Chapter 11. Solutions and Their Properties
Solutions
Solute: material present in least amount
Solvent: material present in most amount
Solution = solvent + solute(s)
What criteria must be satisfied in order to form a solution?
An artist’s conception of how a salt dissolves in water
How could we identify the cation and anion?
Dissolving of
sugar
Solution energy = M+X- (s) + H2O
M+(aq) + X-(aq)
Lattice energy = M+(g) + X-(g)
M+X- (s)
Enthalpy, H
Solvent
aggregated
Solute
aggregated
Hinitial
A Exothermic solution process
Enthalpy, H
Solvent
aggregated
Solute
aggregated
Hinitial
Solution
DHsoln < 0
Hfinal
A Exothermic solution process
DHsolvent
Enthalpy, H
Solvent
separated
Solvent
aggregated
Solute
aggregated
Hinitial
Solution
DHsoln < 0
Hfinal
A Exothermic solution process
Solute
separated
Solvent
aggregated
DHsolute
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
Hinitial
Solution
DHsoln < 0
Hfinal
A Exothermic solution process
Solute
separated
Solvent
aggregated
DHsolute
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
DHsolute
+
DHsolvent
Hinitial
Solution
DHsoln < 0
Hfinal
A Exothermic solution process
Solute
separated
Solvent
aggregated
DHsolute
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
DHsolute
+
DHsolvent
DHmix
Hinitial
Solution
DHsoln < 0
Hfinal
A Exothermic solution process
Enthalpy, H
Solvent
aggregated
Solute
aggregated
Hinitial
B Endothermic solution process
Enthalpy, H
Solution
Solvent
aggregated
Solute
aggregated
Hfinal
DHsoln > 0
Hinitial
B Endothermic solution process
Solution
DHsolvent
Enthalpy, H
Solvent
separated
Solvent
aggregated
Solute
aggregated
Hfinal
DHsoln > 0
Hinitial
B Endothermic solution process
Solute
separated
Solvent
aggregated
DHsolute
Solution
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
Hfinal
DHsoln > 0
Hinitial
B Endothermic solution process
Solute
separated
DHsolute
+
DHsolvent
Solvent
aggregated
DHsolute
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
Solution
Hfinal
DHsoln > 0
Hinitial
B Endothermic solution process
DHmix
Solute
separated
DHsolute
+
DHsolvent
Solvent
aggregated
DHsolute
DHsolvent
Enthalpy, H
Solvent
separated
Solute
aggregated
Solution
Hfinal
DHsoln > 0
Hinitial
B Endothermic solution process
Interactions that must be overcome and those that form
Why do salts with a positive enthalpy of solution form solutions?
Variation of solubility of solids and liquids with temperature
Solubility of gases with temperature
Solvent : H2O
Solubility of gases as a function of pressure
Le Chatelier’s principle
B is a non-volatile component
Vapor pressure of a pure component
Pure solvent A
Solution of A and B
air + vapor
Mole fraction of a = na/(na +nb +nc + ...)
in the presence of a non-volatile solute
Colligative property: a physical property that depends on how the
amount present
Distillation of volatile materials
Raoult’s Law:
PA = xAPAo;
PB = xBPBo
PT obs = xA PAo + xBPBo
for two volatile components
where PAo and PBo are the vapor pressures of the pure
components
Raoult’s Law: PA = XAPAo; PB = XBPBo
PT obs = XA PAo + XBPBo
for two volatile components
25 °C
mol fraction
Boiling occurs when the total pressure = 760 mm
For a 1:1 mol mixture at 93 ° C
560
200
PA = XAPAo; PB = XBPBo
PT obs = XA PAo + XBPBo
93 °C
Two volatile liquids
For an initial 1:1 mixture of benzene-toluene, the initial
composition of the vapor based on its vapor pressure is
approximately
Ptoluene = 200 mm
Pbenzene = 560 mm
Treating the vapor as an ideal gas, if we were to condense the
vapor:
PbV/PtV = nbRT/ntRT
Pb/Pt = nb/nt
nb/nt = 56/20; the vapor is enriched in the more volatile component
xB = 56/(56+20) = 0.73
Starting with a 50:50 mixture: (the composition of the first drop)
A Volatile Liquid and Non-volatile Solid
A recipe for making maple syrup requires adding two cups
sugar (sucrose, mw 342 g/mol) for every cup of water.
