Intermolecular Forces

Three types of forces exist between molecules (intermolecular) they are:

  1. Van der Waal's
  2. Permanent Dipole
  3. Hydrogen Bonding

The properties, such as boiling points and melting points, of molecules are dependent on intermolecular forces, since these forces have to be considered if we are to change between states.

van der Waal's

These forces arise because electrons in atoms or molecules are constantly moving and at very high speeds. At any instant in time, it is possible the one side of the atom or molecules has a greater share of the electrons.

When this happens, an instantaneous electric dipole occurs. This temporary state caused by the imbalance of electrons provides a negative end of a dipole and a positive one.

This temporary dipole then induces the same imbalance in its neighbouring molecules, which hence are now attracted to one another.

This temporary intermolecular force is often referred to as van der Waal's force.

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The strength of a van der Waal's force is determined by its molecular mass, the increase in electrons causes a greater dipole difference to occur, hence a stronger attraction is produced between the molecules.

An example of this effect is the increase in boiling points of Group VII as we descend the group.

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Permanent dipole

The forces occur in polar molecules. A polar molecule arises from the molecule being unsymmetrical in terms of shape or type of atom present.

Example: HCl

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Compare this with a non-polar molecule such as methane, which although having different atoms joined along covalent bonds, its symmetrical shape cancels out any charge which will have risen from the former.

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The size of a permanent dipole is determined upon the electronegativities of the atoms.

The greater the difference in electronegativity (e.g. CsCl) the greater the dipole difference will be.

Solids whose particles are held by permanent dipoles have greater boiling points than those held by van der Waal's, due to their permanent nature.

Hydrogen bonds

The abnormal behaviour of water is explained by the presence of the strongest type of intermolecular force, the Hydrogen Bond.

Water is highly polar due to the large difference in electronegativity between hydrogen and oxygen. This leads to a very strong permanent dipole attraction.

Each water molecule can make two hydrogen bonds between the hydrogen atoms and the lone pair of electrons on the oxygen atoms of the neighbouring water molecules.

This explains the high boiling point of water compared with the other hydrides of Group VI.

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This bond occurs between other molecules that consist of hydrogen atoms and electronegative atoms with lone pairs of electrons.

In diagrams, hydrogen bonds are indicated by dashed lines.

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