Intermolecular Forces

Learning Objectives

  • Understand the kinetic molecular model explaining properties of liquids and solids.
  • Differentiate between types of intermolecular forces (IMFs).
  • Predict potential intermolecular forces in a molecule.
  • Describe key properties of liquids influenced by intermolecular forces:
    • Surface Tension
    • Viscosity
    • Vapor Pressure
    • Boiling Point
    • Molar Heat of Vaporization
  • Analyze the phase changes (solid-liquid, liquid-vapor, solid-vapor) based on energy changes and molecular order.
  • Interpret phase diagrams for water and carbon dioxide.

Kinetic Molecular Theory of Liquids and Solids

  • The kinetic molecular model suggests that the behavior of solids and liquids is determined by:
    • Intermolecular Forces: Forces that attract neighboring particles.
    • Kinetic Energy (KE): Energy that maintains distance and movement of particles.

Properties of Solids and Liquids

Solid
  • Particle Arrangement: Strong forces keep particles fixed, vibrating in place.
  • Movement: Very limited; particles cannot freely move.
  • Density: Very high.
  • Diffusibility: Extremely slow.
  • Compressibility: Slightly compressible.
  • Volume and Shape: Fixed.
Liquid
  • Particle Arrangement: Strong forces but particles can slide past one another.
  • Movement: Moderate.
  • Density: High.
  • Diffusibility: Slow.
  • Compressibility: Slightly compressible.
  • Volume and Shape: Fixed volume but shape of the container.

Types of Forces

Intermolecular Forces (IMFs)
  • Definition: Attractive forces between particles. Crucial for determining states of matter.
  • Types include:
    • Dipole-Dipole Forces: Attractive forces between polar molecules.
    • Hydrogen Bonding: Strong dipole-dipole interactions when H is bonded to N, O, or F.
    • Ion-Dipole Forces: Attraction between an ion and a polar molecule.
    • London Dispersion Forces: Present in all molecules, arise from electron movement.
Intramolecular vs. Intermolecular Forces
  • Intramolecular Forces: Hold atoms within a molecule together (e.g., covalent bonds).
  • Intermolecular Forces: Hold molecules together (weaker than intramolecular).

Strength of Forces

  • Intramolecular forces are generally stronger than intermolecular forces because breaking covalent bonds requires more energy than overcoming intermolecular attractions.

Surface Tension

  • Defined as the tendency of a liquid to minimize its surface area due to cohesive forces among molecules.
  • Water, for example, has high surface tension due to strong hydrogen bonding.

Viscosity

  • The resistance of a liquid to flow.
  • Influenced by:
    • Strength of intermolecular forces (stronger forces increase viscosity).
    • Molecular size and shape (larger molecules increase viscosity).
    • Temperature (increased temperature decreases viscosity).

Vapor Pressure

  • Defined as the pressure of a vapor in equilibrium with its liquid (or solid) phase.
  • Increases with temperature as more molecules escape into the vapor phase.

Boiling Point

  • The temperature at which vapor pressure equals atmospheric pressure.
  • A higher vapor pressure indicates a lower boiling point.

Heat of Vaporization

  • The amount of heat needed to vaporize one mole of a substance at its boiling point.
  • Reflects the strength of intermolecular forces.

Phase Changes

  • Occur when energy changes affect the forces among molecules, resulting in:
    • Freezing (liquid to solid)
    • Melting (solid to liquid)
    • Evaporation (liquid to gas)
    • Condensation (gas to liquid)
    • Sublimation (solid to gas)

Phase Diagrams

  • Graphical representations of pressure-temperature relationships illustrating state equilibriums.
    • Triple Point: All three phases coexist in equilibrium.
    • Critical Point: The temperature and pressure at which distinct liquid and gas phases do not exist.
  • Notably for water, the solid-liquid line slopes downward, which is atypical compared to other substances where it slopes upward.