A level Chemistry · Edexcel 9CH0 · Topic 2E
Intermolecular forces
Key points
- London forces come from instantaneous dipoles inducing dipoles in neighbours; they act between all molecules and increase with number of electrons and surface contact (straight chains beat branched ones).
- Permanent dipole–dipole forces act between polar molecules, in addition to London forces.
- Hydrogen bonds form between an H atom bonded to N, O or F and a lone pair on an N, O or F of another molecule; bond angle around the H is about 180°.
- Hydrogen bonding explains water's high boiling point and why ice is less dense than liquid water (open lattice).
- Boiling points of Group 6/7 hydrides rise down the group (more electrons) except for H₂O and HF, which are anomalously high.
- Solubility: like dissolves like. Alcohols dissolve in water by hydrogen bonding; ionic solids dissolve when ions are hydrated; non-polar substances dissolve in non-polar solvents.
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Intermolecular forces are the weak attractions between molecules. They are much weaker than covalent bonds but they decide boiling temperatures, volatility and solubility of every molecular substance.
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