Hydrogen
Bonding
“Oxygen is more electro negative
than hydrogen, this fact is so important that if it were not
to be true, all living organisms, you and I included would not
have existed. Life as we know would not have been possible.”
- Strange but true. Let us analyse this statement starting with
a look at the polarity of water molecule.
- In this molecule due to the greater electronegativity of Oxygen
there is a polarization of electron density towards the atom
of higher electronegativity.
- This leads to a slight excess concentration of charge density
on the oxygen atom and therefore the hydrogen atom becomes slightly
electron deficient. This can be represented as follows.
In a certain volume of water there are millions
and millions of such molecules. They co-exist in a certain pattern.
The negative part of one molecule will be closer to the positive
part of another. This electrostatic attraction is termed “Hydrogen
bonding”.
“The electrostatic attraction
between a hydrogen atom bonded to an electronegative atom and
another electronegative atom”.
- Another way of looking at it would be
as a proton embedded in the electron cloud of two other atoms
A and B. ( A—H····B).
- Hydrogen bonding occurs between a proton donor group A-H and
a proton acceptor group , where A is an electronegative atom
such as O, N and a halogen, while the acceptor group is a lone
pair of an electronegative atom.
- This attraction does not seem very significant in its magnitude,
sure it is small, just about 5 to 8 KCals per mole while the
regular covalent bond strength is ten times more, but its impact
on the whole system is tremendous.
- Due to the greater electronegativity of Oxygen, water is a
polar molecule, which in turn is the cause of existence of hydrogen
bonding. The unique properties of water are due to hydrogen bonding.
- Hydrogen bonding is the reason for the high heat capacity
of water. It takes a long time for water to get heated up (to
a particular temperature) and an equally long time to cool down
unlike for instance a metal. The heat retaining capacity of water
therefore is high.
- Thus during daytime when we receive solar
radiation, it is water, around three fourth of the globe that
absorbs most of the heat. In the absence of this feature earth’s
temperature would possibly be as high as 200oC as
on the lunar surface (Moon has no water). At night times when
we do not receive the solar radiation directly, it is water
that releases the heat without which the night temperature
would have been probably be –150oC again as on the lunar surface, which
is not facing the sun.
- Water therefore acts as a moderator of temperature on this
planet to within the range in which life as we know is possible.
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