Overview
This lecture explains the different types of van der Waals forces (intermolecular forces), compares them to other bonds, and highlights their significance in real-life phenomena.
Intermolecular Forces Overview
- Intermolecular forces are attractions between molecules, weaker than covalent bonds within molecules.
- They influence macroscopic properties of substances (e.g., boiling points, solubility).
- Types include hydrogen bonds, permanent dipole-dipole, dipole-induced dipole, and London dispersion forces.
Permanent Dipole-Dipole Forces (Pole-Pole)
- Occur between molecules with permanent dipoles (polar molecules).
- Example: HCl molecules attract via partially positive (H) and partially negative (Cl) ends.
- Weaker than hydrogen bonds but stronger than other van der Waals forces.
Dipole-Induced Dipole Forces (Debye Forces)
- Form between a polar molecule (permanent dipole) and a non-polar molecule.
- The polar molecule induces a temporary dipole in the non-polar molecule by redistributing its electrons.
- The attraction is weak and only present while molecules are close.
Instantaneous Dipole-Induced Dipole Forces (London Forces)
- Occur between non-polar molecules due to momentary uneven electron distribution creating a temporary dipole.
- This instantaneous dipole can induce a dipole in neighboring molecules, resulting in attraction.
- These are the weakest, but in large numbers, their cumulative effect is significant.
Real-Life Significance of van der Waals Forces
- Van der Waals forces explain adhesion phenomena, such as how gecko lizards stick to walls.
- At the micro and nano scale, even weak van der Waals forces can become significant.
Comparison of Bond Energies
- Intramolecular bonds (covalent, ionic, metallic) have much higher bond energies than intermolecular forces.
- Van der Waals forces and hydrogen bonds are much weaker, but not negligible in bulk.
Key Terms & Definitions
- Intermolecular Forces — Attractions between different molecules, weaker than atomic bonds.
- Permanent Dipole — A molecule with a consistent separation of charge (permanent positive and negative regions).
- Dipole-Induced Dipole (Debye Force) — Attraction where a polar molecule induces a dipole in a non-polar molecule.
- Instantaneous Dipole-Induced Dipole (London Force) — Temporary dipoles created by momentary electron placement induce attraction between non-polar molecules.
- Hydrogen Bond — A strong type of dipole-dipole attraction involving hydrogen bonded to F, O, or N.
Action Items / Next Steps
- Review lectures/notes on hydrogen bonds and VSEPR theory for background.
- Study diagrams showing electron distribution in dipole interactions.
- Homework: Practice identifying types of van der Waals forces in given molecules.