Order of a chemical reaction : The order of a chemical reaction is defined as the number of molecules (or atoms) whose concentrations influence the rate of the chemical reaction.
OR
The order of a chemical reaction is defined as the sum of the powers (or exponents) to which the concentration terms of the reactants are raised in the rate law expression for the given reaction.
Explanation :
Consider a reaction,
n1A + n2B → Products
Where n1 moles of A react with n2 moles of B.
The rate of this reaction can be expressed by the rate law equation as,
R = k [A]n1 [B]n2
Where k is the rate constant of the reaction, hence, the order of the reaction is n – n1 + n2, (observed, experimentally).
If n = 1,
The reaction is called the first order reaction,
If n = 2,
It is called the second order reaction, etc.
If n = 0,
It is called the zero order reaction.
e.g., photochemical reaction of H2(g) and Cl2(g).
