CBSE Class 12 Chemistry Notes: Chemical Kinetics – Catalyst and Catalysis

Catalyst: A substance that can influence the rate of a chemical reaction but itself does not undergo the permanent chemical change.

Catalyst increases the reaction rate by lowering the activation energy of the reactant.

Eq → Activation energy without catalyst
Eq → Activation energy with catalyst
ER → Energy of reaction
A → Path of reaction without catalyst
B → Path of reaction with catalyst.

Catalysis: The process in which rate of reaction is influenced by catalyst is called catalysis.

Substrate: The reactant on which catalyst acts is called substrate.

Catalyst does not change the heat of reaction it only makes a process faster.

Catalyst does not change the yield of the product

Type of Catalyst:

Positive Catalyst: A catalyst which increase the rate of reaction by lowering the activation energy of reaction.

Example :

Negative Catalyst: A catalyst that retards the chemical reaction by raising the energy barrier or activation energy.

Example:

Types of catalysis

Homogeneous Catalysis: Catalysis in which catalyst and reactants are present in same phase.

Example:

Heterogeneous Catalysis:  Catalysis in which Catalyst and reactants are in different phase.

Example:

Effect of catalyst on the Rate of Reaction:

According to the Arrhenius theory

Thus specific rate of reaction (rate constant k) depends upon two factors

1. Ea
2. T

when we employ catalyst Ea of reaction is lowered and more no. of molecules possesses energy above the threshold temperature rate of reaction increases.

If at temperature T rate constant is K₁ for uncatalyzed reaction and rate case is K₂ for catalyzed reaction

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