Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Dinitrogen pentoxide, $\text{N}_2\text{O}_5$, is placed in an inert solvent (solv), and its decomposition rate is studied. The reaction gives nitrogen dioxide, which stays dissolved, together with oxygen gas. $\text{N}_2\text{O}_5\text{(solv)} \rightarrow 2\text{NO}_2\text{(solv)} + \tfrac{1}{2}\text{O}_2\text{(g)}$
(a)[1]

Suggest which measurements could be taken to monitor the rate of this reaction from the information provided.

(b(i))[1]

Explain what the term half-life of a reaction means.

(b)

In a different experiment, the decomposition rate of $\text{N}_2\text{O}_5(\text{g})$ is examined. $\text{N}_2\text{O}_5(\text{g}) \rightarrow 2\text{NO}_2(\text{g}) + \tfrac{1}{2}\text{O}_2(\text{g})$ The graph displays the data obtained. The reaction is first order with respect to $\text{N}_2\text{O}_5$. This may be verified from the graph by using half-lives.

(b(ii))[1]

Determine the half-life of this reaction. Show how you arrive at your answer on the graph. half-life $= \ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots$ s

(b(iii))[1]

Suggest what happens to the half-life of this reaction if the initial concentration of $\text{N}_2\text{O}_5$ is halved.

(c(i))[2]

Use the graph in 5(b) to find the rate of reaction at $200\,\text{s}$. Show how you worked it out. rate $= \ldots\ldots\ldots\ldots\ldots\ldots\ldots$ units $= \ldots\ldots\ldots\ldots\ldots\ldots\ldots$

(c(ii))[1]

The rate equation for this reaction is given below. rate $= k[\text{N}_2\text{O}_5]$ Using your result from (c)(i), calculate the rate constant, $k$, for this reaction and give its units. $k = \ldots\ldots\ldots\ldots\ldots\ldots$ units $\ldots\ldots\ldots\ldots\ldots\ldots$

(d)[2]

Nitrogen dioxide reacts with ozone, $\text{O}_3$, as shown. $2\text{NO}_2 + \text{O}_3 \rightarrow \text{N}_2\text{O}_5 + \text{O}_2$ The rate equation for this reaction is rate $= k[\text{NO}_2][\text{O}_3]$. Suggest a possible two-step mechanism for this reaction.

Worked solution & mark scheme

This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: Record volume / mass / oxygen evolved (or absorbance) versus time

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