Section 13.6: Colligative Properties

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Section 13.6 Colligative Properties
The presence of solute particles (ions,
molecules) make the physical properties of
a solution different from those of a pure
solvent. It is the of solute particles that
matters, not their chemical identity. Colligativ
e properties vapor pressure lowering
boiling point elevation freezing point
depression osmotic pressure
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Section 13.6 Vapor Pressure Lowering
The vapor pressure of a solution of a nonvolatile
(does not vaporize) nonelectrolyte is always
lower than the vapor pressure of the pure solvent.
Review vapor pressure - the pressure exerted by
a vapor when it has reached equilibrium in a
system that is closed with respect to the vapor
molecules
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Section 13.6 Vapor Pressure Lowering
Entropy argument Pure solvent
vaporizes because the vapor has a higher entropy
than the liquid Solvent in a solution
already has a greater entropy than the pure
solvents
Therefore, less tendency to vaporize to increase
entropy.
Quantitatively Raoults law Psolvent
Xsolvent x Pºsolvent
where Psolvent vapor pressure of the solvent
above the solution Xsolvent
mole fraction of solvent in the solution
Pºsolvent vapor pressure of the pure
solvent Mole fraction moles of solvent /
(moles of solvent moles of solute) Applies
only to ideal solutions (in other words, dilute
solutions). - freshwater versus saltwater
(in ocean or in your body)
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Section 13.6 Vapor Pressure Lowering
How does the amount of solute affect the
magnitude of the vapor pressure lowering (?P)?
If Xsolvent Xsolute 1 and
Psolvent Xsolvent x Pºsolvent
then Pºsolvent -
Psolvent ?P Xsolute Pºsolvent
What is the vapor pressure lowering (?P, in units
of torr) when 10.0 mL of glycerol (C3H8O3) is
added to 500.0 mL of water at 50 ºC? At this
temperature, the vapor pressure of pure water is
92.5 torr and its density is 0.988 g/mL. The
density of glycerol is 1.26 g/mL.
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Section 13.6 Boiling Point Elevation
A solution boils at a higher temperature than the
pure solvent. Why?
Review boiling point - the temperature at which
the vapor pressure of a liquid is equal to the
external pressure.
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Section 13.6 Freezing Point Depression
?Tf Kfm
where ?Tf freezing point depression
Kf molal f.p. dep. constant m
molality of the solution
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Section 13.6 Boiling and Freezing Point
Depression Problem
You add 1.00 kg of C2H6O2 (antifreeze) to your
car radiator, which contains 4450 g of water.
What are the b.p. and f.p. of the solution?
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Section 13.6 Osmotic Pressure
This colligative property appears when two
solutions of different concentrations
are separated by a semipermeable membrane
(solvent passes, solute does not).
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Section 13.6 Vapor Pressure for Volatile
Solutes..
..or how to make Moonshine in Appalachia with
this guy.
How is the vapor pressure affected when the
solute vaporizes too? Daltons Law of Partial
Pressures The total vapor pressure is the sum of
the partial vapor pressures.
Ptotal Psolvent Psolute (Xsolvent x
Pºsolvent) (Xsolute Pºsolute)

(Raoults law) Example A solution contains
equal amounts (moles) of benzene and toluene
Xbenzene Xtoluene 0.50 v.p.pure benzene
95.1 torr, v.p.pure toluene 28.4 torr
How does vapor pressure change for both? Does the
composition of the vapor change?
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Fractional distillation a process used to
separate a mixture of volatile components.
At 20 ºC, the v.p. of pure ethyl alcohol is 9 kPa
and the v.p. of pure water at this temperature is
2.4 kPa. The solution contains equal amounts of
water and ethyl alcohol. What is the partial
pressure of each liquid in the solution? What is
the mole fraction of ethyl alcohol and water in
the vapor?
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Section 13.6 Electrolyte Solutions
Colligative properties vapor pressure
lowering boiling point elevation
freezing point depression osmotic
pressure Equations differ slightly when dealing
with electrolyte solutions (non-volatile)
i particles in solution / solute added
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Optional Homework Problems Even numbered
problems 13.90 13.104 (All answers in back of
book) Come with questions on Optional Homework
Problems from Chapter 13 tomorrow.