1
Chapter 13
Physical Properties of Solutions
Insert picture fromFirst page of chapter
Copyright McGraw-Hill 2009 2
13.1 Types of Solutions
Copyright McGraw-Hill 2009 3
13.1 Types of Solutions
Solution classifications based on amount of solute dissolved relative to the maximum:
• Saturated – maximum amount at a given temperature (This amount is termed the solubility of the solute.)
• Unsaturated – less than the maximum
• Supersaturated – more than a saturated solution but is an unstable condition
Copyright McGraw-Hill 2009 4
unsaturated
supersaturated
saturated
heat
Copyright McGraw-Hill 2009 5
Conversion of a Supersaturated Solution to a Saturated Solution
Copyright McGraw-Hill 2009 6
13.2 A Molecular View of the Solution Process
Factors that determine solubility
• Intermolecular forces present in the formation of a solution– Solute-solute interactions– Solvent-solvent interactions– Solute-solvent interactions
Copyright McGraw-Hill 2009 7
321soln HHHH
Copyright McGraw-Hill 2009 8
321soln HHHH
0soln321 HHHH
0soln321 HHHH
endothermic
exothermic
Copyright McGraw-Hill 2009 9
Solubility can be predicted based on “like dissolves like” in terms of intermolecular forces.For example: water and methanol are both polar
and dissolve in each other
• Two liquids that are soluble in each other in all proportions are term miscible.
• Ions readily dissolve in polar solvent due to solvation by the solvent molecules.
O
HH
Copyright McGraw-Hill 2009 10
Predict whether Vitamin B6 is water soluble
or fat soluble.
Copyright McGraw-Hill 2009 11
Water soluble due to the presence of polar
groups.
polar groups
Copyright McGraw-Hill 2009 12
• Energy and entropy – Exothermic processes are generally more
favorable than endothermic processes– Solutions do form when the overall process is
endothermic– Entropy (randomness or disorder) contributes
to the solution process
• Entropy tends to increase for all process
• A solution is more disordered than the isolated solute and solvent
Copyright McGraw-Hill 2009 13
13.3 Concentration Units
• Molality (m) – number of moles of solute dissolved in 1 kg (1000 g) of solvent
• Percent by Mass – ratio of mass of solute to the mass of the solution times 100
kg)(insolutionofmass
soluteofmolesmolality m
x100solventofmasssoluteofmass
soluteofmassmassby percent
Copyright McGraw-Hill 2009 14
Units analogous to percent by mass (part per hundred) to express very small concentrations
• Parts per million (ppm)
• Parts per billion (ppb)
*masses must be expressed in the same units
6x10solutionof*mass
soluteof*massppm
9x10solutionof*mass
soluteof*massppb
Copyright McGraw-Hill 2009 15
Choice of units depends on the purpose of the experiment
• Mole fraction – used for gases and vapor pressures of solutions
• Molarity – commonly used since volumes of solutions are easy to measure
• Molality – temperature independent• Percent by Mass – temperature independent
and need not know molar masses• Conversion between units requires the use of
density if any mass to volume or volume to mass conversion in needed.
Copyright McGraw-Hill 2009 16
Determine the a) molality, b) percent by
mass and c) the ppm concentrations of a
solution prepared by dissolving 15.56 g of
glucose (C6H12O6) in 255 g of water.
Copyright McGraw-Hill 2009 17
a) molality
Molar mass of glucose = 180.2 g/mol
mol0.086349g180.2
molxg15.56glucosemol
mm 0.339kg
g10x
g 255
1 x mol0.086349
3
Copyright McGraw-Hill 2009 18
b) percent by mass
c) ppm
glucose5.751% x100g 270.56
g 15.56
glucose5.751x10 x10g 270.56
g 15.56 46 ppm
(original slide typo!)
Copyright McGraw-Hill 2009 19
13.4 Factors that Affect Solubility
• Temperature– The solubility of solids may increase,
decrease or remain relatively constant with increasing temperature
– The solubility of gases decreases with increasing temperature• Thermal pollution – a consequence of
the relation between gas solubility and temperature
Copyright McGraw-Hill 2009 20
Temperature Dependence of the Solubility of Selected Solids
Copyright McGraw-Hill 2009 21
• Pressure – significantly affects only the solubility of gases– Henry’s law – the solubility of a gas, c, is
directly proportional to the pressure of the gas, P, over the solution
where c, is in mol/L, k is Henry’s law
constant with units of mol/L. atm and P is in atm.
Pc kPc
Copyright McGraw-Hill 2009 22
Molecular View at Different Pressures
P1 P2
P1< P2
Copyright McGraw-Hill 2009 23
13.5 Colligative Properties
Colligative properties depend only on the number of solute particles in solution and not the nature of the solute• Vapor-Pressure lowering
– Nonvolatile solute (no appreciable pressure)• Vapor pressure of the solvent is decreased
– Volatile solute (exhibit appreciable pressure)• Vapor pressure is the sum of the individual
pressures
Copyright McGraw-Hill 2009 24
– Raoult’s law – quantitative expression of the solution vapor pressure
• Nonvolatile solute (P1 is the solution vapor pressure, P0 is the vapor pressure of the pure substance, and is the mole fraction.)
• Volatile solute (PT is the solution vapor pressure, and are vapor pressures of pure solution components)
0111 PP
0BB
0AAT PPP
0AP 0
BP Volatile solute: not exam material
Copyright McGraw-Hill 2009 25
Calculate the vapor pressure of a solutionmade by dissolving 115 g of urea, a nonvolatilesolute, [(NH2)2CO; molar mass = 60.06 g/mol]in 485 g of water at 25°C. (At 25°C, .)mmHg23.8OH2
P
Copyright McGraw-Hill 2009 26
0OHOHOH 222
PP
0.9336mol1.915mol26.91
mol26.91OH2
mol26.91g18.02
molgx485mol OH2
mol1.915g60.06
mol xg115molurea
mmHg22.2mmHg23.8 x 0.9392OH2P
Copyright McGraw-Hill 2009 27
• Boiling-Point Elevation (Tb)
– The boiling point of a solution, Tb, of a nonvolatile solute will be higher than that of the pure solvent, .
– Elevation is directly proportional to the molal concentration.
– Kb is the molal boiling-point elevation constant.
0bT
mKT bb
0bbb TTT
Copyright McGraw-Hill 2009 28
Effect of Vapor Pressure Lowering
Effect on Boiling Point
Copyright McGraw-Hill 2009 29
Copyright McGraw-Hill 2009 30
• Freezing-Point Depression (Tf)
– The freezing point of a solution, Tf, will be lower than that of the pure solvent, .
– Depression is directly proportional to the molal concentration.
– Kf is the molal freezing-point elevation constant.
0fT
mKT ff
f0
ff TTT
Copyright McGraw-Hill 2009 31
Effect of Vapor Pressure Lowering
Effect on Freezing Point
Copyright McGraw-Hill 2009 32
Calculate a) the freezing point and b) the
boiling point of a solution containing 268 g of
ethylene glycol and 1015 g of water. (The
molar mass of ethylene glycol (C2H6O2) is
62.07 g/mol. Kb and Kf for water are 0.512°C/m
and 1.86°C/m, respectively.)
Copyright McGraw-Hill 2009 33
a) freezing point
C7.914.254C1.86 o
o
f mxm
T
mol4.318g62.07
molxg268glycolethylenemol
mm 4.254kg
g10x
g 1015
1 x mol3184
3
.
foo C0.00C7.91 T
C7.91of T
Copyright McGraw-Hill 2009 34
b) boiling point
C100.00C2.18 ob
o T
C2.184.254C0.512 o
o
b mxm
T
C102.18ob T
Copyright McGraw-Hill 2009 35
Osmosis - Selective passage of solvent molecules through a semipermeable membrane
solvent solution solvent solution
Copyright McGraw-Hill 2009 36
• Osmotic Pressure () – Pressure required to stop osmosis
– Directly proportional to molar concentration, M
= MRT
where R is 0.08206 L.atm/mol . K and T is in
kelvins
Copyright McGraw-Hill 2009 37
• Solutions of Electrolytes
– Dissociation of strong and weak electrolytes affects the number of particles in a solution
– van’t Hoft factor (i) – accounts for the effect of dissociation
solutionindissolvedinitiallyunitsformulaofnumber
ondissociatiaftersolutioninparticlesofnumberactuali
Copyright McGraw-Hill 2009 38
Modified Equations for Colligative Properties
miKT ff
miKT bb
iRTM
Copyright McGraw-Hill 2009 39
Copyright McGraw-Hill 2009 40
Formation of ion pairs affects colligative properties
Copyright McGraw-Hill 2009 41
Copyright McGraw-Hill 2009 42
The freezing-point depression of a 0.100 m
MgSO4 solution is 0.225°C. Determine the
experimental van’t Hoff factor of MgSO4 at
this concentration.
