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ozone (O3)





ozone
Image courtesy NASA's Earth Observatory
The triatomic form of oxygen. Ozone is a poisonous, pungent, faintly blue gas that is produced by the action of electrical discharges (such as lighting) on oxygen molecules. It forms a stratospheric layer in the Earth's atmosphere (see ozone layer) and is responsible for filtering out ultraviolet radiation from the Sun which would otherwise make advanced life on the planet impossible.

Among the strategies which will be used in the future to search for signs of life on extraterrestrial Earth-class planets will be to look for the spectral absorption lines that are characteristic of ozone.


melting point -192.7 °C
boiling point -111.9 °C


Chemical properties of ozone

The ozone molecule is a high-energy form of oxygen. Oxygen molecules have to be energized (usually electrically) before they come together to form ozone:

3O2 + energy = 2O3.

When the ozone molecule is formed it is active and unstable – it easily reverts to ordinary oxygen. It is a very powerful oxidizing agent. For example, lead sulfide is oxidized to lead sulfate by ozone:

PbS + 4O3 = PbSO4 + 4O2

and ferrous sulfate (in the presence of dilute sulfuric acid) to ferric sulfate:

2FeSO4 + O3 + H2SO4 = Fe2(SO4)3 + H2O + O2.

In these reactions, the ozone molecules lose a solitary oxygen atom, but in other reactions the whole molecule takes part. For example, sulfur dioxide is oxidized to sulfur trioxide, using all three oxygen atoms in the process:

3SO2 + O3 = 3SO3.

Although ozone is present in very small proportions in the air its powerful oxidizing action can, over a period of time, cause cracking in rubber tires – to overcome this, anti-oxidants are mixed with the rubber.


Ozone finds the double bond

Important classes of organic compounds contain double or triple unsaturated chemical bonds, and the chemist needs to know where these are situated in the molecule. Ozone is used in analysis because it is taken in at the unsaturated link to form explosively unstable ozonides. The compound under investigation is dissolved in an organic solvent and ozone is bubbled through to form the ozonide. In butene, for example, the ozonide link can be formed either between the second and third carbon atoms at the double bond.
H3C – CH = CH – CH3
or the third and fourth carbon atoms:
H3C – CH2 – CH = CH – CH2
If the resulting compound is heated gently in water it breaks at the ozonide link. In the first reaction, acetaldehyde (CH3CHO) and acetic acid (CH3COOH) are formed, each with two carbon atoms. In the second reaction two compounds, one with three carbon atoms and one with a single carbon atom, are formed. By identifying the resulting fractions, the position of the double bond in the original compound is found.


Related category

   • INORGANIC CHEMISTRY
   • ATMOSPHERIC PHENOMENA AND STRUCTURES