CATALYSIS:
A catalyst is a substance which asters the speed of a chemical reaction without itself undergoing any chemical change and the phenomenon is known as catalysis.
Example:
2KCIO3 ------> 2KCI + 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 generall 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+ins furnished by hydrochloric acid.
CH3COOCH3(1) + H2O(1) ------> 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) --------> 2SO3
The reactants are in gaseous phase while the catalyst is in solid phase.
ii. Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in Haber's process.
N2(g) + 3H2(g) ---------> 2NH3
The reactants are in gaseous phase while the catalyst is in solid phase.
TYPES OF CATALYSTS:
The catalysts have been divided into different types according to their behaviour and pattern of action.
1. POSITIVE CATALYST:
A catalyst which enhances the speed of the reaction is called positive catalyst and the phenomenon in known as positive catalysis. Various examples are geven below:
i. Decomposition of H2O2 in presence of colloidal platinum
2H2O2 --------> 2H2O + O2
ii. Decomposition of KCIO3 in presence of manganese dioxide.
2KCIO3 ---------> 2KCI + 3O2
2. NEGATIVE CATALYST:
There are certain substances which, when added to the reaction mixture, retard the reaction rate instead of increasing it. These are called negative catalysts or inhibitors and the phenomenon is known as negative catalysis. The examples are given below:
i. The oxidation of sodium sulphite by air is retarded by alcohol.
2Na2SO3 + O2 -------> 2Na2SO4
ii. The decomposition of hydrogen peroside decreases in presence of glycerine.
2H2O2 --------> 2H2O + O2
3.AUTO CATALYST:
In certain reactions, it is observed that one of the products formed during the reaction acts as a catalyst for that reaction. Such type of catalyst is called auto catalyst and the phenomenon is known as auto catalysis.
In the oxidation of oxalic acid by potassium permanganate, one of the products MnSO4 acts as a auto-catalyst because it increases the speed of the reaction.
4. INDUCED CATALYST:
When one reactant influences the rate of other reaction, which does not occur under ordinary conditions, the phenomenon is known as induced catalysis.
Sodium arsenite solution in 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. Thus sulphite has induced the arsenite and hence is called induced catalyst.
PROMOTERS:
The activity of a catalyst can be increased by addition of a small quantity of a second material. A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a promoter. Some examples of the promoters are given below:
In the Haber's process for the synthesis of ammonia, traces of molybdenum increase the activity of finely divided iron which acts as a catalyst.
CATALYTIC POISONS:
A substance which destroys the activity of the catalyst is called a poison and the process is called catalytic poisoning. Some of the examples are
(i) The platinum catalyst used in the oxidation of SO2 in contact process is poisoned by arsenious oxide.
(ii) The iron catalyst used in the synthesis of ammonia in Haber process is poisoned by H2S.
ACTIVE CENTRES:
The catalytic surface has unbalanced chemical bonds on it. The reactant gaseous molecules are adsorbed on the surface by these free bonds. This accelerates the rate of the reaction. The distribution of free bonds on the catalytic surface is not uniform. These are crowded at the peaks, cracks and corners of the catalyst. The catalytic activity due to adsorption of reacting molecules is maximum at these spots. These are, therefore, referred to as the active centres. If a catalyst has more active centres, them its catalytic activity is ilncreased.
