What do you mean by homogeneous and heterogeneous catalysts? Give two examples of each.

When the reactants and the catalyst are in the same phase (i.e., liquid or gas), the process is said to be homogeneous catalysis. The following are some of the examples of homogeneous catalysis:
(i) Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process.

2SO₂(g) + O₂(g) $\stackrel{\mathrm{NO}\left(\mathrm{g}\right)}{\to }$ 2SO₃(g)

The reactants, sulphur dioxide and oxygen, and the catalyst, nitric oxide, are all in the same phase.

(ii) Hydrolysis of methyl acetate is catalysed by H+ ions furnished by hydrochloric acid.
CH₃COOCH₃(l) + H₂O(l) $\stackrel{\mathrm{HCl} \left(\mathrm{l}\right)}{\to }$ CH₃COOH(aq) + CH₃OH(aq)

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 sulphur dioxide into sulphur trioxide in the presence of Pt.

2SO₂ $\stackrel{\mathrm{Pt}\left(\mathrm{s}\right)}{\to }$ 2SO₃
 The reactant is in gaseous state while the catalyst is in the solid state.

(ii) Combination between dinitrogen and dihydrogen to form ammonia in the presence of finely divided iron in Haber’s process.

N₂(g) +3H₂(g) $\stackrel{\mathrm{Fe}\left(\mathrm{s}\right)}{\to }$ 2NH₃(g)

The reactants are in gaseous state while the catalyst is in the solid state.