Chapter 21

Colligative Properties:

Boiling Point & Freezing Point

Boiling Point Elevation

• Boiling point elevation is a colligative property related to vapor pressure lowering.

• The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure.

• Due to the lowering of the vapor pressure of the solvent when a solution forms, a solution will require a higher temperature than the pure solvent to reach its boiling point.

Boiling Point Elevation

∆Tb = (Kb)(m)(#particles)

# particles

Molecular solutes always = 1

Example: C6H12O6

Ionic solutes = # of ions in the compound

Example: BaCl2, Al2(SO4)3

What is the boiling point of a 2.4m What is the boiling point of a 2.4m aqueous solution Ba(NOaqueous solution Ba(NO33))22??

BP = 103.708°C

Freezing Point Depression

• Every liquid has a freezing point - the temperature at which a liquid undergoes a phase change from liquid to solid.

• When solutes are added to a liquid, forming a solution, the solute molecules disrupt the formation of crystals of the solvent.

• That disruption in the freezing process results in a depression of the freezing point for the solution.

Freezing Point Depression

∆Tf = (Kf)(m)(#particles)

What is the freezing point of a What is the freezing point of a 1.5m aqueous solution of NaCl?1.5m aqueous solution of NaCl?

FP = -5.559°C

97.5g of C97.5g of C1212HH2222OO1111 are dissolved in 185g of water. are dissolved in 185g of water.

Find the freezing point and boiling point.Find the freezing point and boiling point.

FP = -2.854°C BP = 100.7931°C

14.0g of C14.0g of C1010HH88 are dissolved in 25.0g of benzene. are dissolved in 25.0g of benzene.

Find the freezing point and boiling point.Find the freezing point and boiling point.

FP = -16.867°C BP = 91.169°C

Homework

• Finish Worksheet: Vapor Pressure, Boiling Point and Freezing Point.

• Review and Reinforcement worksheet due tomorrow.