Activity of catalyst:

Catalyst has a special ability that without taking part in the reaction changes the rate of reaction. This ability of catalyst is said to be activity of catalyst. It mainly depends upon adsorption of reactants on the surface of catalyst. The main factor which governs the activity of catalysts is Chemisorption. The bond must not be too strong or too weak which is formed during adsorption between the catalytic surface and the reactants.

It must be strong enough to make the catalyst active whereas, not so strong that the reactant molecules get immobilized on the catalytic surface leaving no further space for the new reactants to get absorbed.

Selectivity of Catalyst:

Catalysts are highly specific compounds. They have the ability to direct the reaction to yield a particular product. The reaction with same reactants but different catalyst may yield different products. This is termed as the selectivity of catalyst. Catalysts are highly selective in nature. They can accelerate a particular reaction while inhibiting another reaction. Hence, we can say a particular catalyst can catalyse one particular reaction only. It may fail to catalyse another reaction of the same type.