The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.
For the reaction aA + bB → products
The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.
Rate = k[A]^x[B]^y
Order of a reaction
The sum of the powers to which the concentration terms are raised in the rate law expression.
For the expression Rate = k[A]^x[B]^y, the order of the reaction is x+y. The order of the reaction is represented by n.
When n = 1, the reaction said to be first order reaction.
n = 2 second order reaction etc.
There are number of reactions where rate of reaction is independent of concentration of reactants. The order of reaction is zero.