Chapter 12

Q. 12.14

Identifying Intermediates and Molecularity in a Reaction Mechanism

The following two-step mechanism has been proposed for the gas-phase decomposition of nitrous oxide (N_2O):

(a) Write the chemical equation for the overall reaction.

(b) Identify any reaction intermediates.

(c) What is the molecularity of each of the elementary reactions?

(d) What is the molecularity of the overall reaction?


To find the overall reaction, sum the elementary steps. To identify intermediates and molecularity, look at the individual steps.

worked example 12.14


Verified Solution

The overall reaction is the sum of the two elementary steps:

Step 1.      \mathrm{N}_2 \mathrm{O}(g) \longrightarrow \mathrm{N}_2(g)+\mathrm{O}(g)  Elementary reactionStep 2.     \mathrm{N}_2 \mathrm{O}(g)+\mathrm{O}(g) \longrightarrow \mathrm{N}_2(g)+\mathrm{O}_2(g)   Elementary reaction


\begin{aligned}2 \mathrm{~N}_2 \mathrm{O}(g)+ \ \cancel{O(g)} & \longrightarrow 2 \mathrm{~N}_2(g)+ \ \cancel{O(g)} +\mathrm{O}_2(g) \\2 \mathrm{~N}_2 \mathrm{O}(g) & \longrightarrow 2 \mathrm{~N}_2(g)+\mathrm{O}_2(g)\end{aligned} Overall reaction

(b) The oxygen atom is a reaction intermediate because it is formed in the first elementary step and consumed in the second step.

(c) The first elementary reaction is unimolecular because it involves a single reactant molecule. The second step is bimolecular because it involves two reactant atoms or molecules.

(d) It’s inappropriate to use the word molecularity in connection with the overall reaction because the overall reaction does not describe an individual molecular event. Only an elementary reaction can have a molecularity.