Learning Target: Refine the design of a chemical
system by specifying a change in conditions that would
produce increased amounts of products at equilibrium.
Agenda:
•Must Do
•WWD
•Demo
•Properties of solutions
•Colligative properties lab (freezing
point depression)
Must Do:
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Polar water molecules.
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3
The heating/cooling curve for water heated or cooled at
a constant rate.
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Heats of Phase Changes
Molar heat of vaporization, DHvap: the heat needed to
vapourize one mole of a substance at its normal boiling
point.
Molar heat of fusion, DHfus: the heat needed to melt one
mole of a substance at its normal melting point.
Molar heat of sublimation, DHsub: the heat needed to
sublime one mole of a substance from the solid phase to
the gas phase (skips the liquid phase).
DHsub DHvap + DHfus
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Temperature and Phase
Changes
As a phase change occurs, temperature remains constant
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Both liquid water and gaseous
water contain H2O molecules.
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7
Microscopic view of a liquid near
its surface.
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8
Figure 14.10: Behavior of a
liquid in a closed container.
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9
shows a higher vapor pressure
than water.
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Vapour Pressure
Vapour pressure (pvap): the pressure at which dynamic
equilibrium is achieved in a closed container.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Temperature and Phase
Changes
As a phase change occurs, temperature remains constant
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Bubble expands as H2O
molecules enter.
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13
The packing of Cl¯ and Na+ ions in solid sodium
chloride.
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14
Dissolving of solid sodium
chloride.
15
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Mifflin Company. All rights
reserved.
Polar water molecules interacting with positive and
negative ions of a salt.
16
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Mifflin Company. All rights
reserved.
9.4 Colligative Properties
The presence of a solute affects some physical properties of the
solvent.
The physical properties are referred to as colligative properties.
Vapour Pressure (reduced)
Freezing Point (reduced)
Boiling Point (increased)
Osmotic Pressure (increased)
The magnitude of the colligative property depends only on the
concentration of the solute, not its identity.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Vapor Pressure Reduction
A pure solvent in a closed
system will reach a dynamic
equilibrium between the
liquid and vapour phases.
The addition of a solute will
decrease the vapour
pressure because the solute
molecules reduce the rate of
escape of solvent molecules.
Raoult’s Law quantifies this:
p vap, solution = X A p vap, A
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Boiling and Freezing Points
Boiling-point elevation – the increase in boiling point of
a solvent by adding a solute.
Freezing-point depression – the lowering of the
freezing-point of a solvent by adding solute.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Freezing Point Depression
DTf = iK f b
where b is the molality (moles of solute/kg of solvent)
Kf is the cryoscopic constant of the solvent
i is the van’t Hoff factor:
e.g. i = 3 for Na2SO4 because Na2SO4 → 2 Na+ + SO42-
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Molality
Useful for applications where the temperature (thus
volume) of a solution changes.
Defined as the number of moles of solute divided by the
mass of the solvent in kg
nsolute
cm =
msolvent
Units are mol/kg
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Boiling Point Elevation
D Tb = iKbb
Kf is the ebullioscopic constant of the solvent.
Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.
Freezing Point Depression
Calculate ΔT (freezing point depression) if 100g NaCl is
dissolved in 1kg of water.
ΔT = i Kf m
Kf water 1.86
Boiling Point Elevation
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A solution will boil at a higher temperature than the pure solvent. This is the colligative property called boiling point elevation.
The more solute dissolved, the greater the effect. An equation has been developed for this behavior. It is:
ΔT = i Kb m
The temperature change from the pure to the solution is equal to two constants times the molality of the solution. The constant Kbis
actually derived from several other constants and its derivation is covered in textbooks of introductory thermodynamics. Its technical
name is the ebullioscopic constant. The Latin prefix ebulli- means "to bubble" or "to boil." In a more generic way, it is called the "molal
boiling point elevation constant." The constant i will be discussed below.
These are some sample ebullioscopic constants:
Substance Kb benzene 2.53 camphor 5.95 carbon tetrachloride 5.03 ethyl ether 2.02 water 0.52 The units on the constant are degrees
Celsius per molal (°C m¯1). There are some variations on the theme you should also know:
1) K m¯1 - the distance between a single Celsius degree and a Kelvin are the same.
2) °C kg mol¯1 - this one takes molal (mol/kg) and brings the kg (which is in the denominator of the denominator) and brings it to the
numerator. This last one is very useful because it splits out the mol unit. We will be using this equation (or the freezing point) to
calculate molecular weights. Keep in mind that the molecular weight unit is grams / mol. Another reminder: molal is moles solute over
kg solvent.
