text{I}_2 + 3\text{H}_2\text{O}$. The initial rate of the reaction is then studied.
(a(i))[1]

Explain what the term order of reaction means.

(a(ii))[1]

Complete Table 3.2 so that it shows the order of reaction with respect to $[\text{IO}_3^-]$, $[\text{H}^+]$, $[\text{I}^-]$, together with the overall order of reaction.

(a(iii))[1]

Use your answer to (a)(ii) to sketch the lines in Fig. 3.1 that show how the initial rates depend on the concentrations of $[\text{IO}_3^-]$ and $[\text{I}^-]$.

(a(iv))[2]

Use data from Table 3.1 to calculate the rate constant, $k$, for this reaction. Include the units of $k$.

(a(v))[1]

Use data from Table 3.1 to calculate the concentration of hydrogen ions, $[\text{H}^+]$, in experiment 2.

(a(vi))[1]

This reaction is carried out again in two separate experiments. The experiments are done at the same temperature and with the same concentrations of $\text{I}^-$ and $\text{IO}_3^-$. One experiment is at pH $1.0$ and the other is at pH $2.0$. Calculate the value of $\dfrac{\text{rate at pH }1.0}{\text{rate at pH }2.0}$.

(b)[3]

In aqueous solution, iron(III) ions react with iodide ions, as shown. $2\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_2$ The initial rate of reaction is first order with respect to $\text{Fe}^{3+}$ and second order with respect to $\text{I}^-$. The mechanism for this reaction has three steps. In each step, only two ions react together. Suggest equations for the three steps of this mechanism. Identify the rate-determining step.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: The exponent to which a reactant concentration is raised in the rate equation

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