Colligative Properties

Mr Nelson - 2010

Colligative Properties• Colligative properties are properties of

solutions that depend only on the number of solute particles present, not on the identity of the solute particles.

• Colligative properties include– Vapor pressure lowering – Boiling point elevation– Freezing point depression– Osmotic pressure

Solutes

• Nonvolatile solute – a solute that has a low chance of becoming a gas

• Nonelectrolyte solute – a solute that will not form ions when dissolved. Example: sugar

• Electrolyte – solute that forms ions when dissolved

Before You Continue

• A simple rule: – If you add solute to a solvent:

• It takes longer to boil, (higher boiling point)

• It takes longer to freeze, (lower boiling point)

• It has a lower vapor pressure (hence the higher boiling point)

Vapor PressureThe solute-solvent intermolecular attraction, the more solute, the harder for solvent to escape to the vapor phase.

The vapor pressure of a solution is lower than that of the pure solvent

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.

Boiling Point Elevation

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

Tb = Kb m (Kb is a constant found in the chart above)

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

Tb is added to the normal boiling point of the solvent.

Freezing Point Depression

• The change in freezing point can be found similarly:Tf = Kf m (Kf is a constant found in the chart above)

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

Tf is subtracted from the normal boiling point of the solvent.

Boiling Point Elevation and Freezing Point Depression

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

Tb = Kb m

Tf = Kf m

Bookwork Problems

• As a note, all of the bookwork problems that I chose involved nonvolatile, nonelectrolyte solutes. So the molality does not need to be multiplied, because the solute just dissolves.