Colligative Properties
Properties that depend upon the concentration
of solute particles are called colligative
properties.
Generally these properties are measured for
dilute solutions (at high concentrations there
are other interactions that take place).
Vapor pressure, boiling point, and freezing point
are changed by particle concentration.
Vapor Pressure
• Vapor pressure can be described as the pressure exerted by
the vapor above a liquid against the opposing atmospheric
pressure. When a nonvolatile substance like sugar or salt are
added to water they lower the vapor pressure.
• How do they lower the vapor pressure?
– Some of the nonvolatile particles are at the surface and they interfere
with the escape of the volatile water molecules
– Since the volatile water molecules need a certain amount of kinetic
energy to escape, the number gaining enough energy to escape is less
because the nonvolatile particles have absorbed some of that energy
Boiling Point Elevation
• Boiling Point is defined as the temperature at which the vapor
pressure equals the opposing standard atmospheric pressure.
• When a nonvolatile particles are added to water to make a
solution, the boiling point of that solution is raised.
Chloride ion
Sodium ion
Water boils at 100°C at 1 atm
NaCl solution at > 100°C at 1 atm
• The elevation of the boiling point depends on the number of
mols of particles present. For each mol of particles in a kg of
water the boiling point of the aqueous solution is elevated by
0.52°C. That is called the boiling point elevation constant for
water (0.52°C/m).
• Molecular compounds such as C6H12O6 do not break up into
ions, so one mol of C6H12O6 would raise the boiling point
0.52°C.
• Ionic Compounds such as NaCl and CaCl2 break up into ions in
aqueous solution. One mol of NaCl actually yields two mols of
ions, one mol of CaCl2 yields three mols of ions. They would
elevate the boiling point more than one mol of C6H12O6.
The increase in boiling point relative to that of
the pure solvent is related to the molality of the
solution by the following expression:
/\ Tb =Kbm
/\ Tb is the change in boiling point
Kb is the molal boiling point constant, which
depends on the solvent
m is the molality of the solution
The boiling point elevation is proportional to the
concentration of solute particles, regardless of
whether the particles are molecules or ions.
When NaCl dissolves in water, 2 moles of solute
particles are formed for each mole of NaCl that
dissolves. As a result, the boiling point elevation
of a 1 m aqueous solution of NaCl is
approximately twice as large as a 1 m solution of
a nonelectrolyte such as sucrose(C12H22O11).
To properly access the effect of a particular
solute on the boiling point (or any other
colligative property), it is important to know
whether the solute is an electrolyte or
nonelectrolyte. In the previous slide, it was
mentioned that NaCl would raise the boiling
point of a solution twice as much as a
nonelectrolyte. This is important to remember
when solving freezing point depression and
boiling point elevation problems.
When solving freezing point depression and
boiling point elevation problems when a strong
electrolyte is the solute, you must use a factor in
your calculations called the van’t Hoff factor,i.
It represents how many ions the electrolyte
forms in solution.
Examples: CaCl2 i = 3
H2SO4 i = 3
NaCl i = 2
Electrolyte Review
The strong electrolytes are all ionic compounds
and strong acids. Most molecular compounds
are nonelectrolytes.
Freezing Point Depression
• When water freezes the hydrogen bonds give water a rigid
structure (water expands as it freezes).
The solid water has a definite
crystalline structure as a result of
the hydrogen bonding
• Adding NaCl or C6H12O6 (any solute) to the water interferes
with the formation of the hydrogen bonds. The freezing point
of an aqueous solution is lowered 1.86°C for each mol of
particles per kg of water (1.86°C/m).
The decrease in freezing point is expressed by
the following expression:
/\ Tf =Kfm
/\ Tf is the change in freezing point
Kf is the molal freezing point constant, which
depends on the solvent
m is the molality of the solution
Summary
Colligative properties depend upon the
concentration of the solute.
When determining the boiling point elevation or
freezing point depression, you must take into
consideration if the solute is an electrolyte or
nonelectrolyte.
Colligative Formulas
Nonelectrolytes
Electrolytes
/\ Tb =Kbm
/\Tb =Kbmi
/\ Tf =Kfm
/\ Tf =Kfmi
i= van’t Hoff factor(how
many ions of solute are
formed)
Study Check
What is the boiling point elevation of a solution
made from 20.1 g of a nonelectrolyte solute and
400.0 g of water? The molar mass of the solute
is 62.0 g, and the Kb for water is 0.51oC/m.
Study Check
What is the freezing point depression of water in
a solution of 17.1 g of sucrose, C12H22O11, in 200.
g of water? What is the actual freezing point of
the solution? The Kf for water is 1.86oC/m.
Study Check
What is the expected change in freezing point of
water in a solution of 62.5 g of barium nitrate,
Ba(NO3)2, in 1.00 kg of water? Ba(NO3)2 is a
strong electrolyte.