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Intermolecular Hydrogen Bonding

When a hydrogen atom is attached to a highly electronegative atom (such as F, O or N) the bond is highly polar.

These hydrogen bonds are unusually strong compared to other dipole- dipole interactions because of the following few reasons:

a)    the small size of the H atom 

b)    the small size leads to the concentration of a partial positive charge of the dipole in a very small volume.

There are TWO broad categories of hydrogen bonding:

(i) The intermolecular hydrogen bonding - Such a hydrogen bond exists between the hydrogen atom of one molecule and an electronegative atom of another molecule as in water, ammonia, hydrogen fluoride, etc...

(ii) The intramolecular hydrogen bonding - Such a hydrogen bond exists within the same molecule as in salicylic acid, orthonitrophenol, etc...

Let us focus our study on intermolecular type of hydrogen bonding with a few examples and impacts on their physical properties.

An Example of Ethanol and Dimethyl Ether :

Both ethanol and dimethyl ether have the same molecular formula C2H6O and thus, also the same molecular weight. The main difference between these two compounds is that in ethanol, there is O-H bonding whereas in ether there are no such bonds.

In ether, the H is not attached to an electronegative atom such as N, F or O. So, there are no hydrogen bonds.

  Dimethyl Ether

But, in ethanol, H being directly in link with O atom, there exists hydrogen bonding. That means ethanol molecules are more strongly bound to one another than ether molecules. Due to this reason, the boiling point of ethanol is much higher than dimethyl ether.

  Ethanol Hydrogen Bonding

The type of bonding formed by ethanol here is “intermolecular” hydrogen bonding, which implies that the hydrogen bonds are formed between two ethanol molecules. Intermolecular hydrogen bonding increases the boiling point whereas “intramolecular hydrogen bonding” reduces the boiling point.

Other Effects of Intermolecular Hydrogen Bonding:

This above discussed is only one of many such examples that are sited in our every day life.
Here are many more that shows you how intermolecular hydrogen bonding affects the physical properties of substances.

1)  Associates of two or more molecules to form larger units:

For example, two molecules of carboxylic acid associate to form a dimer in the vapour state.

Compounds such as glycol, sulfuric acid, honey, etc...form hydrogen bonding that give rise to their higher values of viscosity and surface tension.

2)      Viscosity:
Viscosity is a measure of a liquid’s resistance to flow. The stronger  the intermolecular forces between liquid molecules, the more they hold together and the harder it is for them to flow past one another.

For example, glycerol contains two hydrogen bonding –OH groups, whereas ethanol has only one. Glycerol, therefore, has stronger intermolecular forces, which make glycerol more viscous than ethanol, as well as giving it a lower vapour pressure and higher boiling point.

3)      Solubility:

Solubility of substances are based on the formation of hydrogen bonds between the solute and the solvent or the two substances added.

Let us take an example of p-nitro phenol, it is capable of forming intermolecular hydrogen bonding between water and p-nitrophenol. So p-nitrophenol is highly soluble in water.

4) Molecular weight, Boiling points and volatility of substances- para- nitrophenol:

There is significant change in the molecular weight of substances when intermolecular hydrogen bonds are formed. This in turn, increases its boiling point as well. Higher the boiling point, less volatile the substance will be.
In p-nitrophenol, as there are intermolecular hydrogen bonding, it increases the molecular weight, whereby increasing the boiling point. So p-nitrophenol is less volatile.

5) Surface Tension:

Molecules in the interior of a liquid interact with molecules on all sides, whereas those at the surface interact only with those below them. A net inward force of attraction contracts the surface and makes it act as a “skin”. The toughness of this skin is measured as its surface tension, which is the energy required to break through the surface or to disrupt a drop of liquid and spread the material out as a film. Water has rather strong intermolecular forces resulting in a relatively high surface tension.

6) Density of ice is less than that of water.

Ice floats on water. This is an anomalous behavior of water. When water freezes, it expands. In ice, each water molecule is tetrahedrally bonded to four other molecules through intermolecular hydrogen bonding. The arrangement is extended to a 3-D network forming an open cage-like structure with a large amount of vacant space inside the structure. These spaces therefore, help increase the volume and thus lower the density of ice. 

When ice melts, its regular structure collapses to some extent, but approximately 85% of the hydrogen bonds remain. Liquid water is thus less “open” than ice and so is more dense. Without the hydrogen bonds holding the liquid molecules together, the liquid form would be more open and less dense, as is the case with most other substances.