The action of catalysis in chemical reactions is explained mainly by two important theories. They are (i) the intermediate compound formation theory (ii) the adsorption theory 1. The intermediate compound formation theory A catalyst acts by providing a new path with low energy of activation. In homogeneous catalysed reactions a catalyst may combine with one or more reactant to form an intermediate which reacts with other reactant or decompose to give products and the catalyst is regenerated. 2. Adsorption theory Langmuir explained the action of catalyst in heterogeneous catalysed reactions based on adsorption. The reactant molecules are adsorbed on the catalyst surfaces, so this can also be called as contact catalysis.

1. Theories of Catalysis

2.  For a chemical reaction to occur, the reactants are
to be activated to form the activated complex.
 The energy required for the reactants to reach the
activated complex is called the activation energy.
 The activation energy can be decreased by
increasing the reaction temperature.

3.  In the presence of a catalyst, the reactants are
activated at reduced temperatures in otherwords,
the activation energy is lowered.
 The catalyst adsorbs the reactants activates them
by weakening the bonds and allows them to react
to form the products.

4. As activation energy is lowered in presence of a
catalyst, more molecules take part in the reaction
and hence the rate of the reaction increases.

5. The action of catalysis in chemical reactions is explained
mainly by two important theories. They are
(i) the intermediate compound formation theory
(ii) the adsorption theory.

6. 1. The intermediate compound formation theory
According to this theory, a catalyst must chemically
combine with one or more reactants to form an unstable
intermediate product. It decomposes or combine later with
other reactant to form the product and the catalyst is
regenerated.
A+ [catalyst] [intermediate]
[intermediate] + B Product + [catalyst]

7. Consider the reactions:
A+B → AB (i)
C is the catalyst
The reaction proceeds by following steps:-
A+C → AC (unstable intermediate) (ii)
AC+B → AB+C (iii)
1. The intermediate compound formation theory
C

8. AC+B → AB+C (3)
Activation energies for the reactions (2) and (3) are
lowered compared to that of (1). Hence the formation
and decomposition of the intermediate accelerate the
rate of the reaction.

9. The catalytic oxidation of SO2 to SO3 in lead chamber process
2 NO + O2 → 2NO2
NO2 + SO2 → SO3 + NO
Example 1 Catalyst Intermediate
compound
Catalyst
Product

10. Example 2
Thermal decomposition of KClO3 in presence of MnO2 proceeds as follows.
2KClO3 → 2KCl+3O2
Steps in the reaction
2KClO3 +6MnO2 → 6MnO3 +2KCl
It is an intermediate.
6MnO3 → 6MnO2 +3O2

11. It is an intermediate.
Formation of water due to the reaction of H2 and O2 in the presence of Cu
proceeds as follows:
H2+½O2 → H2O
Cu+ 1/2 O2 → Cu2O
Cu2O+H2 → H2O + 2Cu
Example 3:

12. This theory describes
(i) the specificity of a catalyst and
(ii) the increase in the rate of the reaction with
increase in the concentration of a catalyst.

13. a. The intermediate compound theory fails to explain
the action of catalytic poison and activators
(promoters).
b. This theory is unable to explain the mechanism of
heterogeneous catalyzed reactions
Limitations

14. 2. Adsorption theory

15. Langmuir explained the action of catalyst in heterogeneous catalyzed
reactions based on adsorption. The reactant molecules are adsorbed on the
catalyst surfaces, so this can also be called as contact catalysis.
According to this theory, the reactants are adsorbed on the catalyst
surface to form an activated complex which subsequently decomposes
and gives the product.

16.  Adsorption theory explains the mechanism of reaction between
two gases catalyzed by heterogeneous catalysis/ contant catalysis.
 Reactants adhere to the surface of a catalyst through adsorption,
which enhances the rate of a chemical reaction.
 This theory was developed to explain the mechanism of
heterogeneous catalysis, where the catalyst is in a different phase
(usually solid) than the reactants (usually gases or liquids).
2. Adsorption theory

17. Here are some key points of the adsorption theory of catalysis:
1. Adsorption of reactant molecule
Reactant molecules adsorb (stick) to the surface of the catalyst. This
adsorption can be physical (van der Waals forces) or chemical
(formation of chemical bonds).
2. Formation of activated complex
the particles of reactant adjacent to one another join to form an
intermediate complex (A-B). The activated complex is unstable, it has
fleeting existence.

18. Here are some key points of the adsorption theory of catalysis:
3. Decomposition of the activated complex
The activated complex breaks to form product C and D. the separated
particles of the product hold to the catalyst surface by partial chemical
bonds.
4. Desorption of product
The particles of product areca desorbed or released from the surface.
They are stable and can lead an independent existance.

19.  The surface of a catalyst is not smooth. It bears steps, cracks and corners.
Hence the atoms on such locations of the surface are co-ordinatively
unsaturated. So, they have much residual force of attraction. Such sites are
called active centers.
 The presence of such active centers increases the rate of reaction by
adsorbing and activating the reactants.
Active centers