Ozone in the Stratosphere

1. OZONE DEPLETION
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2. OZONE IN THE STRATOSPHERE
 A thin layer of ozone gas (03) encircles the earth, in
the stratosphere.
 03 is a natural occurring bluish gas which has an
acrid odor. It has harmful effects on organisms
however, it helps block out the harmful UV rays
15km to 45km up in the atmosphere.
 It is located between 12 km to 50 km above the
Earth.
 The function of it is to prevent 99% of the sun’s
ultraviolet light.

3. FORMATION OF OZONE
1) O2 2 O* (Oxygen free radical)
2) O2 + O* O3
The ozone is formed when the double bond in the
oxygen is broken by very short wavelength UV light.
An oxygen free radical is therefore formed, this
oxygen free radical further reacts with another
oxygen atom to form the ozone gas, O3.

5. OZONE DEPLETION
 The reverse process happens
when O3 absorbs longer wave
length UV light. This is because two pi
bonding electrons are shared between
the entire structure therefore it
has weaker bonds compared to O2.
 O3 (g)  O2 (g) + ●O (g)
The oxygen free radicals react with another
resonance structure to form O2.
●O (g) + O3 (g)  O2 (g)
O
O O
O
O O
Resonance structure of
Ozone

6. STEADY STATE
 As a result the rate of which Ozone is produced and
depleted would be equal, this is know as the
steady state.
 The ozone helps protects the earth from dangerous
UV light by this process, however, this steady state
is constantly being interfered by ozone-depleting
pollutants.

7. OZONE LAYER BEING REDUCED

8. OZONE DEPLETING POLLUTANTS
 The main pollutants includes: CFCs
(Chlorofluorocarbons) and oxides of oxygen. (NOx)
Sources
CFCs are mainly found in hair spray or deodorant
cans as a propellant, old refrigerators, air
conditioners e.t.c.
Oxides of nitrogen are formed from high
temperature reactions of N2 and O2 in supersonic
aircraft engines and lightning.
As an effect, the ozone would deplete much quicker
compared to the ozone formed.

9. REACTIONS FROM CFCS AND NOX
 When CFCs are very un
reactive, therefore, it will
steadily rise from the
atmosphere to the stratosphere.
 C-Cl bond enthalpy is only
338kJ mol-1 compared to C-F
bond which is 484 kJ mol-1, it is
weaker and breaks when it
comes into contact with UV
Light. This reactive Cl free
radical contains an unpaired
electron which gladly react with
O3 to produce O2.

10. REACTIONS FROM CFCS AND NOX CONT.
 Nitric oxides are formed from
high temperature reactions in
supersonic aircraft engines and
even in lightning.
 The oxides of nitrogen reacts with O3 to form
nitrogen dioxide and O2.
Consequences are more UV light will reach the earth
increasing the chances for skin cancer, sun burns,
damage to animals and vegetation and even
genetic mutations.

11. ALTERNATIVE TO CFCS
 Even when CFCs are banned it will still remain in the
atmosphere for a very long time because of their low
reactivity.
Properties required for substitutes are low reactivity, low
toxicity, low flammability as well as no weak C-Cl bonds.
 HFCs (Hydro fluorocarbons), HCFCs
(hydrochlorofluorocarbons) and HFAs
(hydrofluoroacetates) are considered to be good
alternatives because it contains no Cl, which is the gas
which catalyzes ozone depletion. An example would be
Tetrafluoroethane (CF3CH2F)
They decompose more easily and do not build up in the
stratosphere.