- As we know that molecules need to collide to bring about a chemical reaction, so the number of molecules participating in an elementary chemical reaction that collides to bring about the chemical reaction is called molecularity of a reaction.
- The value of molecularity of a reaction is always a positive value.
- Let us consider a following reaction:
A + 2B --> C + D
Here reactants are: 1 molecule of A ,2 molecules of B
Products are: 1 molecule of C , 1 molecule of D.
Therefore we can conclude that reaction is trimolecular.
A reaction with molecularity =1 is called unimolecular.
Example, PCl5 --> PCl3 + Cl2
- A reaction with molecularity =2 is called bimolecular.
Example, Cl + CH4 --> HCl + CH3
- A reaction with molecularity =3 is called trimolecular.
2FeCl3 + SnCl2 --> 2FeCl2 + SnCl4
- It is theoretical value and does not determine the rate of reaction. Nor does it depend upon external factors like temperature or pressure, etc.