5/21/2012
Born-Haber Cycle: ΔHhydration
• ΔHsolution,NaCl = ΔHhydration,NaCl(aq) – UNaCl
• ΔHhydration,NaCl(aq) = ΔHhydration,Na+(g) + ΔHhydration,Cl−(g)
Enthalpies of Hydration
1
5/21/2012
Sample Exercise 11.3
Use the appropriate enthalpy of hydration values in
Table 11.3 and ΔHsolution = 0.91 kJ/mol for NaF to
calculate the lattice energy of NaF.
Vapor Pressure
Pressure exerted by a gas in equilibrium with its liquid
Where are we going with this?
Colligative Properties – boiling
point elevation, freezing point
depression, osmosis
rate of evaporation = rate condensation
2
5/21/2012
The tendency for a liquid to evaporate
increases as 1. the temperature rises
2. the surface area
increases
3. the intermolecular
forces decrease
Vapor Pressure
• In general, vapor pressure of a solution is lower
than vapor pressure of pure solvent.
Rate of evaporation < rate of condensation
3
5/21/2012
Vapor Pressure of Solutions
Raoult’s Law:
vapor pressure of solution is proportional to mole
fraction of solvent (non-volatile solute).
Psolution = XsolventPsolvent°
Vapor pressure lowering:
one of the colligative properties of solutions
Ideal Solutions:
solutions that obey Raoult’s Law.
Sample Exercise 11.4
The liquid used in automobile cooling systems is prepared by
dissolving ethylene glycol (HOCH2CH2OH, molar mass 62.07 g/mol)
in water. What is the vapor pressure of a solution prepared by
mixing 1.000 L of ethylene glycol (density 1.114 g/mL) with 1.000 L
of water (density 1.000 g/mL) at 100.0°C? Assume that the
mixture obeys Raoult’s law.
4
5/21/2012
Solutions of Volatile Components
• For mixtures containing more than one
volatile component:
– Partial pressure of each volatile component
contributes to total vapor pressure of solution.
– Ptotal = X1P1° + X2P2° + X3P3° …
– Where Xi = mole fraction of component i, and Pi°
= equilibrium vapor pressure of pure volatile
component at a given temperature.
Real vs Ideal Solutions
 Deviations from Raoult’s Law occur due to differences in
solute-solvent and solvent-solvent interactions (dashed
lines = ideal behavior).
Negative deviations:
Solute-solvent > solvent-solvent.
Positive deviations:
Solute-solvent < solvent-solvent.
5
5/21/2012
Sample Exercise 11.7
Calculate the vapor pressure of a solution prepared by dissolving 13
g of n-heptane (C7H16) in 87 g of n-octane (C8H18) at 25°C. By what
factor does the concentration of the more volatile component in
the vapor exceed the concentration of this component in the
liquid? The vapor pressures of n-octane and n-heptane at 25°C are
11 torr and 31 torr, respectively.
Vapor Pressure vs Temperature
(Increasing attractive forces.)
Normal boiling point: the temperature at which the vapor
pressure of a liquid equals 1 atm (760 mmHg).
6
5/21/2012
Diethyl ether - dipole-dipole
interactions
Water - hydrogen
bonding (stronger)
Boiling Point
“If you have a beaker of water open to the
atmosphere, the mass of the atmosphere is
pressing down on the surface. As heat is
added, more and more water evaporates,
pushing the molecules of the atmosphere
aside. If enough heat is added, a
temperature is eventually reached at which
the vapor pressure of the liquid equals the
atmospheric pressure, and the liquid boils.”
7
5/21/2012
Normal boiling point - the temperature at
which the vapor pressure of a liquid is equal to the
external atmospheric pressure of 1 atm.
Increasing the external atmospheric pressure
increases the boiling point
Decreasing the external atmospheric pressure
decreases the boiling point
Location
San Francisco
Salt Lake City
Denver
Mt. Everest
Elevation (ft)
sea level
4400
5280
29,028
Boiling Point H2O (oC)
100.0
95.6
95.0
76.5
8
5/21/2012
Clausius-Clapeyron Equation
Vapor Pressure vs Temperature
ln Pvap   
H vap  1 
   C
R
T 
Pressure 
liquid
solid
gas
Temperature 
Plot of ln(P) vs 1/T yields
straight line:
– Slope = −ΔHvap/R
– Intercept = constant
9
5/21/2012
Practice Exercise, p. 513
n-Pentane, C5H12, boils at 36 oC. What is its molar
heat of vaporization in kJ/mol if its vapor pressure
at 25 oC is 505 torr?
10