Phase Changes

• Phase changes

  Solid-liquid, liquid-vapor, and solid-vapor
• Liquid-vapor equilibrium

  1. Overcome intermolecular forces by adding energy enough to vaporize the liquid (molar heat of vaporization)

  2. At equilibrium, the rate of evaporation is equal to the rate of condensation
• Molar heat of vaporization


  Energy required to vaporize 1 mole of a liquid
• Intermolecular forces vary from one substance to another, so the molar heat of vaporization is not the same for all substances
• Liquids with strong intermolecular forces have high molar heats of vaporization
• Condensation


  Reverse process of vaporization, requires heat to be released, has a heat change (∆Hcond) with negative sign
• The values of ∆Hcond have the same numeric value of ∆Hvap
• Liquid-solid equilibrium

  1. Molar heat of fusion (∆Hfus) is the amount of energy required to melt 1 mole of a solid

  2. Molar heat of freezing (∆Hfreeze) is the amount of heat released to freeze a liquid, equal in magnitude but opposite in sign to ∆Hfus
• Solid-vapor equilibrium

  1. Sublimation is the process of converting solid to gas, deposition is the reverse process

  2. Molar heat of sublimation (∆Hsub) is the energy required to sublime 1 mole of a solid, it is the sum of the molar heats of vaporization and fusion