Question

Taking two examples of heterogeneous catalytic reactions, explain how a heterogeneous catalyst helps in the reaction.

Answer 1

Heterogeneous catalysts are solid substances that provide a surface for reactant molecules (usually gases) to adsorb. This increases local concentration and weakens bonds in reactants, lowering activation energy and speeding up the reaction. The catalyst is not consumed and is regenerated after the reaction.

Two examples of heterogeneous catalytic reactions are as follow:

(i) Haber’s process for ammonia synthesis: Iron catalyst adsorbs N₂ and H₂, weakening nitrogen’s strong triple bond, enabling easier reaction to form ammonia.

\[\ce{N2_{(g)} + 3H2_{(g)} ->[Fe_{(s)}] 2NH3_{(g)}}\]

(ii) Oxidation of sulphur dioxide to sulphur trioxide (Contact process): The catalyst adsorbs SO₂ and O₂, enabling faster oxidation to SO₃ by lowering activation energy.

\[\ce{2SO2_{(g)} + O2_{(g)} ->[pt_{(s)} or V2O5_{(s)}] 2SO3_{(g)}}\]

In both cases, the heterogeneous catalyst works by adsorbing reactants on its surface, providing an alternative lower energy pathway and regenerates at the end, allowing continuous use.