Hydrogen bond and its importance in life

What is hydrogen bond ? 

A partially covalent bond which is formed by the weak electrostatic attraction between hydrogen atom attached with electronegative atom or 

group of a molecule and the electronegative atom such as fluorine, nitrogen or oxygen of another same or different molecule, is called  hydrogen bond. 

Hydrogen bonding is denoted by dotted line.

The hydrogen bond among water molecule in 3D and normal structure of ice shown below.

Nature of hydrogen bond :

Hydrogen bond is formed by the weak  electrostatic attraction forces between the positive pole of one molecule and the negative pole of another molecule generally of the same substance.

The strength of a hydrogen bond has been found to vary between 10 - 40 KJ per mole while that of a covalent bond has been found to be of the order of 400 KJ per mole.

Thus, a hydrogen bond is very much weaker than a covalent bond.

What is intra-molecular hydrogen bond ? 

If the hydrogen bond is formed by the electrostatic attraction between polar bonded H-atom and an electronegative element in the same molecule, then the bond is called intra molecular hydrogen bond.

for example, ortho nitro phenol shows intra molecular hydrogen bond, which shown below.

What is inter molecular hydrogen bond ? 

When hydrogen bond is formed by the electrostatic attraction between hydrogen atom of a molecule and an electronegative 

element of another molecule in the same or different substances, then the bond is called inter molecular hydrogen bond.

For example,HF molecule form inter molecular hydrogen bond.

Why does hydrogen bond is very much weaker than covalent bond ? 

Hydrogen bond is formed by the weak electrostatic attraction forces between the positive pole of one molecule and the negative pole of another molecule generally of the same substance.

The amount of heat energy required to break per mole of hydrogen bond is 10 ー 40 KJ.

On the other hand, covalent bond is formed by the  overlapping of two atomic orbitals like SーS,  SーP, PーP of same symmetry.

The amount of heat energy required to break per mole of covalent bond is about  400 KJ.

Generally, the hydrogen bond is very much weaker than covalent bond.

What is importance of hydrogen bond ? 

It can be easily realised that without hydrogen bonding water would have existed as a gas like hydrogen sulphide. 

In that case, no life would have been possible on this world.

Hydrogen bonding also exists in all living organisms whether of animal or of vegetable kingdom.

Thus,it exists in various tissues, organs, blood, skin and bones in animal life.

It plays an important role in determining structure of proteins which are so essential for life.

Hydrogen bonding plays an important role in making wood fibres more rigid .

The cotton, silk or synthetics fibres owe their rigidity and tensile strength to hydrogen bonding.

Hydrogen bonding is thus a phenomenon of great importance in everyday life.


Practice problems:


Why water is liquid but hydrogen sulphide is a gas ?
Why boiling point of water is greater than hydrogen sulphide?  
Why ortho nitro phenol is volatile but para nitro phenol is non volatile ?  

Ionic bond definition-properties in chemistry