Chapter 13 – Solutions - part II
Colligative Properties
• Changes in colligative properties
depend only on the number of solute
particles present, not on the identity of
the solute particles.
• Among colligative properties are
Vapor pressure lowering
Boiling point elevation
Melting point depression
Osmotic pressure
Solutions
Vapor Pressure
Because of solute-solvent
intermolecular attraction,
higher concentrations of
nonvolatile solutes make it
harder for solvent to escape to
the vapor phase.
Therefore, the vapor pressure
of a solution is lower than that
of the pure solvent.
Solutions
Raoult’s Law
PA = XAPA
where
• XA is the mole fraction of compound A
• PA is the normal vapor pressure of A at
that temperature
NOTE: This is one of those times when you
want to make sure you have the vapor
pressure of the solvent.
Solutions
Boiling Point Elevation and
Freezing Point Depression
Nonvolatile solutesolvent interactions
also cause solutions
to have higher boiling
points and lower
freezing points than
the pure solvent.
Solutions
Boiling-Point Elevation
DTb = Tb – T b0
T b0 is the boiling point of
the pure solvent
T b is the boiling point of
the solution
Tb > T b0
DTb > 0
DTb = Kb m
m is the molality of the solution
Kb is the molal boiling-point
elevation constant (0C/m)
Boiling Point Elevation
The change in boiling
point is proportional to
the molality of the
solution:
DTb = Kb  m
DTb is added to the normal
boiling point of the solvent.
where Kb is the molal
boiling point elevation
constant, a property of
the solvent.
Freezing Point Depression
• The change in freezing
point can be found
similarly:
DTf = Kf  m
• Here Kf is the molal
freezing point
depression constant of
the solvent.
DTf is subtracted from the normal
freezing point of the solvent.
Solutions
Freezing-Point Depression
DTf = T 0f – Tf
T
0
Tf
f
is the freezing point of
the pure solvent
is the freezing point of
the solution
T 0f > Tf
DTf > 0
DTf = Kf m
m is the molality of the solution
Kf is the molal freezing-point
depression constant (0C/m)
Boiling Point Elevation and
Freezing Point Depression
Note that in both
equations, DT does
not depend on what
the solute is, but
only on how many
particles are
dissolved.
DTb = Kb  m
DTf = Kf  m
Solutions
What is the freezing point of a solution containing 478 g
of ethylene glycol (antifreeze) in 3202 g of water? The
molar mass of ethylene glycol is 62.01 g.
DTf = Kf m
Kf water = 1.86 0C/m
moles of solute
m =
mass of solvent (kg)
478 g x
1 mol
62.01 g
=
= 2.41 m
3.202 kg solvent
DTf = Kf m = 1.86 0C/m x 2.41 m = 4.48 0C
DTf = T 0f – Tf
Tf = T 0f – DTf = 0.00 0C – 4.48 0C = -4.48 0C
Colligative Properties of
Electrolytes
Since these properties depend on the number of
particles dissolved, solutions of electrolytes (which
dissociate in solution) should show greater changes
than those of nonelectrolytes.
Colligative Properties of
Electrolytes
However, a 1 M solution of NaCl does not show
twice the change in freezing point that a 1 M
solution of methanol does.
van’t Hoff Factor
One mole of NaCl in water
does not really give rise to
two moles of ions.
Some Na+ and Cl−
reassociate for a short
time, so the true
concentration of particles
is somewhat less than two
times the concentration of
NaCl.
The van’t Hoff Factor
• Reassociation is more likely at higher
concentration.
• Therefore, the number of particles
present is concentration dependent.
• We modify the previous equations by
multiplying by the van’t Hoff factor, i
DTf = Kf  m  i
Colligative Properties of Electrolyte Solutions
0.1 m Na+ ions & 0.1 m Cl- ions
0.1 m NaCl solution
Colligative properties are properties that depend only on the
number of solute particles in solution and not on the nature of
the solute particles.
0.1 m NaCl solution
van’t Hoff factor (i) =
0.2 m ions in solution
actual number of particles in soln after dissociation
number of formula units initially dissolved in soln
i should be
nonelectrolytes
NaCl
CaCl2
1
2
3
Colligative Properties of Electrolyte Solutions
Boiling-Point Elevation
DTb = i Kb m
Freezing-Point Depression
DTf = i Kf m
Osmotic Pressure (p)
p = iMRT
Solutions
Change in Freezing Point
Which would you use for the streets of
Bloomington to lower the freezing point
of ice and why? Would the temperature
make any difference in your decision?
a)
sand, SiO2
b)
Rock salt, NaCl
c)
Ice Melt, CaCl2
Solutions
Change in Freezing Point
Common
Applications of
Freezing Point
Depression
Propylene glycol
Ethylene
glycol –
deadly to
small
animals
Solutions
Change in Boiling Point
Common Applications
of Boiling Point
Elevation
Freezing Point Depression
At what temperature will a 5.4 molal
solution of NaCl freeze?
Solution
∆TFP = Kf • m • i
∆TFP = (1.86 oC/molal) • 5.4 m • 2
∆TFP = 20.1 oC
FP = 0 – 20.1 = -20.1 oC
Osmosis
• Some substances form semipermeable
membranes, allowing some smaller
particles to pass through, but blocking
other larger particles.
• In biological systems, most
semipermeable membranes allow water
to pass through, but solutes are not free
to do so.
Solutions
Osmosis
In osmosis, there is net movement of solvent from
the area of higher solvent concentration (lower
solute concentration) to the are of lower solvent
concentration (higher solute concentration).
Solutions
Osmotic Pressure
• The pressure required to stop osmosis,
known as osmotic pressure, p, is
p=(
n
)
RT = MRT
V
where M is the molarity of the solution
If the osmotic pressure is the same on both sides
of a membrane (i.e., the concentrations are the
same), the solutions are isotonic.
Solutions
Molar Mass from
Colligative Properties
We can use the
effects of a colligative
property such as
osmotic pressure to
determine the molar
mass of a compound.
Solutions
Osmosis in Blood Cells
• If the solute
concentration outside
the cell is greater than
that inside the cell, the
solution is hypertonic.
• Water will flow out of
the cell, and crenation
results.
Solutions
Osmosis in Cells
• If the solute
concentration outside
the cell is less than
that inside the cell, the
solution is hypotonic.
• Water will flow into the
cell, and hemolysis
results.
Solutions
A cell in an:
isotonic
solution
hypotonic
solution
hypertonic
solution
Solutions
Chemistry In Action:
Desalination
Colloids versus Suspensions
Suspensions of particles
larger than individual ions
or molecules, but too small
to be settled out by gravity.
•These are mixed, but not
dissolved in each other
•Will settle over time
•Particles are bigger than 1
micrometer (larger than
colloid)
•Examples: dust in air,
muddy water
Solutions
Tyndall Effect
• Colloidal suspensions
can scatter rays of light.
• This phenomenon is
known as the Tyndall
effect.
Solutions
Colloids in Biological Systems
Some molecules have
a polar, hydrophilic
(water-loving) end and
a nonpolar,
hydrophobic (waterhating) end.
Solutions
Colloids in Biological Systems
Sodium
stearate is
one
example of
such a
molecule.
These molecules can aid in the
emulsification of fats and oils in
aqueous solutions.
Solutions