Question

How is order of a reaction obtained from the rate law of a reaction?

Answer 1

The order of a reaction is obtained from the rate law by adding the powers (exponents) of the concentration terms of the reactants in the rate law expression.

For example if the rate law is 

Rate = k[A]^m[B]ⁿ

Then,

m is the order with respect to reactant A.

n is the order with respect to reactant B.

The overall order of the reaction is the sum, i.e., overall order = m + n

These values of mmm and nnn are determined experimentally, not from the balanced chemical equation. So, the order of a reaction is the sum of the powers of the concentrations of reactants in the rate law.