Association
The weak electrostatic interactions of intermolecular hydrogen bonding, causes two or more molecules of a compound to exist as aggregates or associated molecules. This is seen in water molecules, which undergo molecular association. Alcohols and carboxylic acids (RCOOH) also exist as a group of molecules. Molecular association results in the increase in the size as well as in the molecular mass of the compound. These above examples are shown below:
Higher melting and boiling points
The compounds containing hydrogen bonds have high melting and boiling points. Due to hydrogen bonding, the intermolecular force of attraction in the compound becomes large. Consequently, larger energy is required in separating these molecules before they can melt or boil. The melting and boiling points of hydrides of elements of groups 14, 15, 16 and 17 show the influence of hydrogen bonding on their melting and boiling points.
Influence on the physical state
Hydrogen bonding also influences the physical state of the substances (solid, liquid or gas).
Illustration:Both oxygen and sulphur belong to same group but H2O is a liquid at ordinary temperature, while H2S is a gas. This is explained on the basis of electronegativity values.
In water, oxygen is highly electronegative so that it forms hydrogen bonds. As a result the molecules of H2O get associated with one another and this raises the boiling point of water. Consequently, water exists as liquid at room temperature. On the other hand, the difference in electronegativity of atoms in H2S is less and hydrogen bonding in H2S is almost negligible. As a result H2S is not associated and exists as a gas at room temperature.
Solubility
Hydrogen bonding also influences the solubility of one substance in another. Covalent compounds do not generally dissolve in water, but those that can form a hydrogen bond with water, readily dissolve in it. For example, alcohols like ethanol, ammonia, amines, lower aldehydes and ketones are soluble in water due to the formation of hydrogen bonds with water molecules.
Water an unusual Liquid
Water is the most common substance on the earth's surface. Water has interesting structure and this accounts for its many unusual properties. One of the most important of these is its ability to dissolve a variety of compounds. It dissolves ionic compounds, many polar inorganic and organic compounds. Water also has a high heat of vaporisation, high heat of fusion, high specific heat, good liquid range (from 0°C, freezing point, to 100°C, boiling point).
The interesting behaviour of water with its many unusual properties canbe explained on the basis of hydrogen bonding.
