Class 12 Chemistry Surface Chemistry Catalysis


Substances which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction are called catalysts.

  • Promoters- Substances which enhance the activity of the catalyst
  • Poisons- Substances which decreasethe activity of the catalyst

Example- In Haber’s process for manufacturing ammonia, molybdenum acts as promoter for the catalyst- iron

N2 (g) + 3H2 (g) --> 2NH3 (g)

Homogenous and Heterogeneous Catalysis

Catalysis can be classified into 2 types.

  1. Homogenous catalysis:

The reactants and the catalyst are in the same phase (that is liquid or gas).

Example- In Oxidation of sulphur dioxide,

 Sulphur dioxide, oxygen and the catalyst nitrogen oxide are all gases

2SO2(g) + O2(g) à 2SO3 (g)

  1. Heterogeneous catalysis:

The reactants and the catalyst are in different phases

Example- In Oxidation of ammonia,

Ammonia and oxygen are gases but the catalyst Platinum is solid

4NH3(g) +5O2(g) --> 4NO(g) + 6H2O(g)

Adsorption theory of Heterogeneous catalysis:

  • This theory explains the mechanism of heterogeneous catalysis.
  • It is combination of the intermediate compound formation theory and the old adsorption theory. The Intermediate Compound Formation Theory is in the chapter of the Chemical Kinetics
  • Old Adsorption theory- The reactants (gases or in solutions) are adsorbed on the surface of the solid catalyst. The increase in concentration of the reactant on the surface also increases the rate of reaction. The heat of adsorption( adsorption is exothermic) also increases the rate of reaction.

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