Freezing Point Depression
Ck12 Science
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Printed: November 17, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Freezing Point Depression
1
Freezing Point Depression
• Define freezing point depression.
• Calculate the freezing point of a solution when given the molal freezing-point depression constant.
Why salt icy roads?
Colligative properties have practical applications, such as the salting of roads in cold-weather climates. By applying
salt to an icy road, the melting point of the ice is decreased, and the ice will melt more quickly, making driving
safer. Sodium chloride (NaCl) and either calcium chloride (CaCl2 ) or magnesium chloride (MgCl2 ) are used most
frequently, either alone or in a mixture. Sodium chloride is the least expensive option, but is less effective because it
only dissociates into two ions instead of three.
Freezing Point Depression
The Figure 1.1 shows the phase diagram for a pure solvent and how it changes when a solute is added to it. The
solute lowers the vapor pressure of the solvent resulting in a lowering of the freezing point of the solution compared
to the solvent. The freezing point depression is the difference in temperature between the freezing point of the pure
solvent and that of the solution. On the graph, the freezing point depression is represented by ∆T f .
When a pure solvent freezes, its particles become more ordered as the intermolecular forces that operate between the
molecules become permanent. In the case of water, the hydrogen bonds make the hexagonally-shaped network of
molecules that characterizes the structure of ice. By dissolving a solute into the liquid solvent, this ordering process
is disrupted. As a result, more energy must be removed from the solution in order to freeze it, and the freezing point
of the solution is lower than that of the pure solvent.
The magnitude of the freezing point depression is directly proportional to the molality of the solution. The equation
is:
∆T f = K f × m
The proportionality constant, K f , is called the molal freezing-point depression constant. It is a constant that is
equal to the change in the freezing point for a 1-molal solution of a nonvolatile molecular solute. For water, the
1
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FIGURE 1.1
The vapor pressure of a solution (blue) is
lower than the vapor pressure of a pure
solvent (pink). As a result, the freezing
point of a solvent decreases when any
solute is dissolved into it.
value of K f is -1.86°C/ m. So the freezing temperature of a 1-molal aqueous solution of any nonvolatile molecular
solute is -1.86°C. Every solvent has a unique molal freezing-point depression constant. These are shown in Table
1.1, along with a related value for the boiling point called Kb .
TABLE 1.1: Molal Freezing-Point and Boiling-Point Constants
Solvent
Normal freezing
point (°C)
Acetic acid
Camphor
Naphthalene
Phenol
Water
16.6
178.8
80.2
40.9
0.00
Molal
freezingpoint depression
constant,
Kf
(°C/m)
-3.90
-39.7
-6.94
-7.40
-1.86
Normal
boiling
point (°C)
117.9
207.4
217.7
181.8
100.00
Molal
boilingpoint
elevation
constant,
Kb
(°C/m)
3.07
5.61
5.80
3.60
0.512
Sample Problem: Freezing Point of a Nonelectrolyte
Ethylene glycol (C2 H6 O2 ) is a molecular compound that is used in many commercial anti-freezes. A water solution
of ethylene glycol is used in vehicle radiators to lower its freezing point and thus prevent the water in the radiator
from freezing. Calculate the freezing point of a solution of 400. g of ethylene glycol in 500. g of water.
Step 1: List the known quantities and plan the problem.
Known
•
•
•
•
•
mass C2 H6 O2 = 400. g
molar mass C2 H6 O2 = 62.08 g/mol
mass H2 O = 500.0 g = 0.500 kg
mass H2 O = 500.0 g = 0.500 kg
K f (H2 O) = −1.86◦ C/m
2Unknown
◦
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Chapter 1. Freezing Point Depression
Step 3: Think about your result.
The freezing point of the water decreases by a large amount, protecting the radiator from damage due to the
expansion of water when it freezes. There are three significant figures in the result.
Summary
• Freezing point depression is defined.
• Calculations involving freezing point depression are described.
Practice
Solve the problems at the site below:
http://home.comcast.net/~cochranjim/PDFS3/COLIGWS1A.pdf
Review
Questions
1. How does a solute affect the freezing of water?
2. How many moles of glucose would be needed to lower the freezing point of one kg of water 3.72°C?
3. How many moles of NaCl would be needed to produce the same amount of lowering of temperature?
• freezing point depression: The difference in temperature between the freezing point of the pure solvent and
that of the solution.
• molal freezing-point depression constant: A constant that is equal to the change in the freezing point for a
1-molal solution of a nonvolatile molecular solute.
References
1. Michael Pereckas (Flickr: Beige Alert). http://commons.wikimedia.org/wiki/File:Salt_truck_Milwaukee.jpg
.
2. CK-12 Foundation - Christopher Auyeung. .
3