Modern inorganic chemistry

264 GROUP VI

At room temperature ozone is a slightly blue diamagnetic gas

which condenses to a deep blue liquid. It has a characteristic smell,

and is toxic. Ozone is a very endothermic compound :

O 3 -+fO 2 :A//= -142kJmor!

It decomposes exothermically to oxygen, a reaction which can be

explosive. Even dilute ozone decomposes slowly at room tempera-

ture; the decomposition is catalysed by various substances (for

example manganese(IV) oxide and soda-lime) and occurs more

rapidly on heating.

Ozone is very much more reactive than oxygen and is a powerful

oxidising agent especially in acid solution (the redox potential varies

with conditions but can be as high as + 2.0 V). Some examples are:

1. the conversion of black lead(II) sulphide to white lead(II)
   sulphate (an example of oxidation by addition of oxygen):

PbS 4- 4O 3 -> PbSO 4 + 4O 2 T

2. the oxidation of iron(II) to iron(III) in acid solution:

2Fe2+ + O 3 + 2H 3 O+ -> 2Fe^3 + + O 2 t + 3H 2 O

The adherence of mercury to glass, i.e. tailing' in presence of ozone,

is probably due to the formation of an oxide. The oxidation of the

iodide ion to iodine in solution is used to determine ozone quanti-

tatively.

H 2 O + O 3 -> 2OH~ + I 2 4- Ot

The liberated iodine is titrated with standard sodium thiosulphate(VI)
solution after acidification to remove the hydroxide ions.
Addition compounds called ozonides are produced when alkenes
react with ozone and reductive cleavage of these compounds is used
extensively in preparative and diagnostic organic chemistry.
The molecular formula of ozone was determined by comparing
its rate of diffusion with that of a known gas. The geometric structure

./°\
of the molecule is angular O O with two equal O—O distances,
which are slightly greater than in the oxygen molecule, and an
O—O—O angle of 116°.
Ozone has long been used on a small scale for water purification
since it destroys viruses, and recent developments suggest that this
use will increase in importance.