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Unformatted text preview: is pure ozone. Actually the
concentration of ozone is extremely low. Ozone is spread thinly at a concentration of about 10 ppm at
elevations between 15 and 35 km. If all the ozone in the atmosphere was collected into a layer of pure ozone at
standard temperature and pressure, it would only be 3 mm thick! The low concentration and unstable nature of
ozone combine to make the ozone layer a fragile part of the atmosphere. Formation and Destruction of Ozone. Ozone is formed in the ozone layer by a two step mechanism
initiated by the action of UV light on oxygen molecules. In the first step, UV-light dissociates molecular
oxygen into oxygen atoms in a process called photodissociation. The energy of the UV-photon is enough to
break the O—O bond. In the second step, the oxygen atom combines with an oxygen molecule, forming ozone. net O2 (g) + h ν → 2O(g)
O(g) + O2 (g) → O 3 (g) (occurs twice)
3O2 (g) → 2O3 (g) Ozone, being an unstable molecule, tends to undergo reactions that convert it to the more stable O 2
molecule. Ozone absorbs UV light with wavelengths in the range 200 to 300 nm. When ultraviolet light is
absorbed by ozone, it too undergoes photodissociation:
O3 (g) + h ν → O2 (g) + O(g)
Atomic oxygen can also destroy ozone.
O(g) + O3 (g) → 2O 2 (g) Catalytic Depletion of Ozone. The concentration of ozone in the stratosphere depends on a balance
between the rate of its production from molecular oxygen and its rate of destruction.
3O2 (g) → 2O3 (g)
2O3 (g) → 3O 2 (g) production of ozone
destruction of ozone Ozone is said to be in a steady state. This refers to the situation where the rate of ozone production is
approximately equal to its rate of destruction. Any process that speeds up the destruction of ozone will cause its
stratospheric concentration to decrease.
The depletion of stratospheric ozone has been associated with man-made substances called
chlorofluorocarbons, or CFCs for short. The CFCs were developed in the 1930s by chemists who were seeking
a new refrigerant to repla...
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- Spring '08