Overview
This lecture covers the different types of intermolecular forces, how they impact phase changes like boiling, and how to identify which forces are present in various substances.
Types of Intermolecular Forces
- Intermolecular forces are electrostatic attractions between molecules.
- Ion-ion interactions occur between formally charged ions and are the strongest intermolecular force.
- Ion-dipole interactions occur between an ion and a molecule with a dipole (partial charges).
- Dipole-dipole interactions occur between molecules with permanent dipoles (polar molecules).
- Hydrogen bonds are strong dipole-dipole interactions specific to N-H, O-H, or F-H bonds.
- Van der Waals (London dispersion) forces are weak attractions due to momentary/induced dipoles and are present in all substances.
Intermolecular Forces and Phase Changes
- Boiling requires enough energy to overcome intermolecular forces holding molecules together in a liquid.
- Stronger intermolecular forces mean higher melting and boiling points.
- Helium, with only van der Waals forces, boils/melts just above absolute zero.
- Water, with hydrogen bonds, has higher melting/boiling points (273 K and 373 K).
- Sodium chloride, held by ion-ion interactions, has very high melting/boiling points (melting at 1074 K).
Identifying Types of Intermolecular Forces
- Nonpolar covalent compounds (no dipole) only exhibit van der Waals forces.
- Polar covalent compounds may have dipole-dipole or hydrogen bonding if N-H, O-H, or F-H bonds are present.
- The molecular geometry determines if a molecule is polar (has a dipole); shapes can cancel out bond dipoles (e.g., CO₂ and BF₃ are nonpolar).
- Formally charged ions have ion-ion interactions.
Key Terms & Definitions
- Intermolecular Forces — Forces of attraction between molecules.
- Ion-Ion Interaction — Strongest force; attraction between oppositely charged ions.
- Ion-Dipole Interaction — Attraction between an ion and a polar molecule.
- Dipole-Dipole Interaction — Attraction between polar molecules with permanent dipoles.
- Hydrogen Bond — Strong dipole-dipole interaction involving N-H, O-H, or F-H bonds.
- Van der Waals (London Dispersion) Forces — Weak attractions from temporary dipoles in all molecules.
Action Items / Next Steps
- Practice identifying the types of intermolecular forces in various compounds given their structure and bonding.
- Review boiling and melting points in relation to intermolecular forces.