Assuming a cup of water contains 300 mL of water and a
cup of sugar contains 300 g of sucrose (C12H22O11), at
what temperature would you expect the syrup to boil given
the following vapor pressures of pure water?
Temp (°C) Vapor pressure (mm) pure H2O
o
Raoult’s
Law
P
=
x
P
obs
H2O H2O
100
760
101
787
Pobs = 760 mm for boiling to occur
102
815
300g/18 g/mol = 16.7 mol H2O
103
845
104
875
600g/342 g/mol = 1.75 mol sugar
105
906
xH2O =16.7/(16.7 + 1.75)= 0.91
PoH20 = Pobs/xH20; 760 /0.91 = 835 mm
Freezing point depression and boiling point elevation (using nonvolatile solutes)
DT = Km
K is a constant characteristic of each substance
m = molality
(mols/1000g solvent)
DT = Km
m = molality, mol/1000g solvent
The boiling point elevation is for non-volatile solutes
Boiling Point Elevation for A Volatile Liquid and Non-volatile Solid
A recipe for making maple syrup requires adding two cups sugar
(sucrose, mw 342 g/mol) for every cup of water. Assuming a cup
of water contains 300 mL of water and a cup of sugar contains 300
g of sucrose (C12H22O11), at what temperature would you expect
the syrup to boil given the following vapor pressures of pure
water?
sucrose, 600/342 g/mol = 1.75 mol
1.75 mol/300 g H2O = x mol/1000gH2O
x = 5.83 m
DT = Kf m;
DT = 0.51*5.83m; DT = 3 °C
Boiling point of water = 103 °C
It is found that 1 g of naphthalene in 100 g of camphor depresses
the freezing point of camphor by 2.945 °C. What is the
molecular weight of naphthalene?
DT = Kf m
DT= 37.7 °C kg/mol*m
2.945 °C = 37.7 °C /molal*m; m = 0.0781molal
If 1 g of naphthalene is dissolved in 10 g of the solvent
camphor, 10 g of naphthalene is dissolved in 1000 g camphor
molality is equal to the number of moles/1000 g of solvent
therefore, 10g/MW = 0.0781 moles; MW = 10/0.0781 = 128 g /mol
Osmosis and osmotic pressure
A simple example of osmosis: evaporation of water from a salt
solution
What acts as the semipermeable membrane?
Osmotic pressure
Osmotic Pressure
π = MRT
where π is the osmotic pressure between a pure solvent and
a solution containing that solvent; M is the molarity of the solution
(mols /liter), R is the gas constant, and T = temperature (K)
The total concentration of dissolved particle in red blood cells in
0.3 M. What would be the osmotic pressure between red cells and
pure water if the blood cells were to be placed in pure water at 37
°C?
π = 0.3moles/liter*0.0821liter atm/(mol K)*(273.15+37 )K
π = 0.3 mol/l*0.0821l atm/(K mol)*298 K
π = 7.6 atm
In what direction would the pressure be directed?
What would happen to the cells?
What is the osmotic pressure developed from a solution of 0.15
M NaCl in contact with a semipermeable membrane of pure
water at T = 37 °C?
NaCl + H2O = Na+ + Clπ = nRT
π = 0.3 mol/l*0.0821l atm/(K mol)*310.2 K
π = 7.6 atm
Reverse Osmosis
Osmotic
pressure
Applied pressure needed to
prevent volume increase
P
P
Pure
solvent
Solution
Net
movement
of solvent
Semipermeable
membrane
Solute
molecules
Solvent
molecules
A
B
C
De-salination of sea water
What happens when polar molecules meet non-polar ones?
What happens when there are incompatible interactions are attached
to the same molecule?
A typical soap molecule:
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CO2- K+
How does soap clear your clothes?
CO2-O C
2
CO2-
-O C
2
CO2-
-O C
2
CO2-
-O C
2
CO2CO2-
micelle
Soap in water forms micelles, small spherical objects suspended in
water
non-polar
interior
----
-
----
ionic exterior
Other examples of self assembly: