Colligative Properties

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Colligative Properties

• Chapter 13, Section 2

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What is a colligative property?

• A property that depends on the concentration of
  solute particles NOT the identity of the
  particle.
• Vapor Pressure Reduction
• Boiling Point Elevation
• Freezing Point Depression
• Osmotic Pressure
• Concentration will be in terms of molality (m)

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• Vapor pressure is pressure caused by molecules in
  the gas phase that are in equilibrium with the
  liquid phase. (in a closed system particles go
  back and forth between phases)

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Vapor Pressure Reduction

• The pressure of the vapor over a solvent is
  reduced when a solute is dissolved in it. Vapor
  pressure reduction is directly related to
  concentration of the solution.

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• Why does this occur?
• Increasing solute particles reduces the
  proportion of solute to solvent. Fewer solvent
  particles will be available to leave the solution
  and enter the gas phase.
• The solution will remain a liquid over a larger
  temp. range.

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Phase Diagram
Shows us what phase the solvent will be in at a
given temperature and pressure.
If we add a solute that lowers the vapor
pressure, then our phase diagram adjusts
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Boiling Point Elevation

• The boiling point of a solvent is raised when a
  solute is dissolved in it. It is directly
  related to the concentration of the solution.
• Boiling point the temp. at which vapor pressure
  of a liquid is equal to atmospheric pressure

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• The change in boiling point is proportional to
  the molality of the solution
• ?Tb Kb ? m
• Kb is the molal boiling point elevation
  constant, a property of the solvent. It has the
  units ºC/m.
• ?Tb is the boiling point elevation. It is the
  difference between the boiling points of the pure
  solvent and that of the solution.

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Freezing Point Depression

• The freezing point of a solvent is lowered when a
  solute is dissolved in it. Freezing point
  depression is directly related to concentration
  of a solution.
• Freezing point the temp. where vapor pressure
  of the solid and liquid phases are the same.

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• The change in freezing point can be found
  similarly
• ?Tf Kf ? m
• Here Kf is the molal freezing point depression
  constant of the solvent.

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Osmotic Pressure (?)

• The pressure required to prevent osmosis.
• Osmosis is the flow of particles from a dilute
  solution to a concentrated one across a semi
  permeable membrane.

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Electrolytes change moles of solute

• Because ionic compounds dissociate in water, they
  will increase the concentration of dissolved
  particles.
• When determining or using moles of an
  electrolyte, use the dissociated amount.
• AlBr3 (s) ? Al3 (aq) 3 Br- (aq)

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• If 50.0 g of BaCl2 are added to 2.5 kg of
  water, compute the change in freezing point and
  state the new freezing point of the solution.
• Water Kf -1.86?C/m Kb.51 ?C/m

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Drill 4/29/11

• Write everything you know about acids and bases

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• Determine the freezing point of a solution of
  60.0g of glucose, C6H12O6, dissolved in 80.0g of
  water.

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• Workbook pg. 226 1, pg. 229 1, pg. 232 1
• (use the table on pg. 223 for Kb or Kf)
• Then pg. 233 3, 11 and pg. 234 17