Intermolecular Forces
High-Yield Summary
- Intermolecular forces act BETWEEN molecules (unlike covalent/ionic bonds, which act within a molecule) and determine melting/boiling point.
- Strength ranking, weakest to strongest: London dispersion forces < dipole-dipole interactions < hydrogen bonds.
- London dispersion forces are the only IMF in nonpolar molecules; they arise from temporary, fluctuating electron distributions.
- Dipole-dipole interactions require a permanent dipole (polar molecules only).
- Hydrogen bonds require H covalently bonded to N, O, or F, interacting with an N/O/F on a neighboring molecule; about 5-10% as strong as a covalent bond but far stronger than typical dipole-dipole.
The Three Intermolecular Forces
| Force (Weakest → Strongest) | Requires |
|---|---|
| London dispersion force | Any molecule (polar or nonpolar) — only IMF for nonpolar molecules |
| Dipole-dipole interaction | Permanent dipole moment (polar molecules only) |
| Hydrogen bond | H covalently bonded to N, O, or F, attracted to N/O/F on a neighboring molecule |
Must Know
- London dispersion force strength scales with molecular size/electron count/polarizability — larger nonpolar molecules have higher boiling points.
- Hydrogen bonding causes anomalously high boiling points relative to molecular weight: H₂O (100°C) vs. heavier H₂S (−60°C), since sulfur isn't electronegative enough for H-bonding.
- Stronger IMFs → more energy needed to separate molecules → higher boiling/melting point.
Common MCAT Trap
- London dispersion forces are present in EVERY molecule, polar or nonpolar — not just nonpolar ones. Nonpolar molecules just don't have anything stronger.
- A heavier molecule doesn't automatically have a higher boiling point — H₂S is heavier than H₂O but boils far lower, because it lacks hydrogen bonding.
- Hydrogen bonding requires H bonded directly to N, O, or F — not just any electronegative atom (e.g., H–Cl does not hydrogen bond by this definition).
Quick Recall
Rank the three intermolecular forces from weakest to strongest.
Why does water boil at a much higher temperature than the heavier H₂S?
What is the only intermolecular force available to a nonpolar molecule, and why does it exist at all?