Chap 13: PROPERTIES of
SOLUTIONS
13.1 TYPES of SOLUTIONS
SOLUTION
SOLUTION
Homogeneous
Homogeneous mixture
mixture of
of at
at least
least 22 components:
components:
the
the SOLUTE
SOLUTE (in
(in lesser
lesser quantity)
quantity) and
and the
the
SOLVENT
SOLVENT (in
(in greater
greater quantity)
quantity)
• Describe the solution process in terms of solutesolvent interactions and thermodynamics
• Calculate solution concentrations in terms of
molarity, molality, and percentages
• Understand the influence of polarity, pressure,
and temperature on solubility
• Understand and predict the effect of
concentration on colligative properties
REVIEW §4.5
CORN
OIL
HOMOGENEOUS
NONHOMOGENEOUS
2 phases
NONHOMOGENEOUS
2 phases
STATE
SOLVENT
SOLUTE
liquid
H 2O
NaCl
CLUB SODA
liquid
H 2O
CO2
14-karat GOLD
solid
Au
Cu
Pd HYDRIDE
solid
Pd
“H2”
AIR
gas
N2
O2
SOLUBLE
SOLUBLE
Dissolves
Dissolves to
to aa significant
significant
extent
extent in
in aa solvent
solvent
ETHYL
ALCOHOL
more alcohol!
more sugar!
SUGAR
SOLUTION
SEA WATER
HOMOGENEOUS
INSOLUBLE
INSOLUBLE
Does
Does not
not dissolve
dissolve to
to aa
significant
significant extent
extent in
in aa solvent
solvent
HOMOGENEOUS
MISCIBLE
MISCIBLE
Liquids
soluble
Liquids soluble in
in all
all proportions
proportions
Opposite:
Opposite: IMMISCIBLE
IMMISCIBLE
Degrees of Solution
SOLUTE
FORMULA
SOLUBILITY
Nitrogen
N2
0.0019
Potassium Chloride
KCl
29
Calcium Carbonate
CaCO3
0.0012
Ascorbic Acid
C6H8O6
33
Ethyl Alcohol
C2H5OH
∞
Sucrose
C12H22O11
179
g/100 g H2O
• An UNSATURATED solution contains less than the
maximum amount of solute that can be dissolved
under existing conditions
SOLUTE + SOLVENT → SOLUTION
• A SATURATED solution contains the maximum
amount of solute that can be dissolved
SOLUTE + SOLVENT ' SOLUTION
• A SUPERSATURATED solution temporarily contains
excess dissolved solute
– Excess solid solute may come out of solution by
CRYSTALLIZATION
SOLUTE↓ + SOLVENT ← SOLUTION
∞ = miscible
Page 13-1
“Like Dissolves Like”
SOLUBILITY
mol/kg H2O
SOLUTE
• The physical state of the solvent determines the
physical state of the solution
∞
CH3OH
CH2CH2CH2CH2OH
1.1
CH2CH2CH2CH2CH2CH2OH
– Ex: Gases are miscible with each other (dispersion forces
only)
– Ex: Hydrocarbon liquids are miscible in each other
(dispersion forces only)
Substances
Substances with
with similar
similar
types
types of
of interparticle
interparticle
forces
forces dissolve
dissolve in
in each
each
other.
other.
– Ex: Low-MW alcohols are miscible with water (H-bonding
predominates in both solvent and solute)
13.2-3 THE SOLUTION PROCESS
Dissolving Ionic Compounds
When an ionic solid dissolves in water, the polar
solvent removes ions from the crystal lattice:
+−+
∆Hsoln = ∆Hsolute + ∆Hsolvent + ∆Hmixing
+
+ −
∆Hsoln = ∆Hsolute + ∆Hsolvation
+−+
+−+
• Solute particles and solvent particles mix
(∆Hmixing < 0)
+−+
−+
• Solvent particles separate (∆Hsolvent > 0)
+
−+
+
Solvated ion
+−+
• Solute particles separate: Reduction of interparticle attractions (ion-ion, dipole-dipole, Hbonds, dispersion forces) among solute particles
(∆Hsolute > 0)
+
− +
• hydrocarbon-like
• nonpolar
• dispersion
+−+
·
·
·
+−+
·
·
·
• Forces established between solvent and solute
must be comparable in strength to interparticle
forces disrupted in both solvent and solute
(§13.2)
0.058
+−+
• water-like
• polar
• H-bonding
The Role of Interparticle Interactions
+−+
+−+
SOLVATION:
SOLVATION: The
The process
process of
of surrounding
surrounding
aa solute
solute particle
particle with
with solvent
solvent particles
particles
Dissolving Covalent Compounds
THE SOLUTION PROCESS, cont.
• As a solid dissolves, some solvated solute particles
collide, associate, and recrystallize.
Covalent compounds do not dissociate:
+
− +
+−+
+
− +
+−+
+−+
+
−+
• As long as ratesoln > ratextln, more solute dissolves.
• The solution process eventually reaches
equilibrium: ratesoln = ratextln
+−+
Solute(undissolved) + solvent ' Solute(dissolved)
+
+ −
+−+
+
− +
• The solution at this point is SATURATED, it
contains the maximum amount of dissolved solute.
• SUPERSATURATED solutions — [Solute] >
[Solute]equil — are unstable.
