Intermolecular forces and physical propertiesEdexcel International A Level Chemistry: Revision notes

A liquid boils when its molecules gain enough energy to overcome the intermolecular forces and escape as a gas. The covalent bonds inside the molecules are not broken.

The stronger the intermolecular forces, the more energy is needed, so the higher the boiling temperature and the lower the volatility. Volatility is how easily a substance evaporates.

To compare substances, work through these steps:

1. Name the forces in each substance (London forces, permanent dipole–permanent dipole, hydrogen bonds).
2. Compare the number of electrons (size) so that you can compare London forces.
3. Say which has the stronger forces and so needs more energy to overcome them.

Alkane molecules are non-polar, so only London forces act between them. London forces depend on the number of electrons, and so on the size of the molecule.

As the chain gets longer, the molecules have more electrons and a larger surface area of contact with their neighbours. The London forces get stronger and more energy is needed to separate the molecules.

Data: methane 112 K, ethane 184 K, propane 231 K, butane 273 K.

The boiling temperature therefore increases with chain length. This is why short-chain alkanes are gases and longer ones are liquids at room temperature.

Isomers have the same molecular formula and so the same number of electrons. Their London forces differ because of shape.

A straight-chain molecule can lie alongside its neighbours with a large area of contact. A branched molecule is more compact and more spherical. It has less surface contact and cannot approach its neighbours as closely, so the London forces are weaker.

Data: pentane 309 K, 2-methylbutane 301 K, 2,2-dimethylpropane 283 K.

More branching means a lower boiling temperature.

Alcohols have an –OH group. Hydrogen is bonded to oxygen, so alcohol molecules form hydrogen bonds as well as London forces.

Compare ethane (18 electrons, 184 K) with methanol (18 electrons, 338 K), and propane (26 electrons, 231 K) with ethanol (26 electrons, 351 K).

With the same number of electrons, the London forces are similar, so the difference arises from the extra hydrogen bonding in the alcohol. The alcohol has a higher boiling temperature and lower volatility.

Within the alcohols, boiling temperature increases with chain length as London forces get stronger.

Boiling temperatures: HF 293 K, HCl 188 K, HBr 206 K, HI 238 K.

From HCl to HI the number of electrons increases, so London forces get stronger. The molecules are polar, but the polarity falls as the halogen becomes less electronegative. The London forces increase more than the dipole attractions decrease, so the boiling temperature rises.

HF is the exception. Fluorine is so electronegative that HF forms hydrogen bonds, which are stronger than the other forces here. HF has the fewest electrons but the highest boiling temperature of the four.