Q.How is ozone prepared from oxygen? Explain its reaction with -

Ozone is made by a silent electric discharge through dry O2 (3O2 -> 2O3). As a strong oxidant it forms ethylene ozonide with C2H4, liberates I2 from KI, causes tailing of Hg, and oxidises black PbS to white PbSO4.

Preparation of ozone from oxygen:

When a slow, silent electric discharge (not a spark, which would decompose ozone) is passed through pure, cold, dry oxygen in an apparatus called an ozoniser, about 10% of the oxygen is converted into ozone, giving 'ozonised oxygen':

3O2 <-> 2O3; delta_H = +142 kJ/mol (endothermic)

Because the reaction is endothermic, low temperature and silent discharge favour a good yield; a spark or high temperature would decompose the ozone formed. Pure ozone can be obtained by liquefaction and fractional distillation of ozonised oxygen.

Ozone is a powerful oxidising agent because it readily gives up a nascent oxygen atom: O3 -> O2 + [O].

Reactions:

1. With ethene (C2H4): ozone adds across the double bond to form ethylene ozonide. C2H4 + O3 -> C2H4O3 (ethylene ozonide). (On hydrolysis this ozonide gives carbonyl compounds - the basis of ozonolysis.)
2. With potassium iodide (moist): ozone oxidises iodide to iodine. 2KI + H2O + O3 -> 2KOH + I2 + O2. The liberation of iodine (which turns starch blue) is used to detect and estimate ozone. …