Colligative Properties of Colligative Properties of SolutionsSolutions

Colligative PropertiesColligative Properties Colligative Property: A property that Colligative Property: A property that

depends only upon the number of depends only upon the number of solute particles (solute particles (concentration), and ), and NOT upon their identity.NOT upon their identity.

Three Important Colligative Three Important Colligative Properties of Solutions.Properties of Solutions. Vapor-pressure loweringVapor-pressure lowering Boiling-point elevationBoiling-point elevation Freezing-point depressionFreezing-point depression

      

 

      

 

Vapor-Pressure LoweringVapor-Pressure Lowering Vapor pressure: is the Vapor pressure: is the

pressure exerted by a pressure exerted by a vapor that is in dynamic vapor that is in dynamic equilibrium with its liquid equilibrium with its liquid (molecules are moving (molecules are moving back and forth between back and forth between liquid and vapor phases)liquid and vapor phases) A solution that contains a A solution that contains a

solute that is not easily solute that is not easily vaporized always has a vaporized always has a lower lower vapor pressure vapor pressure than the pure solvent.than the pure solvent.

This is true because This is true because in a solution, solute in a solution, solute particles reduce the particles reduce the number of free number of free solvent particles solvent particles able to escape the able to escape the liquid.liquid.

Vapor-Pressure LoweringVapor-Pressure Lowering

The decrease in a solution’s vapor The decrease in a solution’s vapor pressure is proportional to the pressure is proportional to the number of particles the solute makes number of particles the solute makes in solution.in solution.

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Why do we spread Why do we spread SALTSALT on the on the roads in the winter?roads in the winter?

Freezing-Point DepressionFreezing-Point Depression

Freezing-Point Depression: The difference Freezing-Point Depression: The difference in temperature between the freezing in temperature between the freezing point of a solution and the freezing point point of a solution and the freezing point of the pure solvent (of the pure solvent (water).). The presence of a solute in water disrupts The presence of a solute in water disrupts

the formation of the orderly pattern of ice. the formation of the orderly pattern of ice. Therefore more kinetic energy must be Therefore more kinetic energy must be withdrawn from a solution than from the withdrawn from a solution than from the pure solvent to cause the solution to solidify.pure solvent to cause the solution to solidify.

Freezing-Point DepressionFreezing-Point Depression

Freezing-Point DepressionFreezing-Point Depression

The magnitude of the The magnitude of the freezing-point depression freezing-point depression is proportional to the is proportional to the number of solute number of solute particles dissolved in the particles dissolved in the solvent and does not solvent and does not depend upon their depend upon their identity.identity. Which would be a better Which would be a better

salt for putting on icy salt for putting on icy roads, NaCl or CaClroads, NaCl or CaCl22??

Boiling-Point ElevationBoiling-Point Elevation

Boiling Point: The temperature Boiling Point: The temperature at which the vapor pressure of at which the vapor pressure of the liquid phase equals the liquid phase equals atmospheric pressure.atmospheric pressure. Because of the decrease in vapor Because of the decrease in vapor

pressure, additional kinetic pressure, additional kinetic energy must be added to raise energy must be added to raise the vapor pressure of the liquid the vapor pressure of the liquid phase of the solution to phase of the solution to atmospheric pressure to initiate atmospheric pressure to initiate boiling.boiling.

Boiling-Point ElevationBoiling-Point Elevation

Boiling-Point Elevation: The Boiling-Point Elevation: The difference in temperature difference in temperature between the boiling point between the boiling point of a solution and the boiling of a solution and the boiling point of the pure solvent.point of the pure solvent. The boiling point of a solution The boiling point of a solution

is higher than the boiling is higher than the boiling point of the pure solvent.point of the pure solvent.

Boiling-Point ElevationBoiling-Point Elevation The magnitude of the The magnitude of the

boiling-point elevation boiling-point elevation is proportional to the is proportional to the number of solute number of solute particles dissolved in particles dissolved in the solvent.the solvent. It takes additional It takes additional

kinetic energy for the kinetic energy for the solvent particles to solvent particles to overcome the overcome the attractive forces that attractive forces that keep them in the liquid.keep them in the liquid.

Boiling-Point ElevationBoiling-Point Elevation

The water molecules are attracted to ions in solution, the ions “hold” the water molecules down instead of allowing them to be released into the vapor phase. The water molecules need more energy to “escape” from the ions in solution!

What happened?What happened?

Initial Final

Conc. dillute

What conditions exist when the equilibrium point is reached and no further change in the volumes

occurs?