Intermolecular Forces and Properties of Liquids

Intermolecular Forces

  • Intermolecular forces (IMFs) are attractions between atoms, molecules, or ions in condensed phases (solids and liquids).
  • IMFs include dispersion forces, dipole-dipole attractions, and hydrogen bonding.
  • The phase of a substance depends on the relative strength of IMFs and the kinetic energy (KE) of its molecules.
  • Increasing temperature (KE) can induce changes in physical state (e.g., solid to liquid to gas).

Forces Between Molecules

  • Intramolecular forces are within a molecule (e.g., covalent bonds).
  • Intermolecular forces are between molecules and determine physical properties.
  • Van der Waals forces are all attractive forces between neutral atoms/molecules.

Dispersion Forces

  • Dispersion forces (London dispersion forces) exist in all condensed phases.
  • Temporary dipoles arise from asymmetrical electron distribution; induce dipoles in neighboring species.
  • Larger, heavier atoms/molecules exhibit stronger dispersion forces.
  • Polarizability is the measure of how easily a molecule's electron cloud is distorted.
  • Molecular shape affects dispersion forces; elongated shapes have greater contact area and stronger forces.

Dipole-Dipole Attractions

  • Polar molecules have partial positive and negative charges (dipoles).
  • Dipole-dipole attractions are electrostatic forces between partially positive and partially negative ends of polar molecules.

Hydrogen Bonding

  • Hydrogen bonding is a strong dipole-dipole attraction between molecules with H bonded to F, O, or N.
  • It significantly affects the properties of condensed phases (liquids and solids).
  • Hydrogen bonds are intermolecular, not intramolecular, and weaker than covalent bonds.
  • DNA's double helix is held together by hydrogen bonds between complementary base pairs.

Properties of Liquids

  • Viscosity is a liquid's resistance to flow; depends on IMFs, molecular size/shape, and temperature.
  • Cohesive forces are IMFs between identical molecules.
  • Surface tension is the energy required to increase a liquid's surface area, resulting from cohesive forces.
  • Adhesive forces are IMFs between different molecules.
  • Capillary action is when a liquid flows in a porous material due to attraction of liquid molecules to the surface of the material and to other liquid molecules.