Intermolecular interactions

Intermolecular Forces

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

Van der Waals potential: $V(r) \propto -\frac{1}{r^{6}}$, attractive at moderate $r$.

Common pitfall: Hydrogen bonds are between molecules, not within them. They are attractions between an H bonded to N/O/F and a neighbouring lone pair, about 20 times weaker than covalent bonds.

Energy, Phase and Boiling

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

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