Question

Define order of reaction. How does order of a reaction differ from molecularity for a complex reaction?

Answer 1

Hint: Order of the reaction increases as number of reacting species present in the rate law expression increases. In simple words, we can say that molecularity of a reaction is the number of species reacting in a given reaction.

Complete answer:
‘ The sum of the powers of molar concentration of all the reacting species in the rate equation is known as order of the reaction.’
- The order of the reaction can be any positive number like 0,1,2,3....The order of the reaction can be a fractional number as well.
e.g. \[aA + bB \to cC + dD\]
The rate law expression of the above given reaction can be written as \[Rate = {[A]^a}{[B]^b}\]
So, order of the reaction is the sum of the powers of all the concentrations of the reacting species, hence order of the reaction will be (a+b).
Differences between order of the reaction and molecularity is as shown below.

Order of the reaction	Molecularity
- The sum of the powers of molar concentration of all the reacting species in the rate equation is known as order of the reaction	- The number of reacting species taking part in the reaction is called molecularity of the reaction.
- Order of the reaction can be a fractional number also.	- Molecularity of a reaction is always a whole number.
- The order of the reaction is derived by practical experiments.	- Molecularity of the reaction is derived with the help of the reaction mechanism.
- It is applicable to elementary as well as complex reactions.	- It is only applicable to elementary reactions. For complex reactions, it has no meaning.

Note: Remember that in complex reactions, we can count the number of species taking part in the reaction but we cannot assign perfect molecularity to them as the reaction is complex and which molecules are colliding is not known precisely.