Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Nitrogen monoxide, NO, reacts with ozone, $\text{O}_3$. $\text{NO(g)} + \text{O}_3\text{(g)} \rightarrow \text{NO}_2\text{(g)} + \text{O}_2\text{(g)}$. The reaction is first order with respect to both NO and $\text{O}_3$. At $298\ \text{K}$, the rate constant is $k = 11\,500\ \text{mol}^{-1}\,\text{dm}^3\,\text{s}^{-1}$.
(a(i))[1]

Fill in the rate equation for this reaction.

(a(ii))[2]

In a reaction, the initial concentrations of NO and $\text{O}_3$ are each $1.20 \times 10^{-6}\ \text{mol dm}^{-3}$. Calculate the initial rate of the reaction. State its units.

(a(iii))[1]

The reaction described in (a)(ii) is followed over time. Predict whether the graph of $[\text{NO}]$ against time, under these conditions, shows a constant half-life or not. Explain your answer.

(b(i))[1]

Nitrous oxide, $\text{N}_2\text{O}$, breaks down into its elements. $2\text{N}_2\text{O(g)} \rightarrow 2\text{N}_2\text{(g)} + \text{O}_2\text{(g)}$. At a high temperature, a small amount of platinum wire is added to a large amount of nitrous oxide. The reaction has zero order kinetics. The platinum wire acts as a catalyst. Sketch, on the axes below, a graph of reaction rate against time for the catalysed decomposition of $\text{N}_2\text{O}$ under these conditions.

(b(ii))[1]

Sketch a graph, on the axes below, of $[\text{N}_2\text{O}]$ against time for this reaction.

(b(iii))[3]

Platinum acts as a heterogeneous catalyst in this reaction. Describe the mode of action of a heterogeneous catalyst.

(b(iv))[1]

Suggest a reason why this reaction has zero order kinetics when the amount of nitrous oxide is large and the amount of platinum is small.

Worked solution & mark scheme

This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: Rate law: $\text{rate} = k[\text{NO}][\text{O}_3]$

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