Intermolecular Forces: London Dispersion, Dipole-Dipole, and Hydrogen Bonding Explained

What are intermolecular forces?

Intermolecular forces are the forces of attraction that exist between molecules. They determine physical properties like boiling point, melting point, viscosity, and solubility.

When you heat water and it boils, you are not breaking the O-H covalent bonds inside the water molecules. You are breaking the intermolecular forces between the water molecules, giving them enough energy to escape as gas (steam).

The stronger the intermolecular forces, the more energy is needed to overcome them, and the higher the boiling point.

Intermolecular vs intramolecular forces

For example, when water boils at 100 °C, the H₂O molecules separate from each other (intermolecular forces broken), but each molecule remains as H₂O (covalent bonds intact). This is why steam is still H₂O, not hydrogen and oxygen.

London dispersion forces (LDF)

London dispersion forces (also called van der Waals forces or induced dipole-dipole forces) are the weakest type of intermolecular force. They exist between all molecules, including non-polar ones.

How they form

1. At any instant, the electrons in a molecule may be unevenly distributed, creating a temporary (instantaneous) dipole.
2. This temporary dipole induces a dipole in a neighbouring molecule (the electrons in the neighbour shift in response).
3. The resulting attraction between the temporary dipole and the induced dipole is a London dispersion force.

What affects their strength?

London forces get stronger with more electrons (larger molecules). This is because larger electron clouds are easier to distort, creating bigger temporary dipoles. This is why:

• F₂ (gas) → Cl₂ (gas) → Br₂ (liquid) → I₂ (solid): boiling point increases as the molecule gets larger.
• Methane (CH₄, bp -161 °C) boils much lower than octane (C₈H₁₈, bp 126 °C) because octane has far more electrons.

Dipole-dipole interactions

Dipole-dipole interactions occur between polar molecules. A polar molecule has a permanent dipole because the atoms have different electronegativities, pulling the shared electrons unevenly.

The slightly positive end (δ+) of one molecule is attracted to the slightly negative end (δ-) of another. These are stronger than London forces because the dipole is permanent, not temporary.

Examples of polar molecules

• HCl: Cl is more electronegative than H, so the bond is polar. The Cl end is δ- and the H end is δ+.
• CHCl₃: The three polar C-Cl bonds do not cancel out, giving the molecule a net dipole.

Note: some molecules have polar bonds but are non-polar overall because the dipoles cancel due to symmetry. CO₂ is linear and symmetric, so the two C=O dipoles cancel. It only has London forces.

Hydrogen bonding

Hydrogen bonding is the strongest type of intermolecular force. It is a special case of dipole-dipole interaction.

Hydrogen bonds are much stronger than normal dipole-dipole forces because N, O, and F are so electronegative that the H atom becomes very δ+, and the lone pairs on these atoms create a strong attraction.

Key examples

• Water (H₂O): Each molecule can form up to 4 hydrogen bonds (2 through its H atoms, 2 through its lone pairs). This explains water's high boiling point (100 °C) compared to H₂S (-60 °C), which only has dipole-dipole forces.
• Ammonia (NH₃): H bonded to N. Boiling point is -33 °C, higher than PH₃ (-87 °C).
• Ethanol (C₂H₅OH): The O-H group forms hydrogen bonds. This is why ethanol (bp 78 °C) has a much higher boiling point than ethane (bp -89 °C), even though they have similar molecular masses.

Summary comparison

Remember: London forces are always present in addition to any other type. A polar molecule has both London forces and dipole-dipole interactions. A molecule with H-bonding has all three.

Boiling point trends

The boiling point of a substance depends on the type and strength of its intermolecular forces. Here is how to predict boiling point from structure:

Exam examples

For full coverage of bonding and structure, see our AQA Topic 5: Energy Changes and IB Chemistry revision notes.

Frequently Asked Questions

What are intermolecular forces?

Intermolecular forces are the forces of attraction between molecules. They are weaker than covalent bonds. The three types are London dispersion forces, dipole-dipole interactions, and hydrogen bonding. They control physical properties like boiling point

What is the difference between intermolecular and intramolecular forces?

Intramolecular forces are the bonds within a molecule (covalent bonds). Intermolecular forces are the attractions between separate molecules. When a covalent substance boils, only the intermolecular forces break. The covalent bonds stay intact. This is why steam is still H₂O.

Which is the strongest intermolecular force?

Hydrogen bonding is the strongest. It requires H bonded to N, O, or F. Dipole-dipole forces are moderate. London dispersion forces are the weakest, but they get stronger with more electrons (bigger molecules).

Why does water have a high boiling point?

Water forms hydrogen bonds between its molecules. The O-H bond is very polar, and the lone pairs on oxygen attract the δ+ hydrogen of neighbouring molecules. Hydrogen bonds are strong, so more energy is needed to break them, giving water a higher boiling point than you would expect for its small molecular size.