Sunday, January 30, 2011

Life and Water_2




The hydrogen inter molecular bond in water is 1.8 A0 long. This is lower than expected value of 2.6 A0, which is the total value of van der Waals radius of H and O The lower value is due to the fact that hydrogen atom is so small and in spite of the repulsion forces it can get much nearer to the core of the oxygen atom of another water molecule. The cohesive nature of water is due to these inter molecular Hydrogen Bonds in water.

Another characteristic that water has is that it has higher melting point, boiling point and heat of vaporization than most of the other organic compounds. At each these points a phase change takes place and at each phase change (solid-> liquid, liquid ->gas) some of the Hydrogen bonds are broken. For this energy is needed. But the energy of Hydrogen bond is only 20 kJ/mol while the O-H covalent bond is 460 kJ/mol. The hydrogen bond energy is so little so the question rises, how come the boiling point, melting point and heat of vaporization so high?

Even though the energy of the Hydrogen bond is very low there are many hydrogen bonds in water. Theoretically each water molecule can form H bonds with as many as 4 neighboring water molecules. At any given moment in liquid water at room temperature each water molecule forms hydrogen bonds with an average of 3.4 other water molecules.

So to break more hydrogen bonds you need more energy.

Lifespan of each hydrogen bond is 1 x 10-19 s.Hydrogen bonds are constantly breaking and forming in a dynamic process, within a very short time.

Ice lattice has an open structure thus the volume is larger and as a result the density of ice is lesser than liquid water.

The ice having lower density than liquid water is really important for the existence of life.
Had ice been more dense than liquid water when temperature decreases in winter the water source would start to freeze from bottom to to top which would make it hard for animals to exist.And when it melts the ice at the bottom would be still frozen in the summer, due to the sun rays being unable to reach the bottom of the water source.This would result with a much cooler atmosphere.

But since ice has a lower density than liquid water it starts to freeze from top to bottom and even in winter some part of water source have liquid water at the bottom thus making it possible for aquatic life to exist.

2 comments:

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    1. no problem. I don't write in any order though. Just things here and there from what I study and learn :)

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