A thin layer of ozone gas (0 3 ) encircles the earth, in the stratosphere.
0 3 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.
FORMATION OF OZONE
1) O 2 2 O * (Oxygen free radical)
2) O 2 + O * O 3
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, O 3.
The reverse process happens when O 3 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 O 2 .
O 3 (g) O 2 (g) + ●O (g) The oxygen free radicals react with another resonance structure to form O 2 . ●O (g) + O 3 (g) O 2 (g)
O O O O O O Resonance structure of Ozone
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.
OZONE LAYER BEING REDUCED
OZONE DEPLETING POLLUTANTS
The main pollutants includes: CFCs (Chlorofluorocarbons) and oxides of oxygen. (NO x ) 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 N 2 and O 2 in supersonic aircraft engines and lightning. As an effect, the ozone would deplete much quicker compared to the ozone formed.
REACTIONS FROM CFCS AND NO X
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 O 3 to produce O 2 .
REACTIONS FROM CFCS AND NO X CONT.
Nitric oxides are formed from high temperature reactions in supersonic aircraft engines and even in lightning.
The oxides of nitrogen reacts with O 3 to form nitrogen dioxide and O 2 .
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.
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 (CF 3 CH 2 F) They decompose more easily and do not build up in the stratosphere.