Order of the reaction

For the reaction

if the rate of the reaction is given as

then the sum $x + y$ gives the overall order of the reaction. Thus the overall order of a chemical reaction is given as the sum of the powers of the concentration terms in the rate law expression. For example:

i. For the reaction $\mathrm{2H_2O_2(g) \longrightarrow 2H_2O({\it l}) + O_2(g)}$ the experimentally determined rate law is rate $= k\mathrm{[H_2O_2]}$. The reaction is of first order.

ii. If the experimentally determined rate law for the reaction $\mathrm{H_2(g) + I_2(g) \longrightarrow 2\,HI(g)}$ is rate $= k\mathrm{[H_2][I_2]}$, the reaction is of first order in $\mathrm{H_2}$ and $\mathrm{I_2}$ each, and hence overall of second order.

Key points about the order of reaction

a. The order of a chemical reaction is experimentally determined.

b. The order can be integer or fractional. Look at the reaction $\mathrm{CH_3CHO(g) \longrightarrow CH_4(g) + CO(g)}$. The rate law for the reaction was found to be rate $= k\mathrm{[CH_3CHO]}^{3/2}$. Here the order of the reaction is 3/2. …