E9 Ozone depletion
1 hour
Assessment statement
E.9.1 Explain the dependence of O2 and O3 dissociation on the wavelength of light.
Oxygen molecule, which is diatomic and the ozone (O3) are allotropes. Between the
two oxygen atoms in the oxygen molecule exists a double bond that holds the two
oxygen atoms together. Ozone, in contrast, is composed of 3 oxygen atoms and the
molecule is covered with 2 pi bonding electrons, thus two resonance structures are
required to represent its Lewis diagram. Each of the atoms in the ozone may be
double bonded or single bonded. According to experimental technique applied to
determine the type of bond between the 3 oxygen atoms in an ozone molecule, each
of the bonds in the ozone is made up of two identical bonds that have similar
strength and length (1.5 bond).
Since such is the case, double bond between two oxygen atoms in a oxygen molecule
has shorter length and greater strength than a single bond between an ozone
molecule (O3). Due to greater strength of a double bond compare to a single bond,
the light being used to dissociate ozone molecule must contain lower energy than
that of a regular ozone molecule.
According to the Planck’s equation (∆𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = ℎ𝑓, 𝑓 = 𝑐/𝜆), as the wavelength of
the light decreases, the energy that the light contains increases. The wavelength
and the frequency are inversely proportional(𝑓 = 𝑐/𝜆). As the wavelength
shortens, the frequency increases. In the aftermath of such relationship, the
dissociation of a double bond between the two oxygen atoms in the oxygen molecule
requires light with 242 nm or less while the dissociation of the bond between the
oxygen atoms in the ozone molecules requires the light with 330nm or shorter
wavelength.
Describe the mechanism in the catalysis of O3 depletion by CFCs and NOx.
Some of the pollutants in the atmosphere such as nitrogen oxide and
chlorofluorocarbons (CFCs) function as catalysts of depletion of the ozone.
For example, in the stratosphere, chlorofluorocarbons (CFCs), which is used for
refrigeration, propellants, UV light is capable of breaking weaker C-Cl bonds by
homolysis to produce chlorine free radicals. CFCs usually remain in the troposphere
but the CFCs molecules eventually diffuse into the upper atmosphere, stratosphere
where they gain higher energy from UV light. When the CFCs is exposed to high
energy UV radiation, photochemical decomposition occurs producing reactive
chlorine gas atoms. Instead of C-F bond, which is more electronegative, thus
stronger than C-Cl, the weaker C-Cl bond is broken down first.
The equation of the initial step of homolysis is shown:
The chlorine free radical produced from the photochemical decomposition
of the CFCs functions as a catalyst in the decomposition of ozone.
Then, the newly formed ClO molecule enters the termination step, by
reacting with oxygen free radical to form oxygen molecule and chlorine
free radical.
Overall, the net equation can be represented,
Since the chlorine free radical is regenerated, one chlorine free radical
can destroy numerous ozone molecules following the same step as shown
above.
The nitrogen oxides, which are used for supersonic aircraft engine, react
similar to how CFCs react with the ozone.
From the initial step, the nitrogen monoxide reacts with ozone to produce
nitrogen dioxide and an oxygen molecule. Then the nitrogen dioxide
reacts with an oxygen free radical to regenerate nitrogen monoxide and
an oxygen molecule.
As a result overall, an ozone molecule reacts with an oxygen free radical
to form 2 oxygen molecules.
Outline the reasons for greater ozone depletion in Polar Regions.
Scientists have found that the concentration of the ozone is decreasing
sharply in the 1980s and 1990s Antarctica compared to that in the past.
They stated increasing ozone depleting pollutants to be the major source
of cause. The scientists have also found that the concentration of the
ozone varies depending on the season. In October, there is the greatest
depletion in the concentration of ozone producing a hole. In November, the
concentration recovers due to cold air.
The coldest spot on the Earth is the lower stratosphere in the South Pole
because of the circular wind preventing warmer air from entering the
region during the winter. This traps the cold air in the region and forms
ice crystals in the atmosphere. In this region, cold air forms ice that
functions as heterogeneous catalyst and provide the surface area for
pollutants to produce reactive gas such as chlorine molecules.
HCl + ClONO2  HNO3 + Cl2
The ice also contains hydrogen chloride and chorine nitrate, which
produce chlorine. In October, when the temperature gets warmer, the
chlorine molecules go through the photo-dissociation with UV light,
depleting the ozone molecules in the atmosphere.
In spring in Antarctica, the higher temperature melts the ice crystals in
the stratosphere and no long provide surface area for the production of
chlorine molecules.