# Catalysis

Many reactions proceed quite slowly when the reactants are mixed alone, but can be made to occur much more rapidly by the introduction of other substances. These substances are called catalysts and are not used up in the reaction. The effect of catalyst is called catalysis. These terms were introduced by Berzilius.

The function of a catalyst is to split up the main reaction into two or more steps. The potential energy barrier in each step is smaller than that for the single step of the main reaction. Hence catalyst helps to cross the barrier in two or more steps.

There are two types of catalytic reactions:

(i) Homogeneous catalytic reactions:

In such catalytic reactions, catalyst and the reactants are in same phase.

E. g.

$2SO_2(g) + O_2(g) \overset{NO(g)}{\rightarrow} 2SO_3 (g)$

$H_2 (g) + Cl_2 (g) \overset{N_2O (g)}{\rightarrow} 2HCl(g)$

$CH_3 COOH_3 (l) + H_2 O (l) \overset{OH^- (l) or }{\underset{H^+ (l)} \rightarrow} CH_3COOH(l) + CH_3 OH(l)$

(ii) Heterogeneous catalytic reactions:

In such catalytic reactions, catalyst and the reactants are in different phases

E.g.

$N_2(g) + 3H_2 (g) \overset{Fe(s)}{\rightarrow} 2NH_3(g)$

$2SO_2(g) + O_2(g) \overset{V_2O_5 (s)}{\rightarrow} 2SO_3(g)$

$CO (g) + H_2O(g) \overset{Fe_2O_3(s}{\rightarrow} CO_2(g) + H_2 (g)$

$\text{Unsaturated oil} (l) + H_2(g) \overset{Ni(s)}{\rightarrow} \text{Saturated fat (s)}$

$CO + H_2 \overset{Fe}{\underset{Co} \rightarrow} \text{Hydrocarbon}$

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