Colligative Properties

1
Colligative Properties
2
Colligative Properties.

• Are properties that depend on the number of
  dissolved particles only.
• The type of dissolved particles does not matter

3
Freezing Point

• The temperature at which a solution freezes from
  a liquid to a solid
• Freezing point of pure water is 0o C
• Adding a solute to pure water lowers the freezing
  point below zero

4
Freezing Point is lowered when

• Salt is added to roads in winter, so they dont
  get icy even if the temp is below zero
• Antifreeze is added to engine fluids so they
  dont freeze inside your engine and your car
  still starts when the temp is below zero

5
Take 1000g of pure water

• And add one mole of dissolved particles, and you
  will lower the freezing point by 1.86 degrees C.
• Remember it does not matter what the dissolved
  particles are, only how many there are!!!

6
Boiling Point

• The temperature at which a solution boils from a
  liquid to a gas
• Boiling point of pure water is 100o C
• Adding a solute to pure water raises the boiling
  point above 100 degrees C

7
Boiling Point is raised when

• Salt is added boiling water to cook pasta faster
  (at a higher temperature) so the pasta does not
  get soggy. Instead it comes out al dente
• Antifreeze is added to engine fluids so they
  dont boil inside your engine and your car wont
  overheat when temp is above 100 degrees C.

8
Take 1000g of pure water

• And add one mole of dissolved particles, and you
  will raise the boiling point by 0.52 degrees C.
• Remember it does not matter what the dissolved
  particles are, only how many there are!!!

9
Some solutes are more effective than others at
raising and lowering boiling and freezing points

• I mole of sugar (covalent compound) dissolves in
  water to form 1 mole of dissolved sugar
  C6H1206(aq)
• 1 mole of NaCl (ionic compound) will dissolve to
  form 2 moles of dissolved particles 1 mole of
  Na (aq) and I mole of Cl- (aq)

10
Some solutes are more effective than others at
raising and lowering boiling and freezing points

• 1 mole of CaCl2 (ionic compound) will dissolve to
  form 3 moles of dissolved particles 1 mole of
  Ca (aq) and 2 moles of Cl- (aq)
• In general, ionic compounds form more dissolved
  particles, so they are a better choice for adding
  to the water to change boiling or freezing points.

11
Vapor Pressure Table H

• some molecules at the surface of a liquid
• may have enough energy to evaporate even
• if the liquid is below its boiling point. These
• escaping gas particles exert a pressure called
  VAPOR PRESSURE.

12
Vapor Pressure

• The more easily the molecules evaporate, the
  higher the vapor pressure
• The weaker the intermolecular forces, the more
  easily the molecules can separate and go into the
  gas phase, therefore the higher the vapor pressure

13
Vapor Pressure

• The more easily the molecules evaporate, the
  higher the vapor pressure
• The stronger the intermolecular forces, the less
  easily the molecules can separate and go into the
  gas phase, therefore the lower the vapor pressure

14
Using Table H

• Which liquid has the highest vapor pressure at 50
  degrees?
• The lowest?
• Which liquid has the weakest intermolecular
  forces?
• The strongest?

15
Vapor pressure

• The boiling point of a liquid is defines as the
  temperature at which the vapor pressure of the
  liquid equals atmospheric pressure (1 atm or
  101.3 kPa)
• In other words, at that temp all the molecules
  change to gas phase (boil), not just the
  molecules at the surface