CBSE Class 12 Chemistry Notes: Chemical Kinetics – Rate Law and Molecularity

RATE LAW

It is a mathematical expression which relates rate of reaction with concentration It can be derived only by performing experiments.

R = k [A] 2
k is called rate constant or specific rate constant

Unit of k varies from reaction to reaction depending on the order of the reaction.

Unit of rate of reaction is mol L^–1 s^–1
Rate = k [A]^x [B]^y

Generally units of K is determined by
    

Units of k are very significant to identify the order of reaction.

The value of k for a particular reaction changes by change in temperature.

For a particular temperature value, k is different for different reactions.

MOLECULARITY

It is a theoretical aspect and it is defined as the sum of the number of molecules undergoing the covalency change in a balanced stoichiometric equation of a reaction

Molecularity of a reaction cannot be zero, negative or infractions.

Molecularity can be determined just by the equation [chemical].

The slowest steps of the reaction determine the molecularity and rate of the reaction. It cannot be greater than 3.

Molecularity of a reaction is an invariable fact for a particular reaction. But order can be variated.

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