Catalysts, Homogeneous and Heterogeneous Catalysis

The substance that remains quantitatively and chemically unchanged at the end of a reaction is known as a catalyst. The acceleration and retardation of reactions by catalysts is referred to as catalysis. There are different types of catalytic reactions:– catalytic combination, catalytic redox reaction, or catalytic reversible reaction. There are two types of catalyzed reactions:– homogeneous catalysis and heterogeneous catalysis.


Homogeneous catalysis: This is a type of reaction whereby the catalyst, reactants and products are all in the same phase. For instance, the oxidation of sulphur(IV) oxide using nitrogen(II) oxide as a catalyst occurs in the gaseous phase.


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                          2SO2(g) + O2(g) →  2SO2(g)



Heterogeneous catalysis: In heterogeneous catalysis, the reactants, products and catalysts are in different phases. Most catalytic reactions are in this category. Examples of heterogeneous catalytic reactions are:



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




                       H2(g)  + I2(g)  →   2HI(g)


The formation of margarine from vegetable oil in the presence of nickel as a catalyst is an example of a heterogeneous catalysis.



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                Vegetable oil(l) + H2(g)   →     Margarine


Oil to be solidified by hydrogenation facilitated by catalysts



                Characteristics of a Catalyst

There are some characteristics and features peculiar with catalysts that make them unique as facilitators of chemical reactions.


  1. A catalyst alters the rate of a chemical reaction
  1. The effect of a solid catalyst is improved by increasing its surface area.
  1. A catalyst remains unchanged in both chemical nature and mass at the end of a reaction. However, its physical features such as color and texture may be changed.
  1. A catalyst does not affect the type of products formed in the reaction.
  1. A catalyst cannot start a reaction. It is effective only in reactions that are already in progress.
  1. A catalyst will affect the rate of a reaction even though it is present in a very small amount. An increase, however, in the amount of a catalyst up to a certain limit, would usually increase the rate of a reaction.
  1. A catalyst has no effect on the equilibrium of a reversible reaction.
  1. A catalyst is specific in action. That is, it can only catalyze a particular reaction.
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