Intermolecular forces
Intermolecular forces are attractions between separate molecules. They are much weaker than bonds inside molecules. The main types are London dispersion forces, dipole-dipole forces and hydrogen bonds.
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Analogy
Bonds inside a molecule are welded joints. Forces between molecules are Velcro: weak one-by-one but strong in large numbers.
Example
CH₄: −162 (London only)HCl: −85 (dipole-dipole)H₂O: 100 (hydrogen bonds)
In practice
These forces decide melting points, boiling points and how a liquid flows.
Practice
Flashcards
- Which intermolecular force exists in all molecules?London dispersion forces.
Interview questions
- Explain why water has a much higher boiling point than hydrogen sulfide.
- Water forms hydrogen bonds
- H₂S has weaker dipole-dipole and London forces
- More energy needed to separate water molecules
- Boiling point reflects intermolecular forces
Questions people ask
What are London dispersion forces?
London forces are weak attractions that exist between all molecules. Electrons move constantly, so for a moment one side of a molecule has slightly more negative charge. This temporary dipole nudges a neighbour into a matching dipole, and the two attract. Larger molecules with more electrons feel stronger London forces.
Why do larger molecules usually boil at higher temperatures?
Bigger molecules have more electrons, and their electron clouds are easier to distort, which is called polarisability. That makes London forces stronger. More energy is needed to pull the molecules apart into a gas, so the boiling point rises. Compare the halogens: F₂ is a gas, I₂ is a solid.