Ozone (O3)
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Ozone is an allotropic form of oxygen. It is too reactive to remain for long in the atmosphere at sea level. It is formed from atmospheric oxygen in the presence of sunlight at a height of about 20 kilometres,. This ozone layer protects the earth’s surface from an excessive concentration of ultraviolet (UV) radiations.

Preparation of ozone

By passing a slow dry stream of oxygen through a silent electrical discharge, oxygen gets converted to ozone (10%). The product is known as ozonised oxygen.

3O2 → 2O3, ΔHo (298 K) = +142 kJ mol–1

Since the formation of ozone from oxygen is an endothermic process, it is necessary to use a silent electrical discharge in its preparation to prevent its decomposition.

If concentration of ozone greater than 10 per cent is required, a battery of ozonisers can be used, and pure ozone (b.p. 385 K) can be condensed in a vessel surrounded by liquid oxygen.

Physical properties of ozone

Pure ozone is a pale blue gas, dark blue liquid and violet-black solid. Ozone has a characteristic smell and in small concentrations it is harmless. However, if the concentration rises above 100 parts per million, breathing becomes uncomfortable resulting in headache and nausea.

Chemical reactions of ozone

(i) Decomposition of ozone

Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat (ΔH is negative) and an increase in entropy (ΔS is positive). These two effects reinforce each other, resulting in large negative Gibbs energy change (ΔG) for its conversion into oxygen. Therefore, high concentrations of ozone can be dangerously explosive.

2O3 → 3O2; ΔH = -284 kJ/mol

(ii) Oxidising action

Due to the ease with which it liberates atoms of nascent oxygen (O3 → O2 + [O]), it acts as a powerful oxidising agent.

PbS(s) + 4O3(g) → PbSO4(s) + 4O2(g)

2FeSO4 + H2SO4 + O3 → Fe2(SO4)2 + H2O + O2

It liberates iodine from neutral KI solution and the liberated I2, turns starch paper blue.

2KI (aq) + H2O(l) + O3(g) → 2KOH(aq) + I2(s) + O2(g)

I2 + Starch → Blue Colour

Nitrogen oxides (particularly nitric oxide) combine very rapidly with ozone and there is, thus, the possibility that nitrogen oxides emitted from the exhaust systems of supersonic jet aeroplanes might be slowly depleting the concentration of the ozone layer in the upper atmosphere.

NO(g) + O3(g) → NO2 (g) + O2 (g)

(iii) Resonance of O3

The two oxygen-oxygen bond lengths in the ozone molecule are identical (128 pm) and the molecule is angular with a bond angle of about 117o.

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Uses of ozone

Ø As a germicide and disinfectant for sterilizing water.

Ø As a bleaching agent for oils, ivory wax and delicate fibres.

Ø For detecting, the position of double bond in unsaturated compounds.

Ø In destroying odours coming from cold storage room, slaughter houses and kitchen of hotels.

Ø In the manufacture of potassium permanganate.