Intermolecular Forces

• Central Idea: Interactions between molecules that determine their physical properties and behavior.

Types of Intermolecular Forces

1. Van der Waals Forces

   • London Dispersion Forces

     • Form between all molecules

     • Temporary dipoles induced by electron movements

     • Weakest intermolecular force

     • Strength based on polarizability (# of electrons i.e. more electrons = more polarizable = stronger)

     • Nonpolar molecules ONLY have these

   • Dipole-Dipole Forces

     • Attraction between permanent dipoles of polar molecules

     • Stronger than London dispersion forces

     • Strength based on how polar the molecule is (i.e. Coulomb’s Law)

   • Hydrogen Bonding

     • Special type of dipole-dipole force, when Hydrogen is bonded to Nitrogen, Oxygen, or Fluorine

     • Very high melting and boiling points

     • Strongest intermolecular force

2. Mixtures

   • Dipole-Induced-Dipole Forces

     • Attraction between a polar molecule and a nonpolar molecule

     • Polar molecule (permanent dipole) induces a temporary dipole in the nonpolar molecule

   • Ion-Dipole Interactions

     • Ionic compound is dissolved in a polar solvent

     • Causes the boiling point to drastically increase

     • Strongest intermolecular forces

Factors Affecting Intermolecular Forces

• Molecular Size

  • Larger molecules have stronger London dispersion forces

• Polarity

  • Polar molecules have stronger dipole-dipole forces and hydrogen bonding

• Shape

  • Molecules with larger surface areas have stronger London dispersion forces

• Temperature

  • Higher temperatures weaken intermolecular forces

• Pressure

  • Higher pressures can affect intermolecular forces, especially in gases

• Solvent

  • Type of solvent can influence intermolecular forces in solutions

• Concentration

  • Higher concentrations can affect intermolecular forces in solutions

• Presence of Catalysts

  • Catalysts can alter intermolecular forces in chemical reactions

• Presence of Impurities

  • Impurities can affect intermolecular forces in mixtures

• External Forces

  • External forces such as pressure or electric fields can influence intermolecular forces.