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Intermolecular Forces

Intermolecular forces (IMFs) are forces of attraction or repulsion between molecules. They are weaker than intramolecular forces (bonds within a molecule) but play a crucial role in determining physical properties such as boiling point, melting point, solubility, and vapor pressure.

Types of Intermolecular Forces

  1. London Dispersion Forces (Van der Waals)

    • Present in all molecules, especially nonpolar.
    • Caused by temporary dipoles due to electron motion.
    • Strength increases with molecular size and polarizability.
  2. Dipole-Dipole Interactions

    • Occur in polar molecules with permanent dipoles.
    • Positive end of one molecule attracts the negative end of another.
  3. Hydrogen Bonding

    • Special case of dipole-dipole.
    • Occurs when hydrogen is covalently bonded to highly electronegative atoms (N, O, F) and attracted to lone pairs on another molecule.
    • Responsible for water’s high boiling point and unique properties.
  4. Ion-Dipole Interactions

    • Occur between ions and polar molecules.
    • Important in solutions where ionic compounds dissolve in polar solvents like water.

Importance

  • Boiling and Melting Points: Stronger IMFs require more energy to overcome.
  • Solubility: “Like dissolves like” depends on similar types of IMFs.
  • Physical Properties: Vapor pressure, surface tension, and viscosity are influenced by IMFs.

Examples

  • Water (H₂O): Hydrogen bonding dominates, leading to high boiling point.
  • Iodine (I₂): Nonpolar, London dispersion forces dominate.
  • NaCl in water: Ion-dipole interactions allow solubility.

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