Overview
This lecture explains the three main types of intermolecular forces—hydrogen bonding, dipole-dipole forces, and London dispersion forces—and how they affect molecular interactions.
Intermolecular Forces Overview
- Intermolecular forces are forces that act between different molecules and hold them together.
- They arise from attractions between oppositely charged regions of molecules.
- Three main types: hydrogen bonding (strongest), dipole-dipole, and London dispersion forces (weakest).
Hydrogen Bonding
- Hydrogen bonding occurs in polar molecules with hydrogen bonded to nitrogen, oxygen, or fluorine.
- Examples: NH₃ (ammonia), H₂O (water).
- These are not actual bonds but strong attractions due to charge differences.
- Water is polar, with oxygen partially negative and hydrogens partially positive, leading to hydrogen bonding between molecules.
Dipole-Dipole Forces
- Dipole-dipole forces occur between polar molecules with permanent dipoles.
- Example: CO (carbon monoxide), where carbon is partially positive, and oxygen is partially negative.
- These forces are attractions between oppositely charged ends of different molecule dipoles.
London Dispersion Forces
- London dispersion forces are present in all molecules, especially non-polar ones.
- Example: N₂ (nitrogen gas), where electrons can momentarily create a temporary dipole.
- This temporary dipole causes a weak attraction that quickly disappears.
- These are the weakest intermolecular forces, but universal due to moving electrons.
Summary of Intermolecular Forces
- Hydrogen bonding: in polar molecules with H bonded to N, O, or F.
- Dipole-dipole: in polar molecules not involved in hydrogen bonding.
- London dispersion: present in all molecules; only type in non-polar molecules.
Key Terms & Definitions
- Intermolecular Forces — forces of attraction between molecules.
- Polar Molecule — molecule with regions of partial positive and negative charge.
- Hydrogen Bonding — strong attraction between H and N, O, or F in polar molecules.
- Dipole-Dipole Force — attraction between oppositely charged regions of polar molecules.
- London Dispersion Force — weak, temporary attractions from electron movement in all molecules.
Action Items / Next Steps
- Review examples of molecules exhibiting each type of intermolecular force.
- Prepare to apply knowledge of intermolecular forces to physical and chemical properties in future lessons.