Q.How is ozone prepared from oxygen? Explain its reaction with a) C2H4 b) KI c) Hg d) PbS

Ozone is prepared by silent electric discharge through O2 (3O2 -> 2O3). As a strong oxidant it forms an ozonide with C2H4, liberates I2 from KI, causes 'tailing' of mercury, and oxidises PbS to PbSO4.

Preparation of ozone from oxygen:

When a slow, silent electric discharge is passed through pure, dry oxygen in an apparatus called an ozoniser, about 10% of the oxygen is converted to ozone, giving 'ozonised oxygen':

3 O2 <-> 2 O3, Delta H = +142 kJ/mol (endothermic).

Since the reaction is endothermic and reversible, a silent (not sparking) discharge is used to prevent the ozone from decomposing back. Higher concentrations can be obtained and ozone separated by liquefaction.

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

1. With ethene (C2H4): Ozone adds across the double bond to form an unstable addition product, ethene ozonide (ethylene ozonide): C2H4 + O3 -> C2H4O3 (ethene ozonide).
2. With potassium iodide (KI): Ozone oxidises iodide to iodine (this is used to detect/estimate ozone): 2 KI + H2O + O3 -> 2 KOH + I2 + O2. …