Molecular Interactions and Reactions
What you must understand
This domain links intermolecular forces to physical behaviour and to reaction pathways. Know the hierarchy: dispersion forces (present in all molecules, stronger with more electrons and greater surface area), dipole–dipole interactions, and hydrogen bonding (N–H, O–H, F–H donors). Relate these to boiling point, viscosity, vapour pressure and solubility, remembering that "like dissolves like" is a competition between solute–solute, solvent–solvent and solute–solvent forces. For reactions, understand collision theory, activation energy, and the effect of concentration, temperature, surface area, pressure and catalysts on rate. Interpret energy profile diagrams, distinguish kinetic versus thermodynamic control, and recognise organic reaction types: substitution, addition, elimination, oxidation and condensation, with functional group interconversion.
Traps in questions
- Hydrogen bonding requires an H atom bonded to N, O or F; HCl has dipole–dipole forces, not hydrogen bonds.
- Larger molecules have stronger dispersion forces even if they are nonpolar, so "nonpolar means low boiling point" is unreliable across a homologous series.
- A catalyst lowers activation energy for both forward and reverse reactions and does not change ΔH or equilibrium position.
- Increasing temperature increases rate mainly by increasing the fraction of collisions exceeding activation energy, not just collision frequency.
- In organic mechanisms, watch for Markovnikov addition and for substitution versus elimination competing under different conditions.
Rule of thumb and worked example
Rule: rank intermolecular forces first, then molecular mass, then shape (branching reduces surface contact and lowers boiling point).
Example: Compare boiling points of pentane (36 °C), 2,2-dimethylpropane (9.5 °C) and butan-1-ol (118 °C). Pentane and 2,2-dimethylpropane are nonpolar and similar in mass, but the branched isomer is more spherical, so weaker dispersion forces give the lower boiling point. Butan-1-ol has an O–H group, so hydrogen bonding dominates and raises the boiling point far above both alkanes.