Ch. 14: Mixtures & Solutions
Sec. 14.4: Colligative Properties
of Solutions
Objectives
• Describe colligative properties.
• Identify four colligative properties of
solutions.
• Determine the boiling point elevation and
the freezing point depression of a solution.
Colligative Properties of
Solutions
• Solutes affect the physical properties of
their solvents.
• Physical properties of solutions that are
affected by the number of particles but not
the identity of dissolved solute particles are
called colligative properties.
• Vapor pressure, boiling point, freezing
point, and osmotic pressure are colligative
properties.
Electrolytes and Colligative
Properties
Ionic compounds are
electrolytes because
they dissociate in water
to form a solution that
conducts electric
current.
•Electrolytes that produce
many ions are strong
electrolytes.
•Electrolytes that produce
only a few ions are weak
electrolytes.
Electrolytes and Colligative
Properties
• NaCl is a strong electrolyte. It almost
completely dissociates in solution,
producing Na+ and Cl- ions.
NaCl
Na+ + Cl• Each mole of NaCl that is dissolved,
therefore, will produce 2 moles of ions.
• A 1 m solution of NaCl will produce a 2 m
solution of ions.
Nonelectrolytes in Aqueous
Solution
Nonelectrolytes in Aqueous
Solution
• Molecular compounds in solution generally
DO NOT ionize. They are nonelectrolytes.
• Therefore, when 1 mole of such a
compound is dissolved in water, there will
only be one mole of molecules in solution.
• A 1 m sucrose solution will contain 1 m of
sucrose molecules.
Remember . . .
• Colligative properties depend on the
number of particles in solution.
• We would therefore expect the compound
producing more particles in solution to
have a greater effect on the colligative
properties.
Practice Problem
• Which of the following substances will
have the greatest effect on the colligative
properties of a solution?
HCl
•C6H12O6
ŽMgCl2
•CuSO4
•Al(NO3)3
Colligative Properties: Solutes
Cause Vapor Pressure Lowering
• Vapor pressure is
the pressure
exerted in a closed
container by liquid
particles that have
escaped the
liquid’s surface
and entered the
gaseous state.
Why does adding a nonvolatile solute to a
solvent lower the solvent’s vapor pressure?
Vapor Pressure Depression
• When a solvent is pure, its
particles occupy the entire
surface area.
• When the solvent contains a
solute, a mix of particles
occupies the surface area.
Fewer solvent particles are
there, so fewer are able to enter
the gaseous state.
• The greater the number of
solute particles, the lower the
resulting vapor pressure.
Colligative Properties: Solutes
Cause Boiling Point Elevation
• A substance boils when its vapor pressure equals
atmospheric pressure.
• If a solution is heated to the boiling point of its
solvent, because the vapor pressure of the
solution is lower than the VP of the pure solvent,
the VP will NOT equal the atmospheric pressure.
• A solution will not boil at the solvent’s boiling
point.
Boiling Point Elevation
• The solution must be heated to a higher
temperature to boil. The vapor pressure of
the solution must be increased (by heating)
to equal the atmospheric pressure.
• The temperature difference between a
solution’s boiling point and a pure
solvent’s boiling point is called the boiling
point elevation (ΔTb).
• ΔTb = Tb solution - Tb solvent
Boiling Point Elevation
• For nonelectrolytes,
ΔTb = Kbm
where:
• ΔTb is the boiling point elevation of the solvent.
• m is the molal concentration of the solution.
• Kb is the molal boiling point elevation constant
(see Table 5, pg. 500). The units for this constant
are 0C/m.
• Note: Every solute has a different Kb.
Boiling Point Elevation
• Since boiling point elevation is a
colligative property, it is dependent on the
number of particles the solute forms in
solution.
• Therefore, for electrolyte solutions, the
particle molality (or # particles in the
solute times m) must be used instead of m
in the equation: ΔTb = Kbm
– The greater the number of solute particles
in the solution, the greater the BPE.
Practice Problems
1 -What is the boiling point of a 0.029 m
aqueous solution of sucrose? (Kb water =
0.512 0C/m)
2 -The BPE of an aqueous solution of a
nonvolatile, nonelectrolyte is 1.12 0C.
What is the solution molality?
3 -How many grams of sucrose (C12H22O11)
must be dissolved in 125 g of ethanol to
raise the boiling point by 4 C0? (Kb ethanol
= 1.22 0C/m)
Colligative Properties: Solutes
Cause Freezing Point Depression
• The freezing
point of a
solution is
always lower
than that of a
pure solvent.
• Why?
Freezing Point Depression
• Normally at the solvent’s freezing point, solvent
particles do not have the KE to overcome
interparticle forces. Therefore, an organized
crystal forms. (The solvent freezes.)
• Added solute particles interfere with these forces
and, so, the formation of the solid at the normal
freezing point. Additional energy must be
removed (the temperature must be lowered
further) before the solution will freeze.
Freezing Point Depression
• The temperature difference between a
solution’s freezing point and a pure
solvent’s freezing point is called the
freezing point depression (ΔTf).
• ΔTf = Tf solution - Tf solvent
(absolute value)
Freezing Point Depression
• For nonelectrolytes,
D Tf = Kfm
(see Table 6, pg. 502 for Kf values)
• For electrolyte solutions, the particle
molality (or # particles in the solute times
m) must be used instead of m in the above
equation.
– The greater the number of solute particles
in the solution, the greater the FPD.
Practice Problems
1 -What is the freezing point of a 0.029 m
aqueous solution of sucrose? (Kf water =
1.86 0C/m)
2 -The FPD of an aqueous solution of a
nonvolatile, nonelectrolyte is 4.65 0C.
What is the solution molality?
3 How many grams of sucrose (C12H22O11)
must be dissolved in 500 g of ethanol to
lower the freezing point by 2.5 C0? (Kf
ethanol = 1.99 0C/m)
Colligative Properties: Solutes
Cause Osmotic Pressure Elevation
• Osmosis is the diffusion of water (solvent)
particles across a semipermeable
membrane from an area of higher solvent
concentration to an area of lower solvent
concentration.
– Semipermeable membranes are barriers with
tiny pores that allow some particles to cross
but not others.
Osmotic Pressure Elevation
Osmotic Pressure Elevation
• The flow of additional solvent molecules to the
solution side of a membrane creates pressure.
This increase in pressure is called osmotic
pressure.
• Osmotic pressure is defined as the amount of
additional pressure caused by water molecules
that moved into the concentrated solution.
• Osmotic Pressure depends upon the number of
solute particles in a given volume of solution.
The greater the number of solute particles in
the solution, the greater the osmotic pressure.
Osmotic Pressure Elevation
Normally, water
flows from your
intestine into
your tissues.
However,
drinking sea
water results in a
reversal in the
direction of water
flow and, so, the
dehydration of
body tissues.