Question

How can Van der Waals forces be used to determine relative boiling point?

Answer 1

The stronger the intermolecular forces between the liquid molecules of a given substance, the higher its boiling point.

Explanation:

The stronger the intermolecular forces between the liquid molecules of a given substance, the higher its boiling point.

Van der Waals forces ( also called London Dispersion Forces `LDF`) exist between non polar molecules such as hydrocarbons and so on. The higher the polarizability, the stronger the Van der Waals forces.

Note that the polarizability increases with the surface area, therefore, for longer chains the surface area of contact increases and so the Van der Waals forces.

For example, the boiling point of pentane `C_5H_12` is higher than that of butane `C_4H_10`. This is easily understandable since butane is gas at room temperature, and pentane is liquid at the same temperature.