Class 12 Chemistry Chemical Kinetics Molecularity


  • 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.

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