Catalysis, Catalyst, its types, activity and selectivity, Enzymes

Catalysis: A substance which changes the speed of a reaction without being used up itself is called a catalyst. The phenomenon of increasing the rate of a chemical reaction by the use of catalyst is called catalysis. The catalyst may be of two types namely-

• Homogeneous Catalyst
• Heterogeneous Catalyst.

Homogeneous Catalyst:

When the catalyst is in the same phase as the reactants and the products, it is called homogeneous catalyst. For examples, Hydrolysis of an ester is catalysed by the presence of an acid.

                                                         _H⁺

CH₃COOC₂H₅  +   H₂O      ———→    CH₃COOH    +  C₂H₅OH

Heterogeneous Catalyst:

When the catalyst is in the different phase than the reactants and the products, it is called heterogeneous catalyst. For examples, reaction between hydrogen and nitrogen to form ammonia is catalysed by the presence if finely divided iron mixed with Mo (promoter).

_{(Fe + Mo)}

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

Nature of Solid Catalysts:

The solid catalysts may be metals, alloys, metal oxides or metal sulphides. The effectiveness of a catalyst depends upon the two important aspects – Activity and Selectivity.

Activity: The ability of catalysts to increase the rate of a chemical reaction is called activity. For example, a mixture of H₂ and O₂ do not react at all. However, in presence of platinum (catalyst) the mixture of H₂ and O₂ reacts and form water.

                                                                                _{Finely divided Pt}

H₂ (g)   +   O₂(g)     ——————→    H₂O(g)

                                                                                         _{Ni or Pt}

CH₂= CH₂   +  H₂ (g)    ——————→    CH₃—CH₃

Selectivity: The ability of the catalyst to direct a reaction to give a particular product is called selectivity. For examples, acetylene on reaction with hydrogen in presence of Pt catalyst to give ethane while in presence of Lindlar’s catalyst (Palladium and BaSO₄ poisoned with quinoline or sulphur) to give ethylene. Thus, a catalyst is highly selective in nature

_{Ni or Pt}

CH ≡ CH   +   H₂      —————→    CH₃—CH₃

_{Lindlar’s Catalyst}

CH ≡ CH   +  H₂    ———————→ CH₂= CH₂

Enzymes: Enzymes are complex proteinous substances, produced by living bodies which act as catalyst in the physiological reactions. The enzymes are also sometimes called bio-catalyst. Enzymes are proteins with high molecular mass ranging from 15,000 to 1,000,000 g/mol. Enzymes possess very high catalytic activity and highly specific in nature. For examples, maltose, invertase, diastase, etc.

Difference between Enzymes and Catalyst: