intermolecular forces

• Intermolecular forces: the forces that act between molecules, which result from their FGs

  • in ionic compounds: electrostatic interactions, very strong forces between ions of opposite charge
  • in covalent compounds - Van der waals forces, dipole-dipole, hydrogen bonding
• van der waals forces

  • induced dipoles: temporarily occurring dipoles due to connectivity of charge with nonpolar molecule
  • temporary fluctuation in electron density in different parts of a molecules
  • weak forces are additive
  • influenced by polarizability, larger atoms = more polarizable (I > Br > Cl > F)
• dipole-dipole interactions

  • between permanent dipoles of polar molecules
  • much stronger than VDW
  • positive polarity connects with negative
• hydrogen bonding

  • attractive force between an H atom attached to an electronegative atom and an unshared electron pair on another (esp. O, N)
  • OH has smaller bond length which means more energy/strength
• Ranking of forces: ionic > H bonding > dipole-dipole > VDW
• Boiling point

  • stronger intermolecular forces means need more energy to separate individual molecules (higher b.p.)
  • for 2 compounds with similar FGs, the larger molecules will have the higher boiling point