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2.2.9Additional lessons

Intermolecular Forces — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 33 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Three Types of Force

2.2.9 Intermolecular Forces

Intermolecular forces act between molecules and control physical properties.

London dispersion
  • Temporary dipoles from moving electrons induce dipoles in neighbours; these forces act between all molecules.
Dipole–dipole
  • The δ+ end of one polar molecule attracts the δ− end of another.
Hydrogen bond
  • A strong attraction between H bonded to N, O or F and a lone pair on N, O or F of another molecule.
Strength order
  • London forces are generally weakest, then dipole–dipole, then hydrogen bonding, but all are far weaker than covalent bonds.

Part 2 of 5Factors Affecting London Forces

Factors Affecting London Forces
More electrons
  • Larger molecules have more electrons, so their electron clouds are more polarisable and forces are stronger.
Molar mass
  • Boiling points of the alkanes rise with chain length: methane −162 °C, ethane −89 °C, propane −42 °C.
Shape
  • Branched molecules pack less closely and have a smaller contact area, so forces are weaker.
Isomers
  • Butane boils at −0.5 °C but its branched isomer 2-methylpropane boils at −11.7 °C.

Part 3 of 5Hydrogen Bonding

1
Hydrogen is bonded to a very electronegative atom, N, O or F, so it carries a strong δ+ charge.
2
The small δ+ hydrogen can approach a lone pair on a neighbouring N, O or F atom closely.
3
This produces an attraction much stronger than ordinary dipole–dipole forces.
4
Water, ammonia and hydrogen fluoride therefore have unusually high boiling points for their size.
5
Ethanol boils at 78 °C, far above propane at −42 °C and dimethyl ether at −24 °C, which have similar molar masses.

Part 4 of 5Effects on Properties

Boiling point
  • Stronger intermolecular forces need more energy to separate molecules, giving higher boiling points.
Volatility
  • Substances with weak forces evaporate easily and have high vapour pressures.
Solubility
  • Molecules that form hydrogen bonds with water, like ethanol, dissolve readily in it.
Boiling a liquid
  • Only intermolecular forces are overcome; the covalent bonds inside molecules stay intact.
CompoundMain forceBoiling point / °C
CH₄London−162
HClDipole–dipole−85
NH₃Hydrogen bonds−33
H₂OHydrogen bonds100

Part 5 of 5Exam Checklist & Tips

2.2.9 – Key Points for the Exam

  • London dispersion forces Weak attractions caused by temporary dipoles in electron clouds.
  • Dipole–dipole force Attraction between permanent dipoles of polar molecules.
  • Hydrogen bond Attraction between H bonded to N, O or F and a lone pair on another N, O or F.
  • Polarisability How easily an electron cloud is distorted.

Common Mistakes

  • Saying that boiling water breaks the O–H covalent bonds.
  • Claiming a hydrogen bond is a covalent bond.
  • Forgetting that every molecule has London forces, even polar ones.
  • Giving hydrogen bonds to any molecule that contains hydrogen.

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