What Is Hydrogen Bonding?
Hydrogen bonding is the electrostatic force of attraction between a hydrogen atom (which is covalently bonded to a small and highly electronegative atom) and the lone pair of electrons of another small and highly electronegative atom.
It is a special type of intermolecular force with some characteristics of dipole-dipole attraction and some of a covalent bond. It is the strongest type of intermolecular force which consists of a hydrogen atom sandwiched between two very electronegative atoms.
For hydrogen bonding to occur between two molecules we need:
- one molecule having a hydrogen atom covalently bonded to a very electronegative atom such as F, O or N (the three most electronegative atoms) as this will produce a strong partial positive charge on the hydrogen atom.
- a second molecule having a F, O or N atom with an available lone pair of electrons and these lone pairs will be attracted to the partially charged hydrogen atom in the other molecule to form the bond.
Since hydrogen is less electronegative than most nonmetals, a hydrogen atom covalently bonded to a very electronegative atom produces a very highly polarised bond. The δ+ charge on the hydrogen atom is high enough to form a bond with a lone pair of electrons on the F, O or N atom of a neighbouring molecule.
Though the hydrogen bond is the strongest type of intermolecular force, its strength is about one-tenth of the strength of a normal covalent bond. For maximum bond strength, the angle between the covalent bond to the hydrogen atom and the hydrogen bond should be 180°.
The average number of hydrogen bonds formed per molecule depends on:
- the number of hydrogen atoms attached to F, O or N in the molecule
- the number of lone pairs present on the F, O or N.
Hydrogen bonding in water
A water molecule has two hydrogen atoms and two lone pairs and is therefore extensively hydrogen bonded with other water molecules, with an average of two hydrogen bonds per molecule.
On average, water can form four hydrogen bonds per molecule.
Hydrogen bonding in ammonia
Ammonia has less hydrogen bonds than water.
As you can see on the diagram above, ammonia can form, on average, only one hydrogen bond per molecule. That is because, though each ammonia molecule has three hydrogen atoms attached to the nitrogen atom, it has only one lone pair of electrons that can be involved in hydrogen bond formation.
Effect of hydrogen bonding on boiling point
Hydrogen bonding explains why some compounds have higher boiling points than expected.
There is a rise in boiling point from HCl to HI is due to the increasing number of electrons in the halogen atoms as we go down the group. More electrons means larger electron clouds which leads to increased van der Waals’ forces.
However, if hydrogen fluoride only had van der Waals’ forces between its molecules, its boiling point would be less than that of hydrogen chloride since fluorine has less electrons than chlorine. But the boiling point of hydrogen fluoride is much higher than expected because of the stronger hydrogen bonds between its molecules. Hydrogen fluoride is the only one there that is extensively hydrogen bonded because only oxygen, nitrogen, and fluorine are the three elements that are electronegative enough to make hydrogen bonding possible.
The first hydride in each group shows a much higher boiling point than expected because these compounds have hydrogen bonds, which are significantly stronger than all other intermolecular forces.
In each group we expect a gradual rise in boiling point due to the gradual rise in strength of van der Waals’ forces and the number of electrons increase. However, due to hydrogen bonding, H2O, HF and NH3 have exceptionally high boiling points compared to other compounds with greater number of electrons.
- H2O has a higher boiling point compared to NH3 and HF, because an H2O molecule has 2 lone pairs of electrons and can form, on average, four hydrogen bonds.
- In NH3, the number of hydrogen bonds formed is restricted by the one lone pair of electrons in the NH3 molecule.
- In HF, he number of hydrogen bonds is restricted by the fewer number of hydrogen atoms.
The peculiar properties of water due to hydrogen bonding
1. Solid ice is less dense than liquid water
Solids of most substances are more denser than their liquids because in the solid state, the molecules are packed closer and occupy less volume. However, solid ice is less dense than liquid water because:
- in the solid state, the water molecules are bonded to each other through hydrogen bonding to form a giant 3-dimensional tetrahedral structure which has a lot of empty spaces in between the molecules.
- when heated, some of the hydrogen bond breaks down and the rigid structure collapses, filling up the spaces in between them, thus decreasing the volume occupied by the molecules and increasing its density.
2. Solubility of substances in water
We expect covalent compounds to insoluble in water since they are non-polar. However, some covalent compounds, such as ammonia and alcohols, are soluble in water because they can form hydrogen bonds with water.
3. High boiling point of water
The boiling point of water is much higher for Group 16 hydride. This is because water molecules have hydrogen bonds between themselves. These hydrogen bonds increases its boiling point significantly.
4. High surface tension and viscosity
Water has a high viscosity because hydrogen bonding reduces the ability of water to slide over each other, making it more viscous. Water also has a high surface tension because hydrogen bonds also exert a significant downward force at the surface of liquid.