“Catalyst is a substance which speeds up and speeds down a chemical reaction without itself being used up.”

Berzelius (1836)introduced the term catalysis and catalyst.

Ostwald (1895)redefined a catalyst as, “A substance which changes the reaction rate without affecting the overall energetics of the reaction is termed as a catalyst and the phenomenon is known as catalysis.”

Types of catalysis

Catalytic reactions can be broadly divided into the following types,

Homogeneous catalysis : When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas). The catalysis is said to be homogeneous. The following are some of the examples of homogeneous catalysis.

(i) In the lead chamber process

(ii) In the hydrolysis of ester

(iii) In the hydrolysis of sugar

Heterogeneous catalysis : The catalytic process in which the reactants and the catalyst are in different phases is known as heterogeneous catalysis. Some of the examples of heterogeneous catalysis are given below.

(i) In contact process for 

(ii) In Haber’s process for 

(iii) In Ostwald’s process for 

Positive catalysis : When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Some examples of positive catalysis are given below.

(i) Decomposition of 

(ii) Oxidation of 

(iii) Decon’s process

Negative catalysis : There are certain, substance which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalyst or inhibitors and the phenomenon is known as negative catalysis. Some examples are as follows.

(i) Oxidation of sodium sulphite

(ii) Oxidation of benzaldehyde

(iii) Tetra ethyl lead (TEL) is added to petrol to retard the ignition of petrol vapours on compression in an internal combustion engine and thus minimise the knocking effect.

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