Page 242 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 16 ELEMENTS: THE CHALCOGENS
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6.8 OZONE

The chemistry of ozone (O ) is unique in the context of group 16. As mentioned, unlike
3
O , which is paramagnetic, ozone is diamagnetic. There are a number of subtleties about its
2
electronic structure, but for our purposes it’s all right to view it as isoelectronic with SO :
2
+ +
O − − O
O O O O

Like SeO (which we will discuss in Section 6.16), O is an oxidant, but a far more
2
3
powerful one. Have a look at Table 1.5 (Section 1.5) to get a sense of its reduction poten-
tial (under acidic conditions) in relation to that of other strong oxidants. Only molecular
−
fluorine, the perxenate anion (HXeO ), hydroxyl radical, atomic oxygen, and a few other
4
species are more powerful as oxidants.
In the laboratory, ozone is generally prepared in an ozonizer by silent or sparkless electric
discharge through oxygen:
Sparkless electric
discharge (6.68)
ΔH = +285.4 kJ
3 O 2 2 O 3
Observe that the reaction is endothermic; that is, ozone is enthalpically uphill relative to
ordinary oxygen. A silent discharge produces less heat and thereby discourages the reverse
step (Le Chatelier’s principle).
Stratospheric ozone is produced from O via a two-step process: photodissociation of
2
O to atomic oxygen by ultraviolet light ( < 240 nm), followed by the reaction of atomic
2
oxygen and O :
2
O → 2 O (6.69)
2
O + O → O 3 (6.70)
2
Ozone itself also reacts with atomic oxygen to regenerate O :
2
O + O → 2O (6.71)
3 2
The reactions are catalyzed by metal ions in cloud droplets and by various free radicals; the
details, unfortunately, are outside the scope of our discussion.
The highest concentration of atmospheric ozone (2–8 ppm), that is, the well-known
ozone layer, is in the lower part of the stratosphere, 20–30 km above the earth’s surface; the
exact range varies significantly with the season. Even here, however, the O concentration
3
is only about 2–8 ppm, which is far lower than that of O . Despite the low concentrations,
2
the ozone layer absorbs most of the solar UV radiation. Together, atmospheric O and O 3
2
almost completely screen the lowest wavelength part of the UV range (<200 nm), which
is the most harmful. The importance of the ozone layer to life and civilization therefore
cannot be exaggerated.
Human-made organohalogens such as chlorofluorocarbons (CFCs; e.g., CFCl and
3
CF Cl ) and bromofluorocarbons, which were widely used as refrigerants and propellants
2
2