William
Henry
1774-1836.
Henry’s Law.
Francois
Raoult
1830-1901.
Raoult’s Law.
1

The Solution Process
• 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 + …

OH
2 and Cl - …

+ H
2
O.
δ-
+
O
H
H
Cl -
δ+
H O
H
2

The Solution Process
3

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:
X O
H
4

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).
H
C
H H H
C
C C
H
H H
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.
5

•
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.
6

Factors Affecting Solubility
ethanol ---- ethanol ethanol ---- water
7

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

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:
Mass fraction of component
= mass of component in soln total mass of soln ppm of component
= mass of component in soln 
10
6 total mass of soln
Mass % = mass fraction

100
9

Ways of Expressing Concentration
Mole fraction of solute
X
= moles of solute total moles of all components
Molarity
=
M moles of solute liters of solution
Molality m
= moles of solute kg of solvent
10

The Composition of the Oceans
Concentrations in ppm and Molarity
Chloride 19350 0.55
Sodium 10760 0.47
Sulfate 2710 0.028
Magnesium 1290 0.054
Calcium 412 0.010
Potassium 400 0.010
Carbon dioxide* 106 2.3 x 10 -4
Bromide 67 8.3 x 10 -4
Boric acid 27 4.3 x 10 -4
Strontium 7.9 9.1 x 10 -5
Fluoride 1.3 7.0 x 10 -5
* present as bicarbonate and carbonate
11

Pressure Effects: solubility of a gas in a liquid is a function of the pressure of the gas as given by:
C g
= kP g
C g is the solubility of gas,
P g the partial pressure,
k = Henry’s law constant.
•
Carbonated beverages are bottled under P
CO2
> 1 atm.
As the bottle is opened, P
CO2 of CO
2 decreases and the solubility decreases. Therefore, bubbles of CO
2 escape from solution.
12

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

•
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 )
14

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
15

Colligative Properties
•
P
A is the vapor pressure of A in a solution
•
•
P
A
 is the vapor pressure of pure A

A is the mole fraction of A in solution,
P
A
= 
A
P
A

16

Colligative Properties
17

Colligative Properties
•
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 (

T b
).
•
Molal boiling-point-elevation constant, K b
, expresses how much

T b changes with molality, m:

T b
=
K b m
K b depends only on solvent
18

Colligative Properties
•
The solution freezes at a lower temperature (

T f
) than the pure solvent.
•
Decrease in freezing point (

T f
) is directly proportional to molality (K f is the molal freezingpoint-depression constant):

T f
=
K f m
K f depends only on solvent
19

ΔT b
=K b m
ΔT f
=K f m
20

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 4.1
o C. Calculate molar mass of lauryl alcohol.
Use ΔT f
= K f m;
ΔT f
= 4.1
o C
ΔT f
=5.5-4.1=1.4
o C m= ΔT f
/K f
K f
= 5.12
o C/m, fp = 5.5° for benzene
= 1.4
o C/(5.12
o C/m) = 0.27 m molality is mol solute/kg solvent
0.27mol
solute x0.100kg
benzene kg benzene
=
0.027mol
solute
MolarMass solute
=
5.00g
0.027mol
=
183g/mol
21

OSMOSIS
Semipermeable membrane more concentrated less concentrated solution solution
Particles of solute more concentrated
H
2
O
H
2
O
Particles of solute
H
2
O
H
2
O
Water less concentrated
22

Osmotic pressure causes a difference in levels of water
23


V
= nRT
 =


 n
V
=



MRT
RT
24

Colligative Properties
•
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.
25

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

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 Na
2
SO
4 provides
1.0 mol of (NH
4
)
3
PO
4
3.0
provides mol of ions.
4.0
mol of ions.
Covalent molecules like sucrose or ethylene glycol do not ionize.
27

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.
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