Intermolecular Forces Summary

Intermolecular Forces

States of Matter

  • The strength of intermolecular forces dictates the state of matter.
  • Stronger forces bring molecules closer.
  • The state of matter depends on the balance between kinetic energies of particles and interparticle attraction energies.

Types of Intermolecular Forces (Weakest to Strongest)

  • Dispersion forces (London dispersion forces)
  • Dipole–dipole forces
  • Hydrogen bonding (special dipole–dipole force)
  • Ion–dipole forces
  • Van der Waals forces include dispersion and dipole-dipole forces.

Dispersion Forces

  • Temporary polarization of nonpolar particles allows dispersion forces to form.
  • Polarizability is the tendency of an electron cloud to distort.

Factors Affecting Dispersion Force

  • More electrons increase dispersion force.
  • Larger atom/molecule size or molecular weight increases dispersion force.
  • For similar masses, compact molecules have less dispersion force.

Dipole–Dipole Interactions

  • Polar molecules have positive and negative ends (dipoles).
  • Oppositely charged ends attract.
  • For similar mass/size, more polar molecules have higher boiling points.

Dipole–Dipole vs. Dispersion Forces

  • Comparable size/shape: dipole-dipole dominates.
  • One molecule much larger: dispersion forces dominate.

Hydrogen Bonding

  • Unusually strong dipole-dipole interactions when H is bonded to N, O, or F.
  • Attraction between H atom attached to a highly electronegative atom and a nearby small electronegative atom.

Ion–Dipole Interactions

  • Found in solutions of ions.
  • Enable ionic substances to dissolve in polar solvents.

Intermolecular Forces Summary

  • Ions present? -> Ionic bonding
  • Polar molecules present & H bonded to N, O, or F? -> Hydrogen bonding
  • Polar molecules present? -> Dipole-dipole forces
  • Otherwise -> Dispersion forces only

Liquid Properties Affected

  • Boiling point and melting point
  • Viscosity
  • Surface tension
  • Capillary action

Viscosity

  • Resistance of a liquid to flow.
  • Increases with stronger intermolecular forces, decreases with higher temperature.

Energy Change & Change of State

  • Heat of fusion: energy to change solid to liquid at melting point.
  • Heat of vaporization: energy to change liquid to gas at boiling point.
  • Heat of sublimation: energy to change solid directly to gas.

Heating Curves

  • Temperature vs. heat added.
  • Heat within a phase: product of specific heat, sample mass, and temperature change.
  • Temperature does not rise during a phase change.
  • Heat for phase changes: product of mass and heat of fusion/vaporization.

Vapor Pressure

  • Boiling point: temperature at which vapor pressure equals atmospheric pressure.
  • Normal boiling point: temperature at which vapor pressure is 760 torr.

Vapor Pressure and Temperature

  • The natural log of the vapor pressure of a liquid is inversely proportional to its temperature. This relationship is quantified in the Clausius–Clapeyron equation.

Phase Diagram of Water

  • High critical temperature and pressure due to strong Van der Waals forces between water molecules.