Chemistry 9701 · AS & A Level · Reaction kinetics

Reaction kinetics — practice question

Dinitrogen pentoxide, $\text{N}_2\text{O}_5$, is put into an inert solvent, and the decomposition of $\text{N}_2\text{O}_5$ is monitored. Nitrogen dioxide is formed and 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 used to track the rate of this reaction from the information given.

(b(i))[1]

Explain what is meant by the term half-life of a reaction.

(b)

In a different experiment, the decomposition of $\text{N}_2\text{O}_5(\text{g})$ is studied. $\text{N}_2\text{O}_5(\text{g}) \rightarrow 2\text{NO}_2(\text{g}) + \tfrac{1}{2}\text{O}_2(\text{g})$ The reaction is first order with respect to $\text{N}_2\text{O}_5$. The graph can be used, via half-lives, to verify this.

(b(ii))[1]

Determine the half-life of this reaction. Show your working on the graph.

(b(iii))[1]

Suggest the effect on 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 determine the reaction rate at $200\,\text{s}$. Show your working.

(c(ii))[1]

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

(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 $\text{rate} = k[\text{NO}_2][\text{O}_3]$. Suggest one possible two-step mechanism for this reaction.

Worked solution & mark scheme

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