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Intermolecular Forces

Review - 3

Explain the trend in the normal boiling points of these liquids in terms of intermolecular forces.

CH4 normal boiling point: -161.5°C
CF4 normal boiling point: -28°C
CCl4 normal boiling point: +77°C
CBr4 normal boiling point: +190°C

Hint

Determine what type(s) of intermolecular force act between pairs of these molecules. All of the molecules are tetrahedral, they do not have permanent dipole moments.

Answer

The stronger the intermolecular forces, the higher the normal boiling point will be. This is because with stronger intermolecular forces the energy required to separate the molecules when they are converted from liquid to gas is larger.

The only intermolecular forces acting between pairs of these molecules are dispersion forces. The magnitude of the dispersion forces depends on the polarisability of the molecules which in turn depends on the volume of the molecules. The covalent radius of the outer atoms in these molecules increases in the order H < F < Cl < Br. This implies that the molecular volume and hence polarisability increases in the order CH4 < CF4 < CCl4 < CBr4. The dispersion forces increase in the same order and this explains the trend in normal boiling points.

 

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