Engineering Chemistry S Datta

574 ENGINEERING CHEMISTRY
Venus is 60,000 times that of earth and average temperature of Venus is 425°C making
existence of life impossible.
It is estimated that atmospheric CO 2
content has increased by 25% during last two
centuries, which has raised the global temperature already. It is estimated that within 50–100
years the temperature may increase by 4 to 5°C which will increase the surface water
evaporation and change the climate which may finally influence the growing of crops.
Global warming may also trigger melting of glaciers resulting in rising of sea level. It
also causes floods, hurricanes, tornadoes, and increased breeding of pests.
To minimise the green-house effect it is required to reduce the use of fossil fuels, use of
alternative source of energy. Conservation of forests, reduction in use of automobiles, ban on
CFC’s and nuclear explosions, encouraging and developing green technologies and checking
population growth and awaring people are the other alternatives to check the green-house
effect or global warming.
Depletion of Ozone Layer (O 3
)
Ozone is an important chemical species present in the stratosphere, i.e., above 20 km
from the earth’s surface and its concentration is around 10 ppm. It is formed in the stratosphere
by photochemical reaction:
O 2
+ hν (242 nm) ⎯⎯→ O + O
O + O 2
+ M (such as N 2
or O 2
) ⎯⎯→ O 3
+ M
The species M absorbs the excess energy liberated in the reaction and the molecule of
ozone is stabilized. It is constantly formed. It is destructive to fabrics, rubber goods, crops etc.
At the same time it acts as a protective shield for the life on earth, by strongly absorbing
UV-radiations coming from the sun (220-230 nm) which is harmful for life on earth.
Ozone is destroyed by Cl 2
, released due to volcanic activity and also by reactions with
(i) NO, (ii) atomic oxygen, (iii) reactive —OH radical present in the atmosphere.
Chlorofluorocarbons (CFCs) come out from jumbo jets and supersonic aircraft, nuclear
explosions, CFCs are also used as coolants in refrigerators, air conditioners, aerosols, plastic
foams. CFC’s escape and accumulate at high altitude, undergo decomposition, under UV light,
the decomposition product being Cl 2
. Each atom of Cl reacts with more than 0.5 molecules of
oxygen in chain reaction converting ozone to oxygen and thereby leads to depletion of ozone
layer.
Cl + O 3
⎯⎯→ ClO + O 2
ClO + O ⎯⎯→ Cl + O 2
The environmental hazards due to the CFCs were recognized in 1970 and the detection
of ozone hole over Antarctica in 1985. The use of CFCs were limited since then.
The SO x
and NO x
also react with ozone
SO 2
+ O 2
⎯⎯→ SO 3
+ O 2
NO + O 3
⎯⎯→ NO 2
+ O 2
Hence emissions of all these pollutants are also needed to be minimised.
The depletion of ozone layer allows the harmful UV radiations to reach earth which
finally causes skin cancer, sunburns, breast cancer, cataract, leukemia, lung cancer, visual
impairment, premature aging, DNA damage and even may lead to death.