Boiling Point Elevation and Freezing Point Depression

  Nonvolatile solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.

•Raises (elevates) the boiling point

•Definition - the temperature at which the vapor pressure of a liquid equals the atmospheric pressure

•More solute increases the temperature at which the vapor pressure equals the atmospheric pressure

•Directly proportional to solute concentration.

DTb = (Tb -Tb°) = i ·m ·kb

  Where, DTb = BP. Elevation 

  Tb = BP of solvent in solution

  Tb° = BP of pure solvent

   m = molality ,     kb = BP Constant

 

   

  The change in boiling point is proportional to the molality of the solution:

DTb = Kb ž m

  where Kb is the molal boiling point elevation constant, a property of the solvent.

•Adding solute to solvent lowers the freezing point of the liquid

•Directly proportional to the solute concentration

•Freezing point depression

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•Salt is spread on roads to melt ice in winter

•Solutes such as antifreeze are added to car radiators in the winter

•BUT!  What else does “antifreeze” do?

•

•Normal Freezing Point:  FP of Substance @ 1atm

•When solute is added, FP < Normal  FP

•FP is depressed when solute inhibits solvent from crystallizing.

When solution freezes the solid form is almost always pure.

Solute particles does not fit into the crystal lattice of the solvent because of the differences in size.  The solute essentially remains in solution and blocks other solvent from fitting into the crystal lattice during the freezing process.

•The change in freezing point can be found similarly:

DTf = Kf ž m

•Here Kf is the molal freezing point depression constant of the solvent.

 

Note that in both equations, DT does not depend on what the solute is, but only on how many particles are dissolved.