General characteristics and Types of catalytic reactions

CATALYSIS

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.

Example,

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) ^{+ O}2(g)  ---- ----- > < --  --- ^2SO3

ii. Hydrolysis of methyl acetate is catalysed by H⁺ ions furnished by hydrochloric acid.

CH₃ COO CH_3(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) ^{+ O}2(g)  ^{--- Pt(s)--- > 2SO}3

ii. Combination between nitrogen and hydrogen to form ammonia in the presence of finely divided iron in Haber's process

^N2(g) ^{+ 3H}2(g) ^--- Fe(s)^{--- > 2NH}3