Question 12.9: The following two-step mechanism has been proposed for the g......
The following two-step mechanism has been proposed for the gas-phase decomposi-tion of nitrous oxide (N_{2}O) :
(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?
STRATEGY
To find the overall reaction, sum the elementary steps. To identify intermediates and
molecularity, look at the individual steps.
(a) The overall reaction is the sum of the two elementary steps:
Step 1. N_{2}O (g)\longrightarrow N_{2} (g) + O_{2} (g) \quad\quad\quad\quad\quad\quad \text{Elementary reaction} \\ \text{Step 2.} N_{2}O (g) + O (g) \longrightarrow N_{2} (g) + O_{2} (g) \quad\quad \text{Elementary reaction}
2 N_{2}O (g) + O (g) \longrightarrow 2 N_{2} (g) + \cancel{O (g)} + O_{2} (g) \\ 2 N_{2}O (g) \longrightarrow 2 N_{2} (g) + O_{2} (g) \quad\quad\quad\quad\quad\quad\quad\quad\quad \text{Overall reaction}
(b) The oxygen atom is a reaction intermediate because it is formed in the first elemen-tary step and consumed in the second step.
(c) The first elementary reaction is unimolecular because it involves a single reactant molecule. The second elementary step is bimolecular because it involves two reactant molecules.
(d) It’s inappropriate to use the word molecularity in connection with the overall reac-tion because the overall reaction does not describe an individual molecular event. Only an elementary reaction can have a molecularity.