A catalyst is a substance which alters the speed of a chemical reaction without itself undergoing any chemical change and the phenomenon is known as catalysis.
2KClO3 -- MnO2-- > < -- MnO2--- 2KCl + 3O2
In the above reaction, MnO2 acts as a catalyst.
General characteristics of catalytic reactions
The following characteristics are generally common to most of the catalytic reactions.
1. The catalyst remains unchanged in mass and in chemical composition at the end of the reaction.
2. Only a small quantity of catalyst is generally needed.
3. A catalyst cannot initiate a reaction. The function of a catalyst is only to alter the speed of the reaction which is already occurring at a particular rate.
4. A catalyst does not alter the position of equilibrium in a reversible reaction.
5. The catalyst is generally specific in its action.
Types of catalytic reactions
Catalytic reactions are classified into two broad types;
1. Homogeneous catalysis
2. Heterogeneous catalysis
1. Homogeneous Catalysis
In these reactions, the reactants and catalyst remain in the same phase.
The following are some of the examples of homogeneous catalysis.
i. Oxidation of SO2 to SO3 with oxygen in the presence of nitric oxide as the catalyst in the lead chamber process.
2SO2(g) + O2(g) ---- ----- > < -- --- 2SO3
ii. Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid.
CH3 COO CH3(l) + H2O(l) < --- HCL(l)--- > CH3COOH + CH3OH
2. 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. Oxidation of SO2 to SO3 in the presence of Pt metal or V2O5 as catalyst in the contact process for the manufacture of sulphuric acid.
2SO2(g) + O2(g) --- Pt(s)--- > 2SO3
ii. Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in Haber's process
N2(g) + 3H2(g) --- Fe(s)--- > 2NH3