Lesson 4
Colligative Properties
of Solutions
Focus Question
Why do we salt the roads when it's cold
outside?
New Vocabulary
colligative property
vapor pressure lowering
boiling point elevation
freezing point depression
osmosis
osmotic pressure
Review Vocabulary
ion: an atom that is electrically charged
Colligative Properties
• Colligative properties are physical properties
of solutions that are affected by the number
of particles but not by the identity of dissolved
solute particles.
• Colligative means depending on the collection.
• Colligative properties include:
• Vapor pressure lowering
• Boiling point elevation
• Freezing point depression
• Osmotic pressure
Electrolytes
• Ionic compounds are electrolytes because they
dissociate in water to form a solution that
conducts electricity.
• Electrolytes that produce many ions are strong
electrolytes.
• Electrolytes that produce only a few ions are
weak electrolytes.
• Many molecular compounds do not ionize when
dissolved and do not conduct electricity. These
are called nonelectrolytes.
• There are some exceptions; molecular
compounds that do ionize are electrolytes.
Vapor Pressure Lowering
• Recall that vapor pressure is the pressure
exerted in a closed container by the particles
that have escaped the liquid’s surface and
entered the gaseous state.
• Adding a nonvolatile solute (one that has little
tendency to become a gas) to a solvent lowers
the solvent’s vapor pressure.
• Vapor pressure lowering depends on the
number of solute particles in solution. It is a
colligative property of solutions.
Vapor Pressure Lowering
• When a solute is
present, a mixture of
solvent and solute
occupies the surface
area, and fewer
particles enter the
gaseous state.
• The greater the
number of solute
particles, the lower
the vapor pressure.
Boiling Point Elevation
• When a nonvolatile solute lowers the vapor
pressure of a solvent, the boiling point is also
affected.
• More heat is needed to supply additional
kinetic energy to raise the vapor pressure to
atmospheric pressure.
Boiling Point Elevation
• The temperature difference between a solution’s
boiling point and a pure solvent's boiling point is
called the boiling point elevation, ΔTb.
• For non-electrolytes, the boiling point elevation
is directly proportional to the solution’s molality.
Boiling Point Elevation
Freezing Point Depression
• At a solvent's freezing point temperature,
particles no longer have sufficient kinetic
energy to overcome interparticle attractive
forces.
• The freezing point of a solution is always
lower than that of the pure solvent.
• Solute particles interfere with the attractive
forces among solvent particles.
Freezing Point Depression
• A solution's freezing point depression, ΔTf, is the
difference in temperature between its freezing
point and the freezing point of the pure solvent.
• Like boiling point elevation, freezing point
depression is a colligative property of a solution
and is directly proportional to molality.
Freezing Point Depression
CHANGES IN BOILING AND FREEZING POINTS
Use with Example Problem 6.
Problem
Sodium chloride (NaCl) is often used to
prevent icy roads and to freeze ice cream.
What are the boiling and freezing points of a
0.029m aqueous solution of sodium
chloride?
Response
ANALYZE THE PROBLEM
You are given the molality of an aqueous
sodium chloride solution. First, calculate ΔTb
and ΔTf based on the number of particles in the
solution. Then, to determine the elevated
boiling point and the depressed freezing point,
add ΔTb to the normal boiling point and
subtract ΔTf from the normal freezing point of
water.
KNOWN
UNKNOWN
molality of solution
= 0.029m
boiling point = ?°C
Kb = 0.512°C/m
freezing point = ?°C
Kf = 1.86°C/m
SOLVE FOR THE UNKNOWN
Determine the molality of the particles. Particle
molality = 2 × 0.029m = 0.058m Determine ΔTb
and ΔTf
•
State the boiling point elevation and freezing
point depression formulas.
ΔTb = Kbm
ΔTf = Kfm
CHANGES IN BOILING AND FREEZING POINTS
SOLVE FOR THE UNKNOWN (continued)
•
Substitute Kb = 0.512°C/m, Kf = 1.86°C/m,
and m = 0.058m.
ΔTb = (0.512°C/m)(0.058m) = 0.030°C
ΔTf = (1.86°C/m)(0.058m) = 0.11°C
Determine the elevated boiling point and
depressed freezing point of the solution.
•
Add ΔTb to the normal boiling point and
subtract ΔTf from the normal freezing point.
boiling point = 100.000°C + 0.030°C =
100.030°C
freezing point = 0.00°C - 0.11°C = -0.11°C
EVALUATE THE ANSWER
The boiling point is higher and the freezing
point is lower, as expected. Because the
molality of the solution has two significant
figures, both ΔTb and ΔTf have two significant
figures. Because the normal boiling point and
freezing point are exact values, they do not
affect the number of significant figures in the
final answer.
Changes in Boiling and Freezing Points
Osmotic Pressure
Osmosis is the diffusion of a solvent through a
semipermeable membrane.
Osmotic Pressure
• Osmotic pressure is the amount of additional
pressure caused by water molecules that
move into the concentrated solution.
• Osmotic pressure depends on the number of
solute particles in a given volume of solution
and is a colligative property of solutions.
Quiz
1. When a nonvolatile solute is added to a solvent, vapor
pressure ______.
A
decreases
B
increases
C
stays the same
D
disappears
CORRECT
Quiz
2. For non-electrolytes, boiling point elevation and
freezing point depression are both:
A
directly proportional to the solution’s molarity
B
inversely proportional to the solution’s molarity
C
directly proportional to the solution's molality CORRECT
D
inversely proportional to the solution's molality
Quiz
3. The freezing point of a solution is always ______ the
freezing point of the pure solvent.
A
higher than
B
lower than
C
the same as
D
unrelated to
CORRECT
Quiz
4. What process does this
figure illustrate?
A
vapor pressure lowering
B
boiling point elevation
C
freezing point depression
D
osmosis
CORRECT