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Chapter: Engineering Chemistry: Surface Chemistry and Catalysis

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General Characteristics of a Catalyst

The following are the general characteristics of a catalyst :

General Characteristics of a Catalyst

 

The following are the general characteristics of a catalyst :

 

(i) A catalyst remains unchanged at the end of the reaction.

 

The amount and the chemical composition of a catalyst remain unchanged in a catalytic reaction. However, the catalyst may undergo a physical change. For example, manganese dioxide, which is used as a catalyst in thermal decomposition of potassium chlorate becomes powder during the course of the reaction.

 

(ii) A small quantity of the catalyst is generally enough.

 

In most of the reactions, only a minute amount of catalyst is required. Only one gram of Cu2+ ions is sufficient to catalyse the oxidation of 109 litres of sodium sulphite solution. In some cases, the rate of reaction is directly proportional to the concentration of catalyst present. Catalysis by acids or bases is usually of this type.

 

(iii) A catalyst does not alter the position of equilibrium state of a reversible reaction.

 

A catalyst allows the equilibrium to be reached faster. However, it does not alter the equilibrium composition of the reaction mixture. It is because, a catalyst increases the rates of forward and backward reaction equally.

 

(iv) Catalysts are generally specific in their action.

 

Generally, one catalyst will change the rate of only one reaction. For example, manganese dioxide catalyses the decomposition of potassium chlorate but not of potassium perchlorate.

 

(v) A catalyst cannot initiate a reaction.

 

A catalyst can change the rate of a reaction which occurs even in the absence of catalyst.

It cannot start a reaction.

 

(vi) The activity of a catalyst can be increased by the presence of promoters and decreased by the presence of poisons.

 

Presence of a promoter increases the activity of a catalyst, while the presence of a poison decreases it.

 

 

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