Catalysts are divided into different types.
- Positive catalyst
- Negative catalyst.
- Auto catalyst.
- Induced catalyst.
A catalyst which enhances the speed of reaction is called positive catalyst. This phenomenon is known as positive catalysis.
R ==> Reactant.
P ==> Product.
Ea ==> Activation energy.
( i ) Decomposition of hydrogen peroxide in presence of colloidal platinum.
( ii ) Decomposition of KClO3 in presence of manganese dioxide.
There are certain substances which, when added to the reaction mixture, retard the reaction rate instead of increase it. This is called negative catalysts, otherwise known as inhibitors. This phenomenon is known as negative catalysis.
( i ) Oxidation of sodium sulphite by air is retarded by alcohol.
2Na2SO3 + O2 → 2Na2SO4 .
In auto catalyst, the reaction is catalyzed by one of its products. This phenomenon is known as auto catalysis.
( i ) Oxidation of oxalic acid by potassium permanganate, one of the products MnSO4 act as a auto catalyst. It increase the rate of the reaction.
In induces catalyst one reactant influences the rate of other reaction, which does not occur under ordinary condition. This phenomenon is known as induced catalysis.
Na3AsO3 + Na2SO3 → Na3AsO4 + Na2SO4
Air is passed through a mixture of solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. Thus sulphite has induced the arsenite. Because sodium sulphite solution readily oxidises in air, but sodium arsenite solution does not oxidise by air. So Na2SO3 act as a induced catalyst.