AP Chemistry - Colligative Properties

Vapor Pressure Lowering (VPL)

  • Vapor Pressure Lowering – Vapor pressure of a solution containing a nonvolatile nonelectrolyte is always lower than that of the pure solvent.
  • Pure solvent – Surface is all solvent, all of which can vaporize.
  • Solution – Some of surface is occupied by nonvolatile solute. Fraction of surface occupied by solvent is smaller. Rate of vaporization in solution must be lower, thus a Lower Vapor Pressure. Also, solute/solvent attractions keep solvent from evaporating, thus lowering the VP.
  • VPL is expressed mathematically through Raoult’s Law

Psolution = XsolventPosolvent

Where Psolution is the vapor pressure above the solution

Xsolventis the mole fraction of solvent in the solution

Posolvent is the vapor pressure of pure solvent

Boiling Point Elevation (BPE) and Freezing Point Depression (FPD)

At a phase change point, the two phases have the same chemical potential meaning that they are energetically equivalent. The chemical potential is dependent on the temperature, and at other temperatures one phase has a lower chemical potential and is more energetically favorable than the other phase. This means that when a solute is added, the chemical potential in one phase is decreased by dilution, but the chemical potential of the other phase is not affected. This means in turn that the equilibrium between the two phases is established at another temperature for a solution than a pure solvent; i.e., the freezing point is depressed or the boiling point is elevated. Or, if it makes more sense… nonvolatile solute “barriers” hinder the boiling and freezing process (they are attracted to the solvent). If boiling, more energy will break the attractions, if freezing, less energy will allow solvent to attract solvent, thus “squeezing out” the solute.

  • Boiling Point Elevation – The phase diagram below shows the difference between the phase diagram of the pure solvent and that of the solution. Focus your attention on Tb below.
  • BPE is expressed mathematically below

Tb = kbm

  • Where Tb is the boiling point elevation (degrees above the normal boiling point the solution boils)
  • mis the molalityof the solution
  • kbis the molal boiling point elevation constant (for water, kb = 0.512 oC/m). We will discuss this more in class.

Freezing Point Depression (FPD)

  • Freezing Point Depression – The phase diagram below shows the difference between the phase diagram of the pure solvent and that of the solution. Focus your attention on Tf below.
  • FPD is expressed mathematically below

Tf = kfm

  • Where Tf is the freezing point depression (degrees below the normal freezing point the solution freezes)
  • m is themolality of the solution
  • kfis the molal freezing point depression constant (for water, kf = 1.86 oC/m). We will discuss this more in class.

Osmotic Pressure () – Discussed in note packet.