1. The Chemistry of Solutes and Solutions
Chapter 15
The Chemistry of Solutes and Solutions
William
Henry
Henry’s Law.
Francois
Raoult
Raoult’s Law. 1 1 1 1

2. The Solution Process Na+ O H Cl- H O
Solutes and solvent are components of the solution.
In the process of making solutions with condensed phases, intermolecular forces become rearranged.
Consider NaCl (solute) dissolving in water (solvent):
the water H-bonds have to be interrupted,
NaCl dissociates into Na+ and Cl-,
ion-dipole forces form: Na+ … -OH2 and Cl- … +H2O.
Na+ O δ- H
Cl- H O δ+

3. The Solution Process

4. The Solution Process “Rule”: LIKE DISSOLVES LIKE
polar solvents dissolve polar solutes.
Non-polar solvents dissolve non-polar solutes.
Water is polar (because it’s bent).
It will therefore tend to dissolve other polar molecules or ions.
For example, most salts, alcohols and sugars dissolve in water.
Alcohols and sugars all contain the O-H part of a molecule which makes them polar: O X
. . H

5. The Solution Process
Most organic substances (compounds of carbon) are non-polar.
That is why they in general do not dissolve in water (which is polar). C H
Generally, carbon chains are non-polar (no dipole moment).
Since the electronegativities of hydrogen and carbon are virtually
the same, hydrocarbons are non-polar. Gasoline is non-polar,
because it is a hydrocarbon.

6. Solute-Solvent Interactions
Miscible liquids: mix in any proportions.
Immiscible liquids: do not mix.
Soluble liquids: mix in certain proportions.
Intermolecular forces are important: water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are re-established in the mixture.
The number of carbon atoms in a chain affect solubility: the more C atoms the less soluble in water.

7. Factors Affecting Solubility
Solute-Solvent Interactions
ethanol ---- ethanol
ethanol ---- water

8. Factors Affecting Solubility
Solute-Solvent Interactions
The number of -OH groups within a molecule increases solubility in water.
Glucose – (a sugar)

9. Ways of Expressing Concentration
All methods involve quantifying amount of solute per amount of solvent or solution.
Amounts are masses, moles or liters.
Qualitatively solutions are dilute or concentrated.
Definitions:
soln in
component of
mass
Mass
fraction of
component =
soln of
mass
total
mass of
component in
soln 6
component of
ppm = 10
total
mass of
soln
Mass % = mass fraction ´ 100

10. Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
moles of
solute
solute of
fraction
Mole =
total
moles of
all
components X M
moles of
solute
Molality = kg of
solvent m

11. The Composition of the Oceans
Concentrations in ppm and Molarity
Chloride
Sodium
Sulfate
Magnesium
Calcium
Potassium
Carbon dioxide* x 10-4
Bromide x 10-4
Boric acid x 10-4
Strontium x 10-5
Fluoride x 10-5
* present as bicarbonate and carbonate

12. Factors Affecting Solubility
Pressure Effects: solubility of a gas in a liquid is a function of the pressure of the gas as given by:
Henry’s Law:
Cg is the solubility of gas,
Pg the partial pressure,
k = Henry’s law constant.
Carbonated beverages are bottled under PCO2 > 1 atm. As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases. Therefore, bubbles of CO2 escape from solution.

13. Temperature effects dissolving gases dissolving solids
opening pop bottles
thermal pollution in lakes
dissolving solids
sugar dissolves better in warm water

14. Lowering the Vapor Pressure
Non-volatile solutes reduce the ability of the surface solvent molecules to escape the liquid.
Therefore, vapor pressure is lowered.
The amount of vapor pressure lowering depends on the amount of solute (this is a colligative property)

15. Colligative Properties
Colligative properties depend on number of (nonvolatile) solute molecules.
Some colligative properties:
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure

16. Colligative Properties
Raoult’s Law (used for vapor pressure lowering)
PA is the vapor pressure of A in a solution
PA is the vapor pressure of pure A
A is the mole fraction of A in solution,

17. Colligative Properties
Boiling-Point Elevation

18. Colligative Properties
Boiling-Point Elevation
At 1 atm (normal boiling point of pure liquid) there is a lower vapor pressure of the solution. Therefore, a higher temperature is required to reach a vapor pressure of 1 atm for the solution (Tb).
Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m:
Kb depends only on solvent

19. Colligative Properties
Freezing-Point Depression
The solution freezes at a lower temperature (Tf) than the pure solvent.
Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant):
Kf depends only on solvent

20. Figure: 13-T04
ΔTb =Kbm
ΔTf=Kfm

21. Colligative Properties
Can be used to determine molecular mass of solute.
Problem: Soln of 5.00 g lauryl alcohol (solute) dissolved in 100 g benzene (solvent) freezes at 4.1oC. Calculate molar mass of lauryl alcohol.
Use ΔTf= Kfm;
ΔTf = 4.1oC
ΔTf= =1.4oC
Kf = 5.12oC/m, fp = 5.5° for benzene
m= ΔTf/Kf = 1.4oC/(5.12oC/m) = 0.27 m
molality is mol solute/kg solvent

22. OSMOSIS Water Semipermeable membrane more concentrated solution
less concentrated
solution
H2O
H2O
Particles of solute
H2O
Particles of solute
H2O
Water
more concentrated
less concentrated

23. Osmotic pressure
causes a difference
in levels
of water
Figure: 13-23

24. Figure: 13-24

25. Colligative Properties
Osmosis
Osmotic pressure, , is the pressure required to stop osmosis:
Isotonic solutions: two solutions with the same  separated by a semipermeable membrane.
Hypotonic solutions: a solution of lower  (less concentrated) than a hypertonic solution (more concentrated).
Red blood cell walls are semipermeable membranes.

26. Salty food causes retention
Colligative Properties
Salty food causes retention
of water and swelling
of tissues (edema)
Osmosis
Crenation and Hemolysis:
(Cell in hypertonic solution)
(Cell in hypotonic solution)
Crenation (shrivels up)
Hemolysis (bursts)

27. TOTAL NUMBER OF PARTICLES
Colligative properties depend on total numbers of particles, and salts give more than 1 mole of particles per mole of compound.
A 1.0 m solution of NaCl provides 1.0 mol of Na+ and 1.0 mol of Cl-
Thus, 1.0 mol of NaCl provides 2.0 mol of ions (particles)
The colligative property is enhanced by a factor of 2.
Other examples:
1.0 mol of Na2SO4 provides 3.0 mol of ions.
1.0 mol of (NH4)3PO4 provides 4.0 mol of ions.
Covalent molecules like sucrose or ethylene glycol do not ionize.

28. Colloids
Colloids are huge molecules which create suspensions with properties between those of mixtures and solutions.
Examples: smoke, fog, mayonnaise, marshmallows, emulsions, colored glass.
The Tyndall effect is used to characterize liquid colloids in water: a beam of light through a colloidal suspension can be seen from the side (because of the scattered light); a beam of light through a true solution is not visible from the side.