Determining the overall reaction and the molecularity of an elementary reaction : The following two - step mechanism hqas proposed for the gas-phase decomposition of nitrous oxide `(N_(2)O)` :
Step 1: `N_(2)O(g)rarrN_(2)(g)+O(g)`
Step 2: `N_(2)O(g)+O(g)rarrN_(2)(g)+O_(2)(g)`
(i) Write the chemical equation for the overall reaction
(ii) Identify any reaction intermediates
(iii) What is the molecularity of each of the elementary reactions ?
(iv) What is the molecularity of the overvall reaction
Strategy : The step-by -step pathway by which a reaction occurs is called its mechanism. Some reactions takes place in a single step, but most reactions occur in a series of elementary steps. To identify intermediates and molecularity, look at the indicidual steps

Explanation

(i) The overall reaction is the sum of the two elementry steps
Step 1: `N_(2)O(g)rarrN_(2)(g)+O(g)`, Elementary reaction
Step 2: `N_(2)O(g)+O(g)rarrN_(2)(g)+O_(2)(g)` , Elementary reaction
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`2N_(2)O(g)+calcel(O(g))rarr 2N_(2)(g)+cancel(O(g))+O_(2)(g)`
`2N_(2)O(g)rarr2N_(2)(g)+O_(2)(g)` overall reaction
(ii) The oxygen atom, O(g), is a reaction intermediate because it is formed in the first elementry step and onsumed in the second step.
(iii) The first elementary reaction is unimolecular because it involves a single reactant molecule while the second elementary reaction is bimolecular because it involves two reactant moleculaes.
(iv) It is inappropriate to use 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.