Ozone is an allotrope of oxygen. It is present in the upper atmosphere, where it is formed by the action of U. V. radiations on

O₃ protects us from the harmful U. V. radiations which causes skin cancer. Now a days, ozone layer in the atmosphere is depleting due to NO released by supersonic aircrafts and chlorofluoro carbons (CFC’S) i.e. freon which is increasingly being used in aerosols and as a refrigerant.

Preparation of ozone: Ozone is prepared by passing silent electric discharge through pure, cold and dry oxygen in a specially designed apparatus called ozoniser. The formation of ozone from oxygen is an endothermic reaction.

3O₂ 2O₃ ΔH = +285.4 kJ

Ozone is prepared in the laboratory by the following two types of ozonisers,

(a) Siemen’s ozoniser, (b) Brodie’s ozoniser

For the better yield of ozone : (a) Only pure and dry oxygen should be used. (b) The ozoniser must be perfectly dry. (c) A fairly low temperature (273 K) must be maintained. (d) The electric discharge must be sparkless.

Structure of O₃: The structure of O₃ molecule is angular as shown in fig. The O — O — O bond angle is 116.8° and O — O bond length is 128 pm.

Uses of ozone

(1) O₃ is used for disinfecting water for drinking purposes because ozone has germicidal properties.

(2) It is used for purifying air of crowded places such as cinemas, underground railway, auditoriums, tunnels, mines etc.

(3) It is used in industry for the manufacture of KMnO₄ , artificial silk, synthetic camphor etc.