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(i) Positive catalysis: The catalyst increases the rate of a reaction.
(ii) Negative catalysis (Inhibitor or retarder): Chemical reactions are sometimes retarded by the presence of a foreign substance the substance is known as a negative catalyst.
Alcohol, Acetanilide: Prevents oxidation of Na2SO3 by air
H3PO4: Prevents decomposition of H2O2
(iii) Auto catalysis: In this type of catalysis, one of the products of the reaction catalyses the reaction. In the oxidation of oxalic acid by KMnO4, Mn2+ ion formed is known to accelerate the reaction. So, when KMnO4 solution is run into warm solution of oxalic acid (+ dil. H2SO4), initially there is a time lag before decolourisation occurs; as more KMnO4 is added, the decolourisation becomes almost instantaneous.
(iv) Homogeneous catalysis: A catalytic process in which the catalyst is in the same phase as the reactant is called homogenous catalysis.
(v) Heterogeneous catalysis: A catalytic process in which the catalyst and the reactants are in different phases is called heterogeneous catalysis. This process is also called contact or surface catalysis.
(vi) Induced catalyst: When one reaction influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis.
Examples of induced catalysis:
(1)Sodium arsenite solution is not oxidised by air. If, however, air is passed through a mixture of the solution of sodium arsenite and sodium sulphite, both of them undergo simultaneous oxidation. The oxidation of sodium sulphite, thus, induces the oxidation of sodium arsenite.
(2)The reduction of mercuric chloride (HgCI2) with oxalic acid is very slow, but potassium permanganate is reduced readily with oxalic acid. If, however, oxalic acid is added to a mixture of KMnO4 and HgCI2, both are reduced simultaneously. The reduction of potassium permanganate, thus, induces the reduction of HgCI2.
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