Sunday, January 30, 2011

Life and Water_1

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great,
And would suffice.
-Robert Frost-

Life evolved in the sea. Earlier life forms were so fragile they could not withstand the harsh terrestrial environment.As life evolved gradually they were able to dominate land too. But we brought a little bit of the 'sea' with us. That is the extra cellular and intra cellular fluids. Both these consists of 70% of water. Most of the reactions that are important for life take place in this water medium.

Water molecule has sp3 hybridization.
The two sp3 half filled orbitals overlap with 1s orbitals of two hydrogen atoms and form the molecule of water. The O-H bonds have an angle of 104.50. Even though usually sp3 hybridization gives a tetrahedral shape, since there are only two bonds, water molecule has a bent shape.

Hydrogen and Oxygen atoms have different electro negativity values. Due to this difference the electron cloud in the O-H bond goes more towards oxygen atom and it becomes more negatively charged than the H atom.It has a 33% of ionic character. Thus creating a dipole moment in the O-H bond.The dipole moment is 1.83 D(debye).

Due to the bent structure and the polarity of the O-H bonds overall water molecule is polar. ( Consider ; had water been a linear molecule the dipole moments would have been cancelled to each other and it would not be a polar molecule)

Water has got extraordinary cohesive ability. The electro static attraction between the two poles of water molecules tend to orient in such a way so that the O-H bond of one molecule points towards the lone pair cloud of the oxygen atom on another water molecule.

This causes a directional inter-molecule association which results in H bonds. In general a hydrogen bond may be D-H---A where D-H is a weakly acidic donor group such as N-H or O-H and A, atoms such as Nitrogen or Oxygen which bears lone pairs, thus weakly basic acceptor.

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