Colligative Properties | AP Chemistry
Colligative Properties
• Colligative properties are
properties that depend solely
upon the number of molecules
being studies
o There are 4 colligative
properties that we must
know
Vapor Pressure
• Recall from chapter 4 that a
nonvolatile solute restricts the
ability of a solvent to vaporize
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Boiling Point Elevation Equation
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Nonvolatile Solute in Water
The addition of nonvolatile solute (yellow)
impedes water’s ability to vaporize.
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The more solute we added, the
more it thwarted water’s ability
to vaporize
Note that the identity of the
solute didn’t matter; all that
mattered was how much solute
we added
o Thus, vapor pressure is a
colligative property
Boiling Point Elevation
• A substance’s boiling point can
be raised by adding a nonvolatile
solute
• The nonvolatile solute hinders
the substance’s ability to escape
the liquid phase
o So more energy- more
heat- will be required to
vaporize the substance
So with the addition of the
nonvolatile solute, the substance
boils at a higher temperature
Boiling point elevation can be
quantified:
∆T = change in boiling point
kb = a constant specific to the
substance being boiled
m = molality of the solution
i = the van’t Hoff factor
The van’t Hoff factor is the
number of constituents into
which a single solute molecule
will dissociate
o i.e. In solution, NaCl
breaks apart into 2
constituents- Na+ and Cl-,
so its van’t Hoff factor = 2
Notice that the equation depends
upon the quantity of solute, but
not its identity
o So boiling point elevation
is a colligative property
Freezing Point Depression
• The equation for freezing point
depression is identical to that for
boiling point elevation, but kb is
replaced with a new constant- kf
Freezing Point Depression Equation
•
The presence of solute molecules
disrupts the compact, crystal
lattices of solids
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Colligative Properties | AP Chemistry
•
(a)
(b)
Pure and Impure Solids
(a) A pure solid has a compact, organized
molecular arrangement. (b) When a solute is
introduced, it disturbs the solid’s structure,
thereby weakening the strength of its bonds.
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By disturbing a substance’s
intermolecular structure, the
solute thwarts the formation of a
compact, solid structure
To overcome this obstacle and
achieve freezing, the temperature
will need to be reduced
o In other words, the
substance’s freezing point
will be lowered
The identity of the solute doesn’t
matter; all that matters is its
quantity
o Thus, freezing point
depression is a colligative
property
Consider the interesting scenario
in which a liquid solute, A, is
mixed into a liquid solvent, B
As A is added, B’s purity
becomes increasingly tainted by
the solute
o And so its freezing point
will drop
Solute A in Solvent B
As more A is added, the freezing point of B
continues to drop.
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•
However, when enough A has
been added, it takes over as the
solvent, and B becomes the
solute
Continuing to add A at this point
will serve only to make the
solution purer
o And so its freezing point
will increase!
Solute B in Solvent A
Continuing to add A to the solution will cause A
to take over as the solvent. B will become the
solute, and continuing to add A will make the
solution purer. As a result, its freezing point will
increase.
Forming a Liquid Mixture
Liquid solute A is added to liquid solvent B.
Osmotic Pressure
• Suppose we took a beaker of
water and inserted a
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Colligative Properties | AP Chemistry
semipermeable membrane down
its center
o The membrane allows
water to pass through, but
not solute
o The flow of water into the
solute side (due to
osmosis) will equal the
flow of water out (due to
water pressure)
Beaker of Water with a Semipermeable
Membrane
The membrane allows water to pass between the
two sides, but not solute molecules.
Equilibrium
Osmosis will bring water into the solute side and
the water pressure buildup in that side will force
water out. The two forces achieve an
equilibrium, in which water flows into each side
at an equal rate.
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We then add solute to one side of
the beaker
In an attempt to equalize the
concentrations in both sides of
the beaker, water will flow into
the side with the solute
o And so the water rises in
that side
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Addition of Solute to One Compartment
When solute is added to one side of the beaker,
water passes into that side in an attempt dilute
the solute.
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Water pressure builds up in the
solute side, which begins pushing
water back to the other side
An equilibrium will ultimately be
established in which:
When that equilibrium has been
established, the solute side has a
greater pressure than the other
side of the beaker
o That extra pressure is
called the osmotic
pressure
Like our other colligative
properties, osmotic pressure
depends upon the quantity of
solute, not its identity
Osmotic pressure can be
calculated according to the
equation:
Osmotic Pressure Equation
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Π = osmotic pressure
i = van’t Hoff factor
M = molarity
R = universal gas constant
J
o R = 8.314
K mol
T = temperature
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