Read Which Force Actually Dominates Before Ranking Attractions
Ranking intermolecular attractions means identifying the STRONGEST force actually available in each option — ion-dipole beats hydrogen bonding, which beats dipole-dipole, which beats dispersion (London) forces, roughly in that order.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Intramolecular vs intermolecular hydrogen bonding changes physical properties oppositely
o-Hydroxybenzoic acid (salicylic acid) forms an intramolecular hydrogen bond within one molecule, which LOWERS its boiling/melting point relative to the para isomer — because that hydrogen bond is 'used up' internally and can't also link separate molecules together the way intermolecular H-bonds do in the para isomer.
Hydrogen bonding explains real anomalies in solubility and volatility
Ammonia's high water solubility (versus phosphine's near-insolubility) and HF's uniquely low volatility among the hydrogen halides both trace back to hydrogen bonding: N-H and H-F bonds are polar and small enough to hydrogen-bond effectively, while P-H is not polar/small enough, and Cl/Br/I are too large and insufficiently electronegative for HCl/HBr/HI to hydrogen-bond the same way.
Ranking attraction strength across different force types
When comparing genuinely different force types in one question (ion-dipole vs dipole-dipole vs dispersion), ion-dipole is strongest whenever an actual ion is present — don't let a familiar-looking dipole-dipole pairing distract from a weaker ion-dipole option elsewhere in the choices, or vice versa; always identify which force TYPE each option actually represents first.
OlympIQ exam lens
- ✓Force strength ranking (strongest to weakest, roughly): ion-dipole > hydrogen bonding > dipole-dipole > dispersion (London) forces.
- ✓Intramolecular hydrogen bonding lowers boiling/melting point relative to an isomer that forms intermolecular hydrogen bonds instead.
- ✓N-H and H-F hydrogen-bond effectively (explaining NH3's water solubility and HF's low volatility); P-H and the heavier hydrogen halides largely don't.
- ✓Identify the force TYPE in each option before ranking — don't assume the most 'familiar-looking' pairing is automatically the strongest.
Don't assume hydrogen bonding is always present just because H and an electronegative atom both appear in the molecule — P-H, C-H and S-H bonds are generally too weakly polar (or the atom too large/insufficiently electronegative) to hydrogen-bond the way N-H, O-H and F-H do.
Ready to try the pattern?
100% coverage across the 15 linked questions.
Practise 15 linked questions