Transcript 8.3 Colligative Properties Objective S3

8.3 Colligative Properties
Objective S3
Chemistry
Essential Questions
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What are the colligative properties?
How does adding a solute affect them?
How do you find the new boiling and freezing
points after the addition of the solute?
Colligative Properties
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Physical properties that are affected by the
concentration of the solute in the solution
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Vapor Pressure Reduction
Boiling Point Elevation
Freezing point depression
Vapor Pressure Reduction
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In a liquid, molecules are constantly moving
from the liquid phase to the gas phase and
back again
Vapor pressure is the pressure of the gas
“pushing” on the liquid
Vapor Pressure Reduction (cont)
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By adding a solute, it prevents some of the
molecules escaping into the gas phase
Because there are fewer molecules in the gas
phase, the pressure is reduced
Vapor Pressure Reduction (cont)
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The decrease in a solution’s vapor pressure
is proportional to the number of particles the
solute makes in solution.
Which would have the lowest pressure?
Boiling Point Elevation
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A liquid boils when its vapor pressure is equal to the
external pressure because at this point the
molecules can escape into the gas phase
Adding a solute reduces vapor pressure and
increases the energy needed for the vapor pressure
to equal the external pressure
The increase in energy results in the boiling point
increasing which is called Boiling Point Elevation
Boiling Point Elevation (cont)
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Boiling point elevation (ΔTb) is proportional to
the molality of the solution
ΔTb=Kbm
Where:
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Kb is the molal boiling point constant (every
solvent has a different Kb value)
m is the molality of the solution
ΔTb is the elevation of the boiling point
For soluble ionic compounds m is multiplied
by the number of ions formed
Boiling Point Elevation (cont)
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Water with sugar added will boil at a higher
temperature than pure water. By how much will the
boiling point of water be elevated if 100g of sucrose
(C12H22O11) is added to 500 g of water? The Kb for
water is 0.52 °C/m.
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First convert solute from grams to moles
100 g C12H22O11/342.34 g= .292 mol C12H22O11
m= .292 mol / .5kg
= .584m
Then use the formula to solve for the change in temp.
ΔTb=Kbm
ΔTb= (.52 °C/m)(.584m)
ΔTb= 0.3 C
Boiling Point Elevation (cont)
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Using Table 16.3 on p 495
Find the boiling point for 1.50 mol NaCl in
6325 g CHCl3. Kb = 3.63 °C/m and boiling
point = 61.2 °C
Find the boiling point for 32 g NH3 in 4353 g
H 2O
Find the boiling point 56.5 g SrBr2 in 542 g
C6H6. Kb = 2.53 °C/m and boiling point = 80.1
°C
What happens?
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What happens to ice when you add salt?
The ice melts
Why do you think it melts?
Freezing Point Depression
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The freezing point of a liquid is when the
vapor pressure of the liquid and the solid are
equal to each other
Adding a solute reduces the vapor pressure
of the liquid which lowers the temperature at
which the solid and liquid pressures are equal
This reduction in temperature causes a lower
freezing point called Freezing Point
Depression
Freezing Point Depression (cont)
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Freezing Point depression (ΔTf) is
proportional to the molality of the solution
ΔTf=Kfm
Where:
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Kf is the molal freezing point constant (every
solvent has a different Kf value)
m is the molality of the solution
ΔTf is the depression of the freezing point
For soluble ionic compounds m is multiplied
by the number of ions formed
Freezing Point Depression (cont)
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Just like it’s name sounds, adding antifreeze protects a car
from freezing. Calculate the freezing point depression of a
solution of 100g of antifreeze, ethylene glycol (C2H6O2), in
0.5 kg of water. Kf for water is 1.86 °C/m.
 Convert solute from grams to moles and calculate
molality
100g C2H6O2= 1.61 mol
m=(1.61 mol)/(.5kg )=3.22m
 Substitute into formula
ΔTf=Kfm
ΔTf=(1.86 °C/m)(3.22m) = 6.01°C
Freezing Point Depression (cont)
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Using Table 16.2 on p 494
Find the freezing point for 7.50 mol LiOH in
6325 g C6H6. Kf = 5.12 °C/m and freezing
point = 5.5 °C
Find the freezing point for 32 g AsO3 in 4353
g H2O
Find the freezing point 56.5 g SrBr2 in 542 g
C2H5OH. Kf = 1.99 °C/m and freezing point =
-114.6 °C
Essential Questions
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What are the colligative properties?
How does adding a solute affect them?
How do you find the new boiling and freezing
points after the addition of the solute?
8.3 Tracked Assignment
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P 490 #25-27
P 496 #38, 41
P 499 #56, 59, 63-64, 67, 76