It is an important parameter for every chemical reaction. It is always determined experimentally and cannot be written from the balanced equation. It refers to the number of reacting particles whose concentration terms determine the reaction rate.
The order of reaction may be defined as, “the sum of powers or exponents to which the concentration terms are raised in the rate law expression.”
For example, for a hypothetical reaction,
aA + bB Products
the rate law expression for this reaction is,
Rate = k [A]m [B]n
The order of the above reaction is equal to (m + n). If the sum of the power is equal to one then the reaction is called first order of reaction. If the sum of powers is two or three, the reaction is second order or third order reaction respectively. The order of reaction can also be zero or fractional.
Example of first order reaction:
(i) 
2N2O (g) 2N2 (g) + O2 (g)
The rate law for this expression is
Rate = k [N2O]; order = 1
Thus it is a first order reaction.
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