Edexcel IAL Chemistry 11.8 Deducing reaction mechanisms

Use the rate equation to constrain the slow-stage species, split high-molecularity overall reactions into elementary steps and verify intermediates cancel.

Exam points

Reject a one-step route when the overall equation requires an improbable multi-particle collision.
Choose a slow step from rate-law species, bond-breaking demand and activation-energy evidence.
Draw a mechanism whose curly arrows, intermediates and summed steps fit the measured rate law.

[Maximum number: 2]

A reaction vessel contained nitrogen monoxide and oxygen in a 2: 1 molar ratio. The mixture was allowed to come to equilibrium forming nitrogen dioxide. The equation for the reaction is shown.

2 \mathrm{NO}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{NO}_{2}

The volume of the vessel was $15 \mathrm{dm}^{3}$ and the reaction was carried out at a constant temperature and at a pressure of 200000 Pa .

At equilibrium there was a total of 0.69625 mol of gas in the reaction vessel and the mass of oxygen was 7.000 g .

Under a different set of conditions, the reaction was carried out to find the initial rate of reaction.

Table for Question 11.8—Deduce a reaction mechanism question 1 — Edexcel A-Level Chemistry A2

The rate equation for this reaction is

\text { rate }=k[\mathrm{NO}]^{2}\left[\mathrm{O}_{2}\right]

A student proposed the mechanism shown for this reaction.

\begin{aligned} 2 \mathrm{NO} & \rightleftharpoons \mathrm{~N}_{2} \mathrm{O}_{2} \quad \text { slow } \\ \mathrm{N}_{2} \mathrm{O}_{2}+\mathrm{O}_{2} & \rightarrow 2 \mathrm{NO}_{2} \quad \text { fast } \end{aligned}

Justify whether or not this mechanism is consistent with the overall equation for the reaction and with the rate equation.

An answer that makes reference to the following points:

- adding the two steps together gives the overall equation

- the steps do not match the rate equation because the slow step should be the second step

Allow the two steps match the overall equation as the reactants and products are the same Allow $\mathrm{N}_{2} \mathrm{O}_{2}$ is formed then reacts / cancels out / is an intermediate Ignore just the overall equation is $2 \mathrm{NO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}$

Allow it does not match because there is no oxygen in the slow step / rate determining step / rds Allow because in this mechanism oxygen is zero order / is not first order

Allow because with these steps the rate equation would be rate $=k[\mathrm{NO}]^{2}$

(2)

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11.8—Deduce a reaction mechanism question 1