Example for second order reaction

The decomposition reaction of ${\mathrm{N}}_2{\mathrm{O}}_5$ is a unimolecular reaction

This is based on the elementary step reaction mechanism. In the given reactions if one of the reactant is water molecule, the order of a reaction is less than the original. Because water is in excess amount there is no change in its concentration. Hence, for the following two reactions

$\mathrm{RCI}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{ROH}+\mathrm{HCl}$

${\mathrm{RCOOR}}^1+{\mathrm{H}}_2\mathrm{O} \mathrm{f} \mathrm{RCOOH} +{\mathrm{R}}^1\mathrm{OH}$

The order should be one (first order) in each case. For the following reaction

$2\hspace{0.17em}{\mathrm{NO}}_2\to 2\mathrm{NO}+{\mathrm{O}}_2$

$\mathrm{Rate}=\mathrm{K}{\left[{\mathrm{NO}}_2\right]}^2$

Therefore, the order of reaction is 2.