The dissociation of $\mathrm{HI}$ molecules as shown below, occurs at a temperature of 629 K The rate constant

$\begin{array}{l}k=3.02\times {10}^{-5}{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}\\ \mathrm{HI}\left(g\right)⟶\frac{1}{2}{\mathrm{H}}_2\left(g\right)+\frac{1}{2}{\mathrm{I}}_2\left(g\right)\end{array}$

What is order of reaction?

The given reaction is,

$\begin{array}{l}\mathrm{HI}\left(g\right)⟶\frac{1}{2}{\mathrm{H}}_2\left(g\right)+\frac{1}{2}{\mathrm{I}}_2\left(g\right)\end{array}$

Rate constant is,

$\begin{array}{l}k=3.02\times {10}^{-5}{\mathrm{M}}^{-1}{\mathrm{s}}^{-1}\end{array}$

Here the unit of rate constant is ${\mathrm{M}}^{-1}{\mathrm{s}}^{-1}$, which is given for the second order reaction.

Thus, the order of the reaction is 2.