Colligative Properties Chapter 13, Section 2

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)

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)

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.

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.

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…

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

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.

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.

The change in freezing point can be found similarly: Tf = Kf  m Here Kf is the molal freezing point depression constant of the solvent.

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.

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)  Al+3 (aq) + 3 Br- (aq)

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

Drill – 4/29/11 Write everything you know about acids and bases

Determine the freezing point of a solution of 60 Determine the freezing point of a solution of 60.0g of glucose, C6H12O6, dissolved in 80.0g of water.

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