Overview
This lesson explains Van der Waals forces, a group of weak intermolecular forces, and compares them to stronger intermolecular and intramolecular bonds.
Intermolecular Forces Overview
- Intermolecular forces (bonds) are attractions between molecules, weaker than intramolecular bonds like covalent bonds.
- These forces significantly affect the physical properties of substances.
- Types include hydrogen bonds, dipole-dipole, dipole-induced dipole, and London dispersion forces.
Dipole-Dipole (Permanent Dipole) Interactions
- Occur between molecules with permanent dipoles (permanently polar molecules).
- Example: In HCl, chlorine attracts electrons more than hydrogen, creating a permanent dipole with partial charges.
- The positive pole of one molecule is attracted to the negative pole of another.
- Dipole-dipole forces are similar to but weaker than hydrogen bonds.
Dipole-Induced Dipole (Debye Forces)
- Form when a polar molecule induces a dipole in a non-polar molecule.
- The polar molecule's electric field pushes electrons in the non-polar molecule, creating a temporary polarity.
- This attraction is weaker and only lasts while the molecules are close together.
London Dispersion (Instantaneous Dipole-Induced Dipole) Forces
- Present between all molecules, especially significant in non-polar molecules.
- Temporary dipoles form when electrons randomly cluster on one side of a molecule.
- These instantaneous dipoles can induce dipoles in neighboring molecules, creating attraction.
- Although individually weak, their effect adds up in large quantities of molecules.
- Also called London forces.
Real-Life Relevance and Comparison
- Van der Waals forces explain phenomena like geckos (gechi) sticking to walls via many tiny contact points maximizing these attractions.
- Intramolecular bonds (covalent, ionic, metallic) are much stronger than intermolecular (Van der Waals and hydrogen) bonds, but the latter still have significant cumulative effects.
Key Terms & Definitions
- Intermolecular Forces — Attractive forces between separate molecules, not within a single molecule.
- Dipole — A molecule with two poles: one positive and one negative.
- Dipole-Dipole Interaction — Attraction between two molecules with permanent dipoles.
- Dipole-Induced Dipole Interaction — Attraction formed when a polar molecule induces polarity in a non-polar molecule.
- London Dispersion Force — Temporary intermolecular force from instantaneous dipoles in non-polar molecules.
- Van der Waals Forces — Collective term for dipole-dipole, dipole-induced dipole, and dispersion forces.
Action Items / Next Steps
- Review previous lessons on hydrogen bonding and VSEPR theory if unclear on dipoles.
- Prepare any questions for class discussion or comment sections.
- Study comparison tables of bond energies for intra- and intermolecular forces.