Intermolecular forces · Year 12
Intermolecular forces are the weak attractions between molecules. They are much weaker than the covalent bonds inside molecules, but they decide the melting and boiling points of simple molecular substances. There are three kinds: London forces (induced dipole–dipole forces), permanent dipole–dipole forces and hydrogen bonds.
Key facts
- London forces act between all molecules: a temporary dipole in one molecule induces a dipole in its neighbour, and the two attract. More electrons make bigger temporary dipoles and stronger forces.
- Long, straight molecules have stronger London forces than branched ones with the same number of electrons, because they touch over a larger surface: pentane boils at 36 °C, 2,2-dimethylpropane at 10 °C.
- Permanent dipole–dipole forces act between polar molecules such as HCl and propanone: the δ+ end of one molecule attracts the δ− end of the next.
- A hydrogen bond forms between a hydrogen atom bonded to nitrogen, oxygen or fluorine and a lone pair on a nitrogen, oxygen or fluorine atom of another molecule. Hydrogen bonds are the strongest intermolecular forces.
- When a simple molecular substance melts or boils, only the intermolecular forces are overcome; the covalent bonds inside the molecules stay.
- Hydrogen bonding explains why water boils at 100 °C (H₂S at −60 °C), why ice is less dense than water, and why alcohols boil at higher temperatures than alkanes of a similar size.