Intermolecular interactions

While intramolecular bonds hold atoms together inside a molecule, intermolecular forces act between molecules and govern macroscopic properties like boiling point and viscosity.

Types of intermolecular forces (weakest $\to strongest)$:

Van der Waals potential

$$V(r) \propto -\frac{1}{r^{6}} \quad \text{(attractive at moderate } r \text{)}$$

Phase and energy

• Gibbs free energy: $\Delta G = \Delta H - T\Delta S$.
• A process is spontaneous when $\Delta G < 0$.
• Boltzmann distribution: $P(\text{state}) \propto e^{-E/k_BT}$.

Connecting forces to phases

• Gases: Weak interactions; molecules move freely.
• Liquids: Moderate interactions; molecules slide past each other.
• Solids: Strong interactions; molecules locked in a lattice.

Physics link: The depth of the intermolecular potential well determines the boiling point — deeper well means more energy needed to vaporise.

Common pitfall: Hydrogen bonds are between molecules, not within them: they are attractions between an H bonded to N/O/F and a lone pair on a neighbouring molecule — about 20× weaker than the covalent bonds inside molecules.