Order of reaction:
The sum of concentration terms on which the rate of a reaction actually depends as observed experimentally is called the order of the reaction i.e. the sum of the powers or exponents to which the concentration or pressure terms are raised in order to determine the rate of the reaction.
Order of a reaction is an experimental quantity. A reaction is said to be of the zero order, first order, second order, and third order if some of concentration terms is equal to 0, 1, 2 and 3 respectively. Example:
Photosynthesis of HCl:
H2 (g) + Cl2 (g) + hν —————> 2HCl
Experimentally, it is found that the rate of reaction is independently of the concentration of H2 and Cl2. Hence it is a reaction of the zero order. Hence for such reactions:
Rate = k
Dissociation of N2O5
2 N2O5 ————> 4 NO2 + O2
Experimentally, it is found that the rate of reaction is directly proportional to [N2O5] i.e. it depends only on one concentration term. Hence the reaction is said to be of first order.