Intermolecular Forces
Chemistry for COMEDK – Beginner Notes
1. What Are Intermolecular Forces?
- Intermolecular means “between molecules.”
- They are tiny pushes or pulls that keep molecules together.
- Think of them like the invisible hand that holds a stack of cards together.
2. Why Do They Matter?
| Feature | How Forces Help | Everyday Example |
|---|---|---|
| Boiling point | Stronger forces → higher boiling point | Water boils at 100 °C, but alcohol boils lower. |
| Stickiness | Forces make liquids cling to surfaces | Soap lathers because molecules stick together. |
| Solubility | Like dissolves like | Sugar dissolves in water because of similar forces. |
3. The Main Types (No “Jargon”)
| Force | What It Looks Like | Quick Analogy | Tiny Example |
|---|---|---|---|
| London Dispersion (Van der Waals) | A tiny, ever‑changing pull that appears in every molecule. | Like two people bumping into each other in a crowded room and briefly holding hands. | Nitrogen (N₂) – only dispersion forces act. |
| Dipole‑Dipole | One molecule has a slight positive side and a negative side; it lines up with another’s opposite side. | Like magnets lining up: north with south. | Chloromethane (CH₃Cl) – permanent dipole. |
| Hydrogen Bond | Very strong pull when hydrogen is attached to O, N, or F and meets another O, N, or F. | Like a special magnetic hook that only a few people can use. | Water (H₂O) – each H bonds to another O. |
Tip: Hydrogen bonds are still just a type of dipole‑dipole, but we name them because they’re super important.
4. “Mini‑Story” of Forces in Action
-
Water in a kettle
- Molecules are pulled together by hydrogen bonds.
- When you heat the kettle, the bonds break one by one until the water turns into steam.
-
Ice vs. Liquid Water
- In ice, hydrogen bonds form a rigid lattice, so the molecules stay in place.
- In liquid water, the bonds form and break quickly, letting the molecules move around.
5. Quick Practice
| Question | Answer |
|---|---|
| Which force is strongest? | Hydrogen bonding (when H bonds to O, N, or F). |
| Why do oils float on water? | Oil molecules have only weak dispersion forces, while water has strong hydrogen bonds, so oil stays separate. |
| Give a molecule that has only London forces. | Neon (Ne) – a noble gas with no dipole. |
6. Bottom Line
- Intermolecular forces are tiny but critical for everyday life.
- They decide how liquids behave, how solids melt, and how substances dissolve.
- Remember: London = any molecule, Dipole‑Dipole = opposite charges line up, Hydrogen bond = the strongest of the three.
Happy studying!