IB Chemistry›Structure 2›2.2›2.2.8
2.2.8
Structure 2.2 SL & HL ⏱ ~4 min revision

Intermolecular Forces

London dispersion, dipole-dipole, and hydrogen bonding (the forces between molecules).

Intermolecular forces (IMFs) are weak electrostatic attractions between molecules. They are not chemical bonds (they are broken during changes of state, not during chemical reactions).

IMF Type Cause Strength Present In
London Dispersion Forces (LDF) Temporary, instantaneous dipoles from random electron fluctuations → induced dipoles in neighbours Weakest All molecules (polar and non-polar)
Dipole-Dipole Permanent dipoles attract each other (δ⁺ → δ⁻) Moderate Polar molecules only (e.g. HCl)
Hydrogen Bonding H bonded to F, O, or N creates an extremely polar bond; lone pair on F/O/N of another molecule attracts the δ⁺ H Strongest H-F, H-O, H-N systems (e.g. H₂O, NH₃, HF)
Three Types of Intermolecular Forces 1. London Dispersion (LDF) Temporary & induced dipoles Temporary Dipole δ− δ+ attraction Induced Dipole δ− δ+ Electron cloud shifts randomly Induces opposite dipole in neighbour Present in ALL molecules 2. Permanent Dipole-Dipole Between permanent polar molecules δ+ H δ− Cl attraction δ+ H δ− Cl Permanent electrostatic attraction Between opposite δ+ and δ− ends Polar Molecules (e.g. HCl) 3. Hydrogen Bonding H bonded to F, O, or N δ− O lone pair : H H H-BOND δ+ H δ− O H δ+ H attracted to lone pair Extremely strong dipole interaction H₂O, NH₃, HF Systems
Key Concept

Why Water Has Anomalous Properties

Water's unusually high boiling point, high specific heat capacity, and ice's lower density than liquid water are all due to extensive hydrogen bonding between H₂O molecules. Each water molecule can form up to 4 hydrogen bonds (2 donor H atoms + 2 lone pairs on O).

Examiner Trap

Naming Intermolecular Forces

  1. London forces exist in ALL substances (even polar ones). Don't say "non-polar molecules only have London forces" and forget that polar molecules also have them.
  2. Hydrogen bonding requires H bonded directly to F, O, or N (not just any electronegative element). H-Cl does NOT show hydrogen bonding.
IB Spec Note

Van der Waals Nomenclature

The IB uses "van der Waals forces" as an umbrella term covering London (dispersion) forces, dipole-induced dipole, and dipole-dipole forces. Hydrogen bonding is not classified under van der Waals in the IB syllabus.

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