Catalysis

Ask a Question, Get an Answer!

Type your question here

Hundreds of tutors are online and ready to help you right now!

It has been mentioned earlier that a catalyst usually increases the rate of reaction. A catalyst is not consumed in the reaction, it is used up in one step and then released in the next step.

Catalytic reactions can be classified into two types. They are

i) Homogenous reactions and

ii) Heterogenous reactions.

In homogenous reactions, the catalyst and the reactants are of the same phase. Oxidation of sulphur dioxide (SO₂) to sulphur trioxide (SO₃) in the presence of nitric oxide (NO) is an example of homogenous catalytic reaction.

In heterogenous reaction, the catalyst exists in a different phase from the reactants. Usually a solid catalyst is used and the reactants can be either in the liquid or gaseous phase. In heterogenous catalysis the surface area of the catalyst affects the rate of the reaction, especially under low reactant concentrations. An example of heterogenous catalytic reaction is hydrogenation of unsaturated hydrocarbon. For e.g., the reaction

is carried out on the surface of platinum catalyst.

The reason why a catalyst increases the rate of a reaction is that it lowers the activation energy of the reaction by altering the mechanism of the reaction. This is illustrated with the example of oxidation of sulphur dioxide (SO₂) in presence of nitric oxide (NO) in below figure.

fig 6.16(a) - Potential energy diagram after the reaction of SO₂ with O₂ - In the absence of NO

fig 6.16(b) - Potential energy diagram after the reaction of SO₂ with O₂ - B in the presence of NO