1. Colligative Properties of Solutions
Chemistry

2. How do you get from this…

3. …to this?

4. Add an ionic compound!

5. Colligative Properties
Properties that depend only on the number of solute particles and not on their identity.

6. Some Colligative Properties are:
Vapor pressure lowering
Boiling point elevation
Freezing Point depression
Osmosis

7. Vapor Pressure

8. Vapor Pressure Lowering
The particles of solute are surrounded by and attracted to particles of solvent.
Now the solvent particles have less kinetic energy and tend less to escape into the space above the liquid.
So the vapor pressure is less.

9. Ionic vs Molecular Solutes
Ionic solutes produce two or more ion particles in solution.
They affect the colligative properties proportionately more than molecular solutes (that do not ionize).
The effect is proportional to the number of particles of the solute in the solution.

10. How many particles do each of the following give upon solvation?
NaCl
CaCl2
Glucose
Electrolytes break apart into ions. Each ion has an effect on boiling point and freezing points. If a solution has more ions it will change the boiling points and melting points even more.

11. Freezing Point Depression

12. Example
Salt is added to melt ice by reducing the freezing point of water.

13. Did you know?
Some animals are able to survive freezing temperatures by producing natural antifreezes, substances that lower the freezing point of a liquid. In many freeze-tolerant insects, the antifreezes are alcohols and sugars.
Examples are box turtles, painted turtles and garter snakes, sugar maples and white spruce.

14. Boiling Point Elevation
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the prevailing atmospheric pressure.
Vapor pressures of solutions containing nonvolatile solute is lower than the vapor pressure of the pure solvent.

15. This means that more heat will be required to raise the vapor pressure of the solution. Thus, the boiling point of a solution is higher than the boiling point of the pure substance.

16. Example
Addition of ethylene glycol C2H6O2 (antifreeze) to car radiators.

17. Osmotic Pressure
The net movement of solvent is always toward the solution with the higher solute concentration.
The flow of solvent to the solution increases the height of the column. The column exerts pressure on the membrane to cause more solvent particles in the solution to pass to the pure solvent side.
When the rate of flow of solvent particles are in equilibrium between the two sides the volume of the solution stops increasing.

18. Freezing Point Depression and Boiling Point Elevation
∆Tb =mkb (for water kb=0.51 oC/m)
Freezing Point Depression
∆Tf=mkf (for water kf=1.86 oC/m)
Note: m is the molality of the particles, so if the solute is ionic, multiply by the #of particles it dissociates to.

19. Which is more effective for lowering the freezing point of water?
NaCl or CaCl2

20. Example 1:
Find the new freezing point of 3m NaCl in water.

21. Example 2:
Find the new boiling point of 3m NaCl in water.