phase equilibrium

• Phase


  A region of space throughout which all physical properties of a material are essentially uniform
• Phase (synonym)


  State of matter
• Phase Equilibrium


  • The rate of passage from one phase to another depends on:
  • The surface area of contact between the two phases
  • The concentration of substance in the phase of origin
• Evaporation


  1. Only takes place on the surface of the liquid

  2. In a closed container there is constant evaporation from the surface

  3. Particles continue to break away from the surface of the liquid – but are trapped in the space above the liquid
• Equilibrium


  Particles leaving the surface are equal to the number of particles returning to the surface
• Vapour Pressure


  The pressure exerted by a vapour in dynamic equilibrium with its liquid
• Volatile


  Liquids with high vapour pressures at room temperature
• Non-volatile


  Liquids with very low vapour pressures
• Boiling point


  The temperature at which a liquid's vapour pressure is equal to the pressure of the gas above it
• Normal boiling point


  The temperature at which a liquid's vapour pressure is equal to one atmosphere (760 mm Hg/torr)
• Vapour pressure of a liquid

  The higher the vapour pressure, the lower the boiling point
• Vapour pressure of a liquid

  High vapour pressure indicates a high concentration of molecules leaving the surface and entering the vapour phase
• Intermolecular forces of a liquid


  Weak intermolecular forces require less heat to boil the liquid, resulting in a low boiling point
• Raoult's Law


  The vapour pressure of a solution of a non-volatile solute is equal to the vapour pressure of the pure solvent at that temperature multiplied by its mole fraction
• Ideal mixture


  A mixture that obeys Raoult's Law, where intermolecular forces before and after mixing are approximately the same
• There is actually no such thing as an ideal mixture!
• Raoult's Law & Binary Mixtures

  1. In a pure liquid, the more energetic molecules have enough energy to escape the surface and form a vapour

  2. In an ideal binary mixture, the tendency of the two different molecules to escape is unchanged
• Raoult's Law (applied to binary mixtures)


  The partial vapour pressure of a component in a mixture is equal to the vapour pressure of the pure component at that temperature multiplied by its mole fraction in the mixture
• Raoult's Law only works for ideal mixtures
• The more volatile substance escapes to form a vapour more easily than the less volatile one when boiling a liquid mixture
• The vapour over the surface of the boiling liquid has a composition which is much richer in the more volatile component
• Positive deviation from Raoult's Law

  The vapour pressure of the mixture is always higher than expected from an ideal mixture
• Negative deviation from Raoult's Law

  The vapour pressure of the mixture is less than expected from Raoult's Law