Page 13-2
Environmental Effects
13.4 QUANTIFYING CONCENTRATION
EFFECT on SOLUBILITY
SOLUTE
PHASE
∆T > 0
∆P > 0
SOLID
increases
no change
LIQUID
varies
no change
GAS
decreases
increases
• When describing a solution, we need a way to tell
how much solute is present
• We refer to the solute’s CONCENTRATION
• Concentrations may be described in terms of
—
—
—
—
Henry’s Law (gas solute):
—
Molarity (M, mol/L)
Molality (m, mol/kg)
Parts by mass (w/w)
Parts by volume (v/v)
Mole fraction (χ, molsolute/molsolute + molsolvent)
Csolute = kH × Psolute
where Psolute is partial pressure
of the gas above the solution.
[X]
[X] read
read as
as “the
“the concentration
concentration of
of X”
X”
MOLARITY
MOLALITY
Mass
Mass of
of solute
solute (mol)
(mol)
Moles
Moles of
of solute
solute (mol)
(mol)
M
M == ———————————
———————————
m
m == ———————————
———————————
Volume
Volume of
of solution
solution (L)
(L)
Mass
Mass of
of solvent
solvent (kg)
(kg)
Accounts for differences in formula weights:
Temperature independent
Equal volumes of 1M (“1 molar”) glucose and 1M
ethyl alcohol contain the same number of
M ≈ m for dilute solutions
molecules
Concentrations Expressed as Percents
TYPE
WEIGHT/WEIGHT
PERCENT*
SOLUTE SOLUTION
UNITS
UNITS
Mass,
g
13.5 COLLIGATIVE PROPERTIES
FORMULA
Properties
Properties that
that depend
depend on
on the
the number
number
of
of solute
solute particles
particles
Mass,
g
wsolute
100 × ————
wsolution
• Depends on how much, but not what type of solute
• Contribution of electrolytes (ionic materials) is
based on the effective number of ions produced
WEIGHT/VOLUME
PERCENT
Weight,
g
Volume,
mL
wsolute
100 × ————
vsolution
VOLUME/VOLUME
PERCENT
Volume,
mL
Volume,
mL
vsolute
100 × ————
vsolution
VAPOR PRESSURE LOWERING
BOILING POINT ELEVATION
FREEZING POINT DEPRESSION
OSMOTIC PRESSURE
6
** ppm
/w
ppm == 10
106 ×× wwsolute
solute/wsolution
solution
Page 13-3
Vapor Pressure Lowering
Boiling Point Elevation
This colligative property is described by Raoult’s
Law: Solution vapor pressure decreases in
proportion to the concentration of solute.
mole
fraction*
The magnitude of boiling point elevation increases
with increasing solute concentration:
Psolvent = χsolvent × P°solvent
∆
∆TTbb == KKbb ×× m
m
Psolvent = (1 − χsolute) × P°solvent
∆P = χsolute × P°solvent
EXAMPLES: Making maple syrup
Antifreeze
Kb, °C/m
Solvent
CCl4
5.03
3.63
CHCl3
CH3CH2OH 1.22
H2O
0.51
*§5.4
Freezing Point Depression
Osmosis
Diffusion
Diffusion of
of aa solvent
solvent through
through aa
semipermeable
semipermeable membrane
membrane from
from aa more
more dilute
dilute
solution
solution to
to aa more
more concentrated
concentrated one
one
The magnitude of freezing point depression
increases with increasing solute concentration:
∆
∆TTff == KKff ×× m
m
SEMIPERMEABLE MEMBRANE
EXAMPLES: Salting roads in winter
Making ice cream
Antifreeze
• Small molecules (such as solvent) go through,
large molecules (such as hydrated ions) do not
RESULT: PRESSURE due to one-way solvent
flow
• Cell walls are semipermeable membranes
Semipermeable Membrane
SEMIPERMEABLE
MEMBRANE
Cl−
Na+
PURE
ELECTROLYTE
SOLVENT
SOLUTION
Na+
Cl−
Water flows “uphill” until pressure equalized
Page 13-4
Osmotic Pressure
Ionic vs Covalent Compounds in Solution
1 mol NaCl
Pressure
Pressure required
required to
to stop
stop osmosis
osmosis
1 mol glucose
Π
Π == MRT
MRT
OSMOLARITY (mol particles/L)
~1 mol of Na+
+
~1 mol of Cl−
⇓
~2 mol of PARTICLES
Colligative Properties of Non-Ideal
Solutions
1 mol of PARTICLES
van’t Hoff Factor (i)
• The ratio of an observed colligative property to
the value calculated assuming the substance to be
a nonelectrolyte:
Property(obsd)
i = ———————
Property(calcd)
• Observed colligative properties are less than many
calculated values — most prominently for solutions
of strong electrolytes. Ex: For 0.100 m NaCl,
∆Tf(obsd) = 0.372°C < ∆Tf(calcd) = 0.384°C
• Difference results from ion pair formation:
Result: < 2 mol particles
Na
per mol NaCl
Cl−
• It’s concentration, charge dependent:
+
Cmpd
Sucrose
NaCl
K2SO4
MgSO4
• Similar behavior (stacking
interactions) by certain
nonelectrolytes:
Mobsd > Mcalcd
• Incorporated into colligative property equations:
∆P = i(χsoluteP°solvent)
∆Tb = i(Kbm)
∆Tf = i(Kfm)
Π = iMRT
The
The effective
effective number
number of
of solute
solute particles
particles
determines
determines colligative
colligative properties.
properties.
Page 13-5
10−1
1.00
1.87
2.32
1.21
[X],
10−2
1.00
1.94
2.70
1.53
m
10−3
1.00
1.97
2.84
1.82
→ 0
1.00
2.00
3.00
2.00