OZONE DEPLETION KKCC

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.

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.

OZONE DEPLETION The reverse process happens

when O3 absorbs longer wavelength UV light. This is because two pibonding 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

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.

OZONE LAYER BEING REDUCED

OZONE DEPLETING POLLUTANTS The main pollutants includes: CFCs

(Chlorofluorocarbons) and oxides of oxygen. (NOx)

SourcesCFCs 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.

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.

REACTIONS FROM CFCS AND NOX CONT.

Nitric oxides are formed from high temperature reactions in supersonic aircraft engines andeven 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.

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.