OZONE
DEPLETION AND
BOND
ENTHALPIES
Oxygen is present in two forms in nature, normal
oxygen (O2) and ozone (O3) and both forms a key role
in protecting the life on Earth from harmful ultraviolet
radiation, by forming a protective screen. When they
absorb UV, their bonds are broken. The double bond in
O2 is stronger than the 1.5 bond in ozone, so is broken
by radiation of higher energy and shorter wavelengths.
The energy of a photon of light is related to its frequency
by Planck’s equation:
Natural formation and
depletion of ozone
The strong covelent double bond in
normal oxygen O2 is broken by UV
light with wavelength smaller than 242
nm. As oxygen has the strongest bond,
shorter wavelength radiation is needed
to break its bonds.The oxygen atoms
have unpaired electrons. These reactive
free radicals react with another oxygen
molecule to form ozone. It is exothermic
as bonds are formed, which produces
heat that maintains the relatively high
temperature of the stratosphere. The
level of ozone there stays at a constant
level if the rate of formation of ozone
is balanced by the rate of removal. This
is known as steady state. The whole
process is described by the Chapman
Cycle.
CHEMISTRY
DEPARTMENT
The breakdown of O2 involves shorter wavelength light than the breakdown of O3. This means that the
breakdown of oxygen needs more energy than the ozone. The difference is due to the relative strengths
of the O-O bonds in the two molecules. Oxygen contains stronger bonds than ozone due to ozone’s
resonance structure, and so is dissociated by light of shorter wavelength.
Ozone absorbs radiation of wavelengths in the range 200 nm to 315 nm is very significant. This corresponds
to the higher range of ultraviolet light, known as UV-B and UV-C, which can cause damage to living tissues.
The absorption of UV light helps to trap heat.
Aybüke Nazlı GÜLTEKİN
11-A
THE CLAPPER 2014 - 2015
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