Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Intermolecular (interparticle) forces act between particles. They are Coulombic and much weaker than covalent bonds. Boiling a molecular liquid breaks none of its covalent bonds.
- London dispersion forces come from temporary dipoles that induce dipoles in neighbours. Every molecule has them. In large molecules they are often the strongest net force.
- A polar molecule adds dipole–dipole forces (with another polar molecule) or dipole–induced dipole forces (with a nonpolar one).
- Hydrogen bonding is a strong dipole interaction between H bonded to N, O or F and a lone pair on N, O or F in another molecule or another part of the same large molecule.
- Ion–dipole forces link an ion to the oppositely charged end of a polar molecule. They are usually stronger than dipole–dipole forces.
- In proteins and DNA, these noncovalent forces act between different parts of one molecule and between molecules, and set the molecule’s shape.
Key relationships
| Force | Needs | Gets stronger when |
|---|---|---|
| London dispersion | any particles | more electrons, larger cloud, π bonds, more contact area (less branching) |
| Dipole–induced dipole | polar + nonpolar | bigger dipole; more polarizable partner |
| Dipole–dipole | polar + polar | bigger dipoles; head-to-tail orientation |
| Hydrogen bonding | H–N, H–O or H–F donor; N, O or F lone pair acceptor | more hydrogen bonds per molecule (more donor H atoms and acceptor lone pairs) |
| Ion–dipole | ion + polar molecule | larger ion charge; bigger dipole |
Assumptions behind a ranking
- The molecules being compared have similar numbers of electrons before you let dipoles or hydrogen bonds decide.
- You have the correct shape, so you know whether bond dipoles cancel.
- Boiling point is used as the evidence for force strength (melting point can mislead; see Topic 3.2).
Mistakes to avoid
- Breaking bonds on boiling. Only intermolecular forces are overcome.
- H on carbon. CH₃OCH₃ and (CH₃)₂CO have H and O but cannot hydrogen bond with themselves.
- Forgetting acceptors. Acetone cannot donate, but it accepts a hydrogen bond from water.
- “Polar always wins.” CCl₄ (nonpolar, 74 electrons) boils far above CH₃F (polar, 18 electrons).
- Mass as the cause. Polarizability (electrons, cloud size) and contact area are the causes.
- “Van der Waals = London.” Name the specific force.
- Wrong orientation. Cations face water’s O; anions face water’s H.
Quick self-check
- Which force is the only one between Br₂ molecules? (London dispersion)
- Which can hydrogen bond with itself: CH₃NH₂ or CH₃F? (CH₃NH₂: it has H on N)
- Which has the higher boiling point: pentane or neopentane, and why? (Pentane: the straight chain has more contact area, so stronger dispersion forces)
- Name the force between Ca²⁺ and water. (Ion–dipole)
Next: practice questions.