W5

• Intermolecular forces


  The forces that exist between the molecules of a compound
• Intermolecular forces are much weaker than intramolecular forces
• Intermolecular forces


  Help to determine the physical properties of a molecule such as melting point, boiling point, surface tension, etc.
• Types of intermolecular forces

  • Ion-dipole
  • H-bonding
  • Dipole-dipole
  • Dipole-induced dipole
  • London forces of attraction
• Strength of intermolecular forces

  • Strongest: Ion-dipole
  • H-bonding
  • Dipole-dipole
  • Dipole-induced dipole
  • Weakest: London forces of attraction
• Van der Waals forces

  Consist of dipole-dipole interaction, dipole-induced dipole interaction, and dispersion forces
• Dipole-dipole attraction


  Present among polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule
• Dipole-induced dipole interaction


  Present among polar and nonpolar molecules, where the partial charges in the polar molecule cause polarization or distortion of the electron distribution in the nonpolar molecule, resulting in an induced dipole
• London dispersion forces


  Temporary forces of attraction that exist between the electrons of two adjacent atoms, the weakest of all intermolecular forces
• All interacting substances exhibit the presence of London dispersion forces in addition to other forces of attraction
• London dispersion forces increase as the molecular mass of a substance increases
• Ion


  An atom or molecule that has gained or lost one or more of its valence electrons, giving it a net positive or negative electrical charge
• Ion-dipole interaction


  Attraction between a cation (+) or anion (-) and a permanent polar molecule
• 1. bonding


  A special type of dipole-dipole interaction between the H-atoms in a polar bond, requiring the H-atoms to be bonded with more electronegative atoms such as O, N, or F
• Summary of intermolecular forces

  • Ion-dipole: Ion (cation or anion) and a polar molecule
  • Hydrogen bonding: Polar molecules containing H chemically bonded to a small and highly electronegative nonmetal atom such as N, O, and F
  • Dipole-dipole: Polar molecules
  • Dipole-induced dipole: Polar and nonpolar molecules
  • London dispersion forces: All substances and solely for nonpolar molecules
• SO2 exhibits dipole-dipole and London dispersion forces
• Na+ and CH2O exhibit ion-dipole and London dispersion forces
• CO2 exhibits only London dispersion forces
• NH3 and H2O exhibit H-bonding, dipole-dipole, and London dispersion forces
• H2O and Cl2 exhibit dipole-induced dipole and London dispersion forces
• Boiling point, melting point, viscosity and surface tension increase as the strength of intermolecular forces increases
• Vapor pressure and volatility decrease with increasing strength of intermolecular forces
• CCl4 exhibits only London dispersion forces, while CHCl3 exhibits dipole-dipole and London dispersion forces. CHCl3 has a higher boiling point and melting point.
• NH3 exhibits H-bonding, dipole-dipole, and London dispersion forces, while CH3F exhibits dipole-dipole and London dispersion forces. NH3 has a higher vapor pressure.
• F2 and Br2 both exhibit only London dispersion forces. Br2 has a higher boiling point and melting point, but a lower vapor pressure.