Catalysis

When hydrogen and oxygen gases are kept in contact with each other, no observable reaction occurs. If you add a small piece of platinum gauge in the mixture of these gases, the reaction occurs readily. Here, platinum gauge speeds up the reaction and is called a catalyst.

A catalyst is a substance which changes the rate of a reaction but remains chemically unchanged at the end of the reaction.

The phenomenon of change of reaction rate by addition of a substance which itself remains unchanged chemically is called catalysis.

Auto Catalysis

In certain reactions, one of the products of the reaction acts as the catalyst. The phenomenon in which one of the products of a reaction acts as a catalyst is known as auto-catalysis.

Negative Catalysis

Some catalysts retard a reaction rather than speed it up. They are known as negative catalysts. For example, Glycerol retards the decomposition of hydrogen peroxide, Phenol retards the oxidation of sulphurous acid.

Promoters and Poisons

Certain substances increase or decrease the activity of the catalyst, although, by themselves they do not show any catalytic activity. The substances which increase the activity of a catalyst are called promoters and those which decrease the activity of a catalyst are called poisons.

For example, in Haber’s process for the manufacture of ammonia, the catalytic activity of iron is enhanced by molybdenum which acts as promoter.