In certain reactions one of the products formed acts as a catalyst to the reaction. Initially the rate of reaction will be very slow but with the increase in time the rate of reaction increases.
Auto catalysis is observed in the following reactions.
CH3COOC2H5+H2O → CH3COOH+C2H5OH
Acetic acid acts as the autocatalyst
2AsH3 → 2As+3H2
Arsenic acts as an autocatalyst
In certain reactions, presence of certain substances, decreases the rate of the reaction.
Ethanol is a negative catalyst for the following reaction.
(i) 4CHCl 3 +3O 2 → 4COCl 2 +2H2O+2Cl2
Ethanol decreases the rate of the reaction
(ii) 2H2O2 → 2H2O+O2
In the decomposition of hydrogen peroxide, dilute acid or glycerol acts as a negative catalyst.