INTERMOLECULAR FORCES OF ATTRACTION

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• Kinetic Molecular Theory of Solids and Liquids:

  • Explains properties of solids and liquids in terms of intermolecular forces of attraction and the kinetic energy of the individual particles
  • All matter is made up of tiny particles
  • These particles are in constant motion
  • The speed of particles is proportional to temperature, with an increase in temperature leading to greater speed
  • Solids, liquids, and gases differ in distances between particles, freedom of motion of particles, and the extent to which the particles interact
• Intermolecular Forces of Attraction:

  • Attractive forces between molecules or particles in the solid and liquid state
  • Collectively known as Van der Waals forces
  • Types of intermolecular forces include dipole-dipole, hydrogen bonding, ion-dipole, London dispersion, and dipole-induced dipole force
• Dipole-Dipole Forces:

  • Exist between polar molecules
  • One end of a dipole attracts the oppositely charged end of the other dipole
• Hydrogen Bonding:

  • Special and very strong type of dipole-dipole force
  • Exists between a hydrogen atom bound to a small and highly electronegative non-metal atom
  • Occurs in polar molecules containing H and highly electronegative elements like Nitrogen, Fluorine, and Oxygen
• Ion-Dipole Force:

  • Acts between an ion (positive or negative) and a polar molecule
  • Explains the solubility of ionic compounds in water, which is a polar molecule
• London Dispersion Forces:

  • Weakest type of intermolecular force
  • When two non-polar molecules approach each other, an instantaneous dipole moment forms
  • Sometimes called an induced dipole-induced dipole attraction
• Surface Tension:

  • Measure of the elastic force in the surface of a liquid
  • Amount of energy required to stretch or increase the surface of a liquid by a unit area
• Capillary Action:

  • Tendency of a liquid to rise in narrow tubes or be drawn into small openings
  • Result of intermolecular attraction between the liquid and solid materials
• Cohesion:

  • Attraction of molecules to other molecules of the same kind
• Adhesion:

  • Attraction of one kind of molecule to a different kind
  • Can be quite strong for water, especially with other molecules bearing positive or negative charges
• Viscosity:

  • Resistance of liquids to flow
  • Increases with stronger intermolecular forces and decreases with increasing temperature
• Evaporation:

  • Process by which an element or compound transitions from its liquid state to its gaseous state
• Vapor Pressure:

  • Pressure exerted by the gas in equilibrium with a liquid in a closed container at a given temperature
  • Relates to the strength of intermolecular forces
• Boiling Point:

  • Temperature at which the vapor pressure of a liquid is equal to the external pressure
• Molar Heat of Vaporization:

  • Amount of energy required to vaporize one mole of a substance at its boiling point
• Properties of Water:

  • Colorless, odorless, and tasteless liquid at room temperature
  • Solid forms at 0°C and gas at 100°C
  • Good solvent, high specific heat, and unique in having a high boiling point
• Unique Properties of Water:

  • Solid water is less dense than liquid water and floats
  • Molecules in solid water are farther apart than in liquid water, resulting in an open structure with unoccupied spaces
• The strength of intermolecular forces is related to the polarity of molecules.
• The strength of the intermolecular forces is directly related to the polarity of the molecule.
• Van der Waals forces arise from temporary dipoles that form due to the motion of electrons.
• Nonpolar substances have weak London dispersion forces, while polar substances have stronger dipole-dipole interactions.
• Nonpolar molecules have weak London dispersion forces, while polar molecules have stronger dipole-dipole interactions.
• They increase with increasing molecular size and increasing molecular polarity.