Significance of Homogeneous and Heterogeneous Catalysts

The substance that remains quantitatively and chemically unchanged at the end of a reaction is known as a catalyst. The acceleration and retardation of reactions by catalysts is referred to as catalysis. There are different types of catalytic reactions:– catalytic combination, catalytic redox reaction, or catalytic reversible reaction. There are two types of catalyzed reactions:– homogeneous catalysis and heterogeneous catalysis.

Homogeneous catalysis: This is a type of reaction whereby the catalyst, reactants and products are all in the same phase. For instance, the oxidation of sulphur(IV) oxide using nitrogen(II) oxide as a catalyst occurs in the gaseous phase.

                            NO(g)

2SO₂(g) + O₂(g) →  2SO₂(g)

Heterogeneous catalysis: In heterogeneous catalysis, the reactants, products and catalysts are in different phases. Most catalytic reactions are in this category.

Examples of heterogeneous catalytic reactions are:

                         Fe(s)

N₂(g) + 3H₂(g)      2NH₃(g)

                          Pt(s)

H₂(g)  + I₂(g)  →   2HI(g)

The formation of margarine from vegetable oil in the presence of nickel as a catalyst is an example of a heterogeneous catalysis.

                                         Ni(s)

Vegetable oil(l) + H₂(g)   → Margarine

Characteristics of a Catalyst

There are some characteristics and features peculiar with catalysts that make them unique as facilitators of chemical reactions.

1. A catalyst alters the rate of a chemical reaction

2. The effect of a solid catalyst is improved by increasing its surface surface

3. It remains unchanged in both chemical nature and mass at the end of a reaction. However, its physical features such as color and texture may be changed.

4. It does not affect the type of products formed in the reaction.

5. It cannot start a reaction. It is effective only in reactions that are already in progress.

6. It will affect the rate of a reaction even though it is present in a very small amount. An increase, however, in the amount of a catalyst up to a certain limit, would usually increase the rate of a reaction.

7. It has no effect on the equilibrium of a reversible reaction.

8. It is specific in action. That is, it can only catalyze a particular reaction.