Intermolecular Forces

Revise Intermolecular Forces for Chemistry 2 (IAL) WCH12 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Chemistry · Topic 2.2

Intermolecular Forces

The Big Idea: Molecules stick to their neighbours using weak "sticky" forces (much weaker than the covalent bonds holding atoms together inside them) — and the type and strength of these sticky forces explains almost everything about a substance's melting point, boiling point, and whether it dissolves in water.

Quick Summary

Intramolecular forces are inside a molecule (covalent bonds) — strong. Intermolecular forces are between molecules — weak.
There are three types of intermolecular force: induced dipole–dipole (London/van der Waals), permanent dipole–dipole, and hydrogen bonding (a special, extra-strong type of permanent dipole–dipole).
Strength order: Covalent bond > Hydrogen bond > Permanent dipole–dipole > Induced dipole–dipole.
Hydrogen bonding needs H directly bonded to N, O, or F (small, highly electronegative atoms with lone pairs).
Water's hydrogen bonds explain its unusually high boiling point, high surface tension, and why ice floats (is less dense than liquid water).
More electrons = stronger London forces = higher boiling point. Branching reduces surface contact = weaker London forces = lower boiling point.
"Like dissolves like": polar substances dissolve in polar solvents (water), non-polar substances dissolve in non-polar solvents.

1. Intramolecular vs Intermolecular Forces

Think of a box of magnets glued together in pairs. Each glued pair is stuck together really tightly — that's like a covalent bond, an intramolecular ("within the molecule") force. But when you tip the box, the pairs slide around and cling weakly to each other — that clinging is the intermolecular ("between molecules") force.

Intramolecular forces — forces within a molecule, holding its own atoms together. These are covalent bonds (single, double, triple, or co-ordinate/dative). They're formed when atoms share their outer electrons.

Intermolecular forces — weaker forces between neighbouring molecules. These are what you have to overcome to melt or boil a substance — you are NOT breaking covalent bonds when water boils, you're only breaking the hydrogen bonds between H₂O molecules.

INTERMOLECULAR force (weak, dashed) H-bond between two water molecules H H- - - - - -H H \ / \ / O O / / H H \___________/ INTRAMOLECULAR force (strong, solid line) the O-H covalent bond inside ONE molecule

Key Fact to Remember Intramolecular forces (covalent bonds) are roughly 10× stronger than intermolecular forces (like hydrogen bonds). This is WHY boiling water doesn't break it into H and O atoms — it just separates the molecules from each other.

Q1. When ice melts into liquid water, are covalent O–H bonds broken? Explain.

2. The Three Types of Intermolecular Force

All intermolecular forces are fundamentally about positive and negative charges attracting each other. The three types just differ in how permanent and how strong that charge separation is.