Notes on Intermolecular Forces and States of Matter

Intermolecular Forces (IMFs)

  • Definition: Forces that exist between molecules, weaker than ionic or covalent bonds.
  • Effects on Properties: IMFs influence melting point, boiling point, and vapor pressure.
  • Types of IMFs:
    • Dipole-Dipole
    • London Dispersion
    • Hydrogen Bonding

Dipole-Dipole Forces

  • Occurrence: Effective in polar molecules.
  • Attraction: Positive end of one dipole attracts the negative end of another.
  • Strength: Increases with polarity of molecules.

London Dispersion Forces

  • Weakest IMF: Present in all molecules, including nonpolar.
  • Mechanism: Instantaneous dipoles due to electron movement.
  • Polarizability: Larger molecules can be more easily polarized, having greater strength in London dispersion forces.

Hydrogen Bonding

  • Strongest IMF: Occurs between hydrogen and highly electronegative atoms (F, O, N).
  • Nature: Strong dipole-dipole attraction.

Comparison of IMFs

  • Hydrogen Bonding:
    • Strength: Very Strong
    • Used: In polar molecules with H and N, O, or F.
  • Dipole-Dipole:
    • Strength: Strong
    • Used: In polar molecules.
  • London Dispersion Forces:
    • Strength: Weak
    • Used: In nonpolar molecules; present in all molecules.

Properties Influenced by IMFs

  • Boiling Point & Melting Point: Higher IMF correlates with higher boiling/melting points.
  • Vapor Pressure: Lower IMFs lead to higher vapor pressures, especially in low molar mass liquids.

Properties of Liquids

  • Viscosity: Resistance to flow; increases with IMF strength and molecular complexity.
  • Surface Tension: Energy needed to increase surface area; higher with stronger IMFs.
  • Capillary Action: Movement of liquid in narrow tubes.

States of Matter vs Energy

  • Solids: Higher IMFs
  • Gases: Weaker IMFs

Phase Changes**

  • Heating Curves: Phases transition at specific temps (melting, freezing, vaporization).
  • Energy Changes:
    • Molar Heat of Fusion (ΔHfus): 6.01 kJ/mol for ice.
    • Molar Heat of Vaporization (ΔHvap): 40.7 kJ/mol for water.
  • Reason for ΔHvap > ΔHfus: Vaporization requires breaking all intermolecular interactions, unlike melting.