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Colligative Properties of
Solutions
Chemistry
How do you get from this…
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…to this?
Add an ionic compound!
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Colligative Properties
• Properties that depend only on the number
of solute particles and not on their identity.
Some Colligative Properties are:
• Vapor pressure lowering
• Boiling point elevation
• Freezing Point depression
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Vapor Pressure
Vapor Pressure Reduction
• Molecules of nonvolite solute take up space, thus
preventing some solvent molecules to vaporize.
• Condensation continues at the same rate, but
vaporization slows down.
• Since condensation > vaporization, vapor pressure is
reduced.
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Ionic vs Molecular Solutes
• Ionic solutes produce two or more ion
particles in solution.
• They affect the colligative properties
proportionately more than molecular
solutes (that do not ionize).
• The effect is proportional to the number of
particles of the solute in the solution.
How many particles do each of the
following give upon solvation?
• NaCl
• CaCl2
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Freezing Point Depression
A disruption of crystal formation
Example
• Salt is added to melt ice by reducing the
freezing point of water.
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Boiling Point Elevation
Boiling Point Elevation (∆Tb)
• Adding a nonvolatile solute, reduces
vapor pressure, more energy is
needed to make the solutions boil,
raising the boiling point.
• Boiling point elevation = BP of the
solution - BP of the solvent.
• Directly proportional to the number of
solute molecules.
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Example
• Addition of ethylene glycol C2H6O2
(antifreeze) to car radiators.
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Freezing Point Depression and
Boiling Point Elevation
Boiling Point Elevation
• ∆Tb =mkb
(for water kb=0.51 oC/m)
• Freezing Point Depression
• ∆Tf=mkf
(for water kf=1.86 oC/m)
• Note: m is the molality of the particles,
so if the solute is ionic, multiply by the #of
particles it dissociates to.
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