Copyright McGraw-Hill 2009 43
miKT ff
One approach:
mxm
i 0.100C1.86
C0.225o
o
1.21i
Note, at this concentration the dissociation of MgSO4 is not complete.
Copyright McGraw-Hill 2009 44
Ideal freezing point depression
C0.1860.100x C1.86 o
o
f mm
T
Compare ideal and real freezing point depression
211C0.186
C0.225o
o
.i
Another approach to the same problem:
Copyright McGraw-Hill 2009 45
A solution made by dissolving 25.0 mg ofinsulin in 5.00 mL of water has an osmoticpressure of 15.5 mmHg at 25°C. Calculatethe molar mass of insulin. (Assume that there is no change in volume when theinsulin is added to the water and that insulinis a nondissociating solute.)
Copyright McGraw-Hill 2009 46
K298
1
atmL0.08206
Kmolxatm2.039x10 2 x
RTM
Calculate the M of the solution
atm2.039x10mmHg760
atmxmmHg15.5 2
L
mol10x8.33810x8.338
44
MM
Copyright McGraw-Hill 2009 47
Calculate the moles of insulin
mol x104.169mL
L10xmL5.00x
L
mol10x8.340mol 6
34
Molar mass is ratio of grams to moles
mol x104.169
1x
mg
g10xmg25.0
6
3
M
mol
g10x6.00 3
M
Copyright McGraw-Hill 2009 48
13.7 ColloidsA colloid is a dispersion of particles of onesubstance throughout another substance.• Intermediate between a homogenous and heterogeneous
mixture• Range of particle size: 103 – 106 pm• Categories
– Aerosols – Foams– Emulsions– Sols– Gels
Copyright McGraw-Hill 2009 49
Aerogel
• Lowest density solids known
• Good thermal, electric, sound insulators
Copyright McGraw-Hill 2009 50
Copyright McGraw-Hill 2009 51
A 2.5-kg brick supported by a 2-g piece of aerogel
(source: Wikipedia)
Copyright McGraw-Hill 2009 52
• Exhibit the Tyndall effect
colloid solution
Copyright McGraw-Hill 2009 53
Fog – A Familiar Manifestation of the Tyndall Effect
Copyright McGraw-Hill 2009 54
• Many important colloids are aqueous and can be further classified.
– Hydrophilic (water loving)
– Hydrophobic (water fearing)
Hydrophilic groups on the surface of a protein stabilize the molecule in water
Copyright McGraw-Hill 2009 55
Stabilization of Hydrophobic Colloidal Particles by Ion Adsorption
Copyright McGraw-Hill 2009 56
Removal of Grease by Soap (Sodium Stearate)
Copyright McGraw-Hill 2009 57
Emulsification – • stabilization of an unstable colloid• accomplished by the addition of an emulsifier or emulsifying agent
Sodium glycoholate – a bile salt or biological emulsifying agent for ingested fats
Copyright McGraw-Hill 2009 58
Lecithin is a mixture of substances found in natural sources such as egg yolk. It contains molecules such asphosphatidylcholine (shown right) that have both hydrophobic (nonpolar) and hydrophilic (polar; charged) regions. Lecithin acts as an emulsifier.
This is why egg yolk is used to make mayonnaise. The egg yolk is typically whisked with a small amount of vinegar or lemon juice (the aqueous phase). Vegetable oil is then added gradually while whisking. Mayonnaise is the resulting emulsion of vegetable oil and aqueous acid.
Top Related