When hydrogen and oxygen gases are kept in contact with each other, no observable reaction occurs. If we 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. The following are some more examples of catalysis:
(i) Decomposition of potassium chlorate occurs at high temperature. If a small amount of the manganese dioxide is added, the decomposition occurs at much lower temperature. Here, manganese dioxide acts as catalyst.
(ii) The evolution of hydrogen by the reaction between zinc and hydrochloric acid is catalysed by Cu2+(aq) ions.
(iii) The oxidation of hydrogen chloride gas by oxygen occurs more quickly if the gases are passed over cupric chloride.
In certain reactions, one of the products of the reaction acts as the catalyst. For example, the oxidation of oxalic acid by acidified potassium permanganate occurs as
2KMnO4(aq) + 3H2SO4(aq) + 5(COOH)2(aq) → K2SO4(aq) + 2MnSO4(aq) + 8H2O( ) + 10 CO2(g)
At room temperature, the reaction is quite slow in the beginning. Gradually it becomes fast due to the catalytic action of Mn2+ ions which are one of the products as MnSO4 in the reaction.
The phenomenon in which one of the products of a reaction acts as a catalyst is known as auto-catalysis.
Some catalysts retard a reaction rather than speed it up. They are known as negative catalysts. For example :
(i) Glycerol retards the decomposition of hydrogen peroxide.
(ii) Phenol retards the oxidation of sulphurous acid.
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 :
(i) In Haber’s process for the manufacture of ammonia, the catalytic activity of iron is enhanced by molybdenum which acts as promoter.
(ii) Copper promotes the catalytic activity of nickel during hydrogenation of oils.
(iii) In Haber’s process the catalyst iron is poisoned by hydrogen sulphide H2S.
(iv) In contact process for the manufacture of sulphuric acid, the catalyst platinum is poisoned by even the traces of arsenious oxide As2O3.