Intermolecular Hydrogen Bonding

 When a hydrogen atom is bonded to a highly electronegative atom through a covalent bond, the electronegative atom tries to pull the shared electrons towards its own side resulting in the development of a large partial negative charge on the electronegative atom a corresponding partial positive charge on the hydrogen atom. Now, the positively charge hydrogen atom of one molecule may attract the negatively charged atom of some other molecule and the two can be linked together through a weak force of attraction. This weak force of attraction is known as the hydrogen bond or hydrogen bonding. 

It is represented by a dotted line. For example, in the molecule HF, F atom is highly electronegative and acquires a partial negative charge due to pulling of the shared pair. Therefore in HF, H atom possesses a partial positive charge and the molecule of HF can be represented as H-F. 

Due to the presence of partial positive and negative charges, several molecules of the HF link together through hydrogen bonds as shown below.

Due to the presence of hydrogen bonding, hydrogen fluoride exists as a cluster of several HF molecules and is represented as (HF)n. 

Hydrogen bonding may be defined as follows.

The weak attractive force which binds the partially positively charged hydrogen atom of one molecule, with the partially negatively charged atom of some other molecule of similar or different type, or with some other negative center of the same molecule, is referred to as hydrogen bond or hydrogen bonding.

Hydrogen bond is much weaker as compared to a covalent bond. The bond energy of a hydrogen bond lies between 3.5-40 kJ/mol (the bond energy of a covalent bond is usually of the order 400 kJ/mol). The bond length of a hydrogen bond is greater than that of a covalent bond. In HF the hydrogen bond length is 255 pm while that of covalent bond is only 92 pm.

Intermolecular hydrogen bonding

When hydrogen bonding exists between two molecules of the same or different substances, it is termed as intermolecular hydrogen bonding. 

Some examples of the compounds containing this type of hydrogen bonding are given below.

Hydrogen fluoride

In the solid state, hydrogen fluoride consists of zig-zag chains of HF molecules linked together through hydrogen bonds.

These chains persist to a large extent in the liquid and gaseous phases also. However, in the liquid and gaseous states, the length of the chain shortens.

Water

In water molecule, the central oxygen atoms is covalently linked to two H atoms. Each H can form a hydrogen bond. In addition to these two hydrogen bonds, the central oxygen atom also forms two hydrogen bonds with the neighboring molecules. Therefore four hydrogen bonds are formed by single molecule of water.

Ice

In ice, water molecules are clustered through hydrogen bonding. The X-ray diffraction studies of ice suggest that in it, each H2O molecule is tetrahedrally surrounded by the oxygen atom of four neighboring H2O molecules.

Each oxygen atom is thus surrounded tetrahedrally by four hydrogen atoms, two of which are covalently linked to the oxygen atoms at a distance of 100 pm while the remaining two are associated by hydrogen bonds at a distance of 175 pm from the oxygen atoms.

In the lattice of ice, H2O molecules are not packed closely and vacant spaces exist between the molecules. This is why for a given mass, ice occupies more volume than the liquid water. Consequently the density of ice is less and it floats over water. Water has a maximum density at 4 C and not at ) C. This is because on heating the ice, the hydrogen bonds break up progressively and the molecules start packing more closely in the vacant spaces. This happens up to 3.98 C, when water acquires maximum density. Above this temperature, the normal expansion of the liquid more than compensates the increase in density due to close packing of molecules. Hence above 4 C, the density decreases with increase in temperature.

Ammonia

In NH3 molecule, the more electronegative nitrogen atom is covalently linked to three hydrogen atoms. In the liquid state, several molecules of ammonia get associated through hydrogen bonds.

Hydrogen bonding between alcohol and water

Intermolecular hydrogen bonding is also possible between molecules of different substances. For example, ethyl alcohol is dissolved in water, the molecules of the two substances get associated through intermolecular hydrogen bonding.

 

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