Define order and molecularity.


Order:

Order of a reaction is defined by the number of atoms or molecules, whose concentration change during the reaction and determine the rate of the reaction.

For example,

It is a first order reaction.

It is a second order reaction.



Molecularity:

The molecularity of an elementary reaction is defined as the number of reactant molecules involved in a reaction. Such as,

 Reaction
#
Molecularity
i
1
ii
2
iii
3

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The molecularity of a complex reaction may be defined as the number of molecules or atoms taking part in the rate-determining steps. Such as, the decomposition of  occurs by the following steps.

Step 1: 
Step 2: 
Step 3: 

Step 1 + Step 2 + Step 3:  (overall reaction)

Here, step 2 is rate determining and has molecularity ‘2’, which could be considered as the molecularity of the decomposition reaction of 

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