1. Properties of Solutions
Chapter 11
Properties of Solutions

2. Solution Basics
Solution – a homogeneous mixture (can contain gases, liquids, or solids)
Solute – the substance being dissolved
Solvent – the substance doing the dissolving

3. Enthalpy of Solution (DHsoln)
Dissolving a solute in a solvent can be viewed in 3 discreet steps.
DHsoln = DH1 + DH2 + DH3

4. Step 1: Expanding the solvent Step 2: Expanding the solute
Step 3: Allowing solute and solvent interactions
DHsoln can be either endo or exothermic depending upon the values of each step.
Endothermic
Exothermic

5. Factors Affecting Solubility
Structure – “Like Dissolves Like”
Solutes and solvents with similar polarities are most likely to dissolve.
Both substances are polar, therefore they will dissolve together.
One substance is polar, one is nonpolar, therefore they do not dissolve together

6. Factors Affecting Solubility
Pressure
Pressure has little effect on the solubility of solids and liquids.
Henry’s Law: The partial pressure of a gaseous solute is directly related to the concentration of dissolved gas
C=kP where:
C = concentration of dissolved gas
k = a constant
P = partial pressure of the gas

7. Factors Affecting Solubility
Temperature
For gaseous solutes – the higher the temperature, the lower the solubility.
For solid solutes – the rate of dissolving always increases as temperature increases and for MOST solids, the solubility of the solid also increases.

8. Vapor Pressure of Solutions
A solution containing a nonvolatile solute has a lower vapor pressure than the pure solvent.
Raoult’s Law
Psoln = XsolventPsolvent

9. Using Raoult’s Law:
Predict the vapor pressure of a solution containing grams of water and 20.0 grams of magnesium chloride at 25ºC. The vapor pressure of pure water at 25ºC is torr.
Find moles of solvent
Find moles of solute
Use Raoult’s Law

10. Raoult’s Law for Liquid-liquid solutions
When both the solute and solvent are liquids, both substances contribute to the vapor pressure.
For ideal solutions (ones in which the solute-solute, solvent-solvent, and solute-solvent attractions are all similar) the following form of Raoult’s law is used.
Ptotal = XAPA + XBPB

11. Negative Deviation from Raoult’s Law
When the DHsoln is a large exothermic number, this means that the solute and solvent have strong attraction for each other making the vapor pressure of the solutionlower than that predicted by Raoult’s Law.

12. Positive Deviation from Raoult’s Law
When the DHsoln is endothermic, this means that the solute and solvent have realtively weak attraction for each other as compared to the attractive forces within the pure liquids. This makes the vapor pressure of the solution higher than that predicted by Raoult’s Law.

13. Colligative Properties
Colligative properties depend upon the concentration of solute particles, NOT their identity.
Examples include
Boiling point elevation
Freezing point depression
Osmotic pressure

14. Boiling Point Elevation
The boiling point occurs when the vapor pressure of the liquid is equal to the atmospheric pressure.
A solution containing a nonvolatile solute has a lower vapor pressure than the pure solvent, therefore the boiling point of a solution will be higher than that of a pure solvent
DTb = Kbmi

15. Freezing Point Depression
A solution has a freezing point that is lower than that of the pure solvent.
DTf = Kfmi

16. Osmotic Pressure
Osmosis is the flow of solvent through a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.

17. Osmotic Pressure (cont)
Osmotic pressure is the pressure required to stop osmosis .

18. Calculating Osmotic Pressure
= iMRT
P = Osmotic Pressure
i = van’t Hoff factor
M = molarity of the solution
R = L atm/K mol
T = temp in Kelvin