When a silent electric discharge is passed through dry oxygen, ozone is formed. Oxygen is never converted into ozone completely and we always obtain a mixture of oxygen and ozone.This mixture is called ozonized oxygen.
'Ozoniser', is the apparatus used to prepare ozone by the passage of silent electrical discharge. Two types of ozonizers are used:
It consists of two co-axial glass tubes fused together. Tin foil is used to coat the inner-side of the inner tube and the outer-side of the outer tube. The inner and outer tin coatings are connected to the terminals of an induction coil, which produces current of high voltage. A slow current of pure and dry oxygen is passed through the annular space. On subjecting oxygen to silent electrical discharge, ozonised oxygen containing 10-15% ozone is formed. By taking the following precautions, the yield of ozone can be increased in the ozonised oxygen:
- Only pure and dry oxygen should be used.
- The ozoniser should be perfectly dry.
- A fairly low temperature (around 0°C) should be maintained.
- There should be no sparking.
Fig: 13.9 - Siemen's ozoniser
In principle, this ozoniser is like the Siemen's ozoniser but dilute sulphuric acid replaces the tin foil. Two carbon electrodes are dipped in the acid and connected to an induction coil. A current of dry oxygen is passed through the space between the tubes. Ozonised oxygen containing about 5% O3 comes out at the other end. If the apparatus is kept cool, the proportion of ozone may go up 20-25%.
Fig: 13.10 - Brodie's ozoniser
11. How is ozone formed in the upper atmosphere?
Ozone is formed in the upper atmosphere in two steps:i) Photodissociation of oxygen by ultraviolet radiations of wavelength less than 240 nm.
ii) Combination of highly reactive oxygen atoms with oxygen molecules.
12. What is the principle of preparation of ozone?
The preparation of ozone needs a source of pure, cold and dry oxygen. The formation of ozone from oxygen is a reversible endothermic reaction
In the reaction, 142.7 kJ of energy is absorbed for the formation of one mole of ozone.The reaction is initiated by a sparkless or silent electric discharge, to produce less heat, as ozone is prone to decomposing back into oxygen with a rise in temperature (Le Chatelier's principle). Hence, ozone is prepared in a specially designed apparatus called an Ozoniser to facilitate the above conditions.