The rate of reaction for ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$ may be represented as

 In rate law, the rate of the reaction is expressed as a function of the concentration of the reactants and products present in the reaction.

The given reaction is:

${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$
For this reaction, the rate of reaction is written as:-
$-\frac{\mathrm{d}\left[{\mathrm{N}}_2\right]}{\mathrm{dt}}=-\frac{1 \mathrm{d}\left[{\mathrm{H}}_2\right]}{3 \mathrm{dt}}=+\frac{1}{2}\frac{\mathrm{d}\left[{\mathrm{NH}}_3\right]}{\mathrm{dt}}$