Notes On Group 16: Oxygen - Ozone - CBSE Class 12 Chemistry
Ozone is an unstable triatomic allotropic form of oxygen. It is present in the atmosphere for about 20 kilometres above sea level. It is formed from the oxygen present there, under the influence of ultraviolet light from the sun. Ozone is of vital importance in protecting the earth’s surface from excessive exposure to ultraviolet radiations. The use of chlorofluorocarbons in aerosols and refrigerators, and their subsequent release into the atmosphere, is blamed for making holes in the ozone layer over the Arctic and Antarctic regions. It is feared that this will allow excessive UV light to reach the earth, which can cause skin cancer in humans. Oxides of nitrogen, particularly nitric oxide, combine rapidly with ozone to form nitrogen dioxide and oxygen. Thus, the oxides of nitrogen released from the exhaust systems of supersonic jet aeroplanes slowly deplete the ozone layer.   Preparation of ozone: Ozone is prepared by passing a silent electric discharge through dry, pure and cold oxygen in a special apparatus called the ozoniser. Concentrations of up to 10% of ozone are obtained in this way. The formation of ozone is an endothermic reaction; it must be carried out at high temperature. Hence, it is prepared by means of silent electric discharge.   Higher concentrations or pure ozone can be obtained by fractional liquefaction of an oxygen-ozone mixture.   Physical properties of ozone: Ozone is a pale blue gas with a characteristic fishy smell. It condenses at -1200C as a dark blue liquid. This, on further cooling, gets solidified to violet black crystals. Ozone is thermodynamically unstable, and decomposes to oxygen. The decomposition is exothermic, and is catalysed by many materials. High concentrations of ozone are dangerously explosive.   Oxidising action of ozone: Due to the ease with which it liberates atoms of nascent oxygen, ozone acts as a powerful oxidising agent. It oxidises,   1)      Lead sulphide to lead sulphate.            4O3 + PbS → 4O2 + PbSO4 2)      Nitrogen dioxide to dinitrogen pentoxide            2NO2 + O3 → N2O5 + O2 3)      Iodide ions to iodine            2Ki + H2O + O3 → 2KOH + I2 + O2 Structure of ozone: The ozone molecule is symmetrical and bent with a bond angle of about 1170.   Ozone is considered a resonance hybrid of the canonical forms   Uses of ozone: Ozone is used as a germicide. Ozone is also used as a disinfectant. For example, it is used to purify drinking water.  It is used as a mild bleaching agent for bleaching oils, ivory, starch, flour, wax and delicate fabrics It is used in the manufacture of potassium permanganate from potassium manganate. It is also used in the manufacture of artificial silk and synthetic camphor.

#### Summary

Ozone is an unstable triatomic allotropic form of oxygen. It is present in the atmosphere for about 20 kilometres above sea level. It is formed from the oxygen present there, under the influence of ultraviolet light from the sun. Ozone is of vital importance in protecting the earth’s surface from excessive exposure to ultraviolet radiations. The use of chlorofluorocarbons in aerosols and refrigerators, and their subsequent release into the atmosphere, is blamed for making holes in the ozone layer over the Arctic and Antarctic regions. It is feared that this will allow excessive UV light to reach the earth, which can cause skin cancer in humans. Oxides of nitrogen, particularly nitric oxide, combine rapidly with ozone to form nitrogen dioxide and oxygen. Thus, the oxides of nitrogen released from the exhaust systems of supersonic jet aeroplanes slowly deplete the ozone layer.   Preparation of ozone: Ozone is prepared by passing a silent electric discharge through dry, pure and cold oxygen in a special apparatus called the ozoniser. Concentrations of up to 10% of ozone are obtained in this way. The formation of ozone is an endothermic reaction; it must be carried out at high temperature. Hence, it is prepared by means of silent electric discharge.   Higher concentrations or pure ozone can be obtained by fractional liquefaction of an oxygen-ozone mixture.   Physical properties of ozone: Ozone is a pale blue gas with a characteristic fishy smell. It condenses at -1200C as a dark blue liquid. This, on further cooling, gets solidified to violet black crystals. Ozone is thermodynamically unstable, and decomposes to oxygen. The decomposition is exothermic, and is catalysed by many materials. High concentrations of ozone are dangerously explosive.   Oxidising action of ozone: Due to the ease with which it liberates atoms of nascent oxygen, ozone acts as a powerful oxidising agent. It oxidises,   1)      Lead sulphide to lead sulphate.            4O3 + PbS → 4O2 + PbSO4 2)      Nitrogen dioxide to dinitrogen pentoxide            2NO2 + O3 → N2O5 + O2 3)      Iodide ions to iodine            2Ki + H2O + O3 → 2KOH + I2 + O2 Structure of ozone: The ozone molecule is symmetrical and bent with a bond angle of about 1170.   Ozone is considered a resonance hybrid of the canonical forms   Uses of ozone: Ozone is used as a germicide. Ozone is also used as a disinfectant. For example, it is used to purify drinking water.  It is used as a mild bleaching agent for bleaching oils, ivory, starch, flour, wax and delicate fabrics It is used in the manufacture of potassium permanganate from potassium manganate. It is also used in the manufacture of artificial silk and synthetic camphor.

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