1. 13.2 Colligative Properties of Solutions

2. POINT > Define colligative properties
POINT > Describe how solutes affect the equilibrium vapor pressure of a solution
POINT > Describe boiling point elevation effects
POINT > Describe freezing point depression effects
POINT > Compare electrolyte and non-electrolyte effects
POINT > Define effects on osmotic pressure

3. POINT > Define colligative properties
Colligative properties are properties of solutions that depend on the concentration of solute, but not on the identity of the solute
We use molal (m) concentration in calculations involving colligative properties
Colligative properties include: vapor pressure effects, boiling point elevation, freezing point depression and changes in osmotic pressure

4. WB CHECK:
Which would have a greater effect on the boiling point of a solution, a 1.0m concentration of glucose (molar mass = 180.0g) or a 1.0m concentration of sucrose (molar mass = 342.3g)?
a) glucose
b) sucrose
c) same effect
d) cannot be determined

5. POINT > Describe how solutes affect the equilibrium vapor pressure of a solution
Solute-solvent intermolecular attractions make it harder for solvent particles to escape to the vapor phase
Ex. the equilibrium vapor pressure of pure water is greater than that of salt water

6. POINT > Describe how solutes affect the equilibrium vapor pressure of a solution
Since the vapor pressure of a solution is lower than that of the pure solvent, the boiling point of the solution is higher than that of the pure solvent
(Recall that boiling point is when vapor pressure = atm. pressure)

7. POINT > Describe boiling point elevation effects
Solute-solvent interactions cause solutions to have higher boiling points and lower freezing points than the pure solvent

8. WB CHECK: Which would have the lowest equilibrium vapor pressure?
a) a 1.0m aqueous solution of NaCl
b) a 2.0m aqueous solution of NaCl
c) they would be the same
d) pure water

9. POINT > Describe boiling point elevation effects
The change in boiling point, Tb, is proportional to the molality of the solution:
Tb = Kb  m  i
Kb = the molal boiling point elevation constant
i = the van’t Hoff factor
The van’t Hoff factor = the number of particles the solute would be expected to dissociate into in solution
Ex. for NaCl i = for CaBr2 i = 3
Kb = 0.51°C/m for water

10. POINT > Define non-electrolyte boiling point elevation effects
Ex. What is the boiling point of a solution comprised of 45.0g glucose in 100.0g water?
Tb = Kb  m  i
Kb = 0.51°C/m for water
Tb = 1.3°C Boiling point = 101.3°C

11. POINT > Describe freezing point depression effects
Solutions also have lower freezing points than the pure solvent
Solvent-solute interactions interfere with solid formation/crystallization
The change in freezing point can be found with:
Tf = Kf  m  i
Kf is the molal freezing point depression constant of the solvent Kf = -1.86°C/m for water

12. WB CHECK:
Determine the freezing point of a 2.75m aqueous solution of glucose °C Determine the boiling point of a 0.75m aqueous solution of sucrose °C An aqueous solution of glucose has a boiling point of °C. What is the molality of the solution? 1.22m

13. POINT > Compare electrolyte and non-electrolyte effects
All the examples so far have been molecular compounds
Since colligative properties depend on the number of particles dissolved, ionic solutions show greater colligative changes than those of non-electrolytes

14. POINT > Compare electrolyte and non-electrolyte effects
The expected changes in boiling point or freezing point for NaCl would be double that for glucose
The expected changes in boiling point or freezing point for CaCl2 would be triple that for glucose...

15. WB CHECK:
Determine the freezing point of a 1.50m aqueous solution of potassium chloride °C Determine the boiling point of a 3.80m aqueous solution of sodium sulfide °C An aqueous solution of sodium chloride has a freezing point of -1.62°C. What is the molality of the solution? 0.435m NaCl

16. POINT > Compare electrolyte and non-electrolyte effects
However, a 1.0m solution of NaCl does not show twice the depression in freezing point that a 1.0m solution of glucose does. Why?...

17. POINT > Compare electrolyte and non-electrolyte effects
One mole of NaCl in water does not quite give rise to two moles of ions
The ions interact and are not completely solitary particles
(except in very dilute solutions)

18. POINT > Compare electrolyte and non-electrolyte effects
Na+ and Cl− ions interact for short times, so the true concentration of particles is somewhat less than two times the concentration of NaCl
The colligative effects are not quite doubled...

19. POINT > Compare electrolyte and non-electrolyte effects
Ion interactions are more likely at higher concentration
Therefore, the number of “colligative particles” present is concentration dependent

20. POINT > Define effects on osmotic pressure
Semi-permeable membranes allow some particles to pass through, but block other particles In biological systems, membranes allow water to pass through, but some solutes are not free to do so Osmosis is the diffusion of water through a semi-permeable membrane

21. POINT > Define effects on osmotic pressure
In osmosis, there is net movement of water from the area of higher water concentration (lower solute concentration) to the area of lower water concentration (higher solute concentration)
Osmotic pressure is the pressure needed to oppose this diffusion of water

23. POINT > Define effects on osmotic pressure
In living cells:
If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic
Water will flow out of the cell, and crenation results (the cell shrinks)

24. POINT > Define effects on osmotic pressure
If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic
Water will flow into the cell, and hemolysis results (the cell swells/bursts)

25. WB CHECK:
A semi-permeable membrane separates two aqueous solutions. Solution 1 is 0.27m calcium chloride. Solution 2 is 0.27m sodium chloride Which way will the net movement of water be?
a)Toward Solution 1
b)Toward Solution 2
c) No net movement of water
(be able to explain this)

26. Homework:
Read 13.2 pages
Practice #1-4 page 426
Practice #1-4 page 427
Practice #1-3 page 430
Practice #1-2 page 433
FA #1-4 page 432