Chem 106
Tues, 2-1-2011
Liquids Section 12.4
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12.4 Summary of Intermolecular Forces
Type of Species
Ion
Polar
Nonpolar
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Type of Force
Ion force
Dipole Force
Induced Dipole Force
2
Which is elemental Iodine is more soluble in,
water or dichloromethane (CH2Cl2)?
1. water
2. dichloromethane
52%
di
ch
lo
ro
m
w
et
ha
n
...
at
er
48%
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3
Overhead demo – top-down view of a beaker with ~1/2 in of water
Added several mL
of dichloromethane.
The low-polarity
CH2Cl2 is not
miscible with water.
Moved the
dichloromethane
bubble over near the I2.
I2 begins to dissolve in
dichloromethane.
Added several crystals of iodine. I2 is a
dark purple crystalline solid. Very little,
if any, solubility in water.
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Which is elemental Iodine is more soluble in,
water or dichloromethane (CH2Cl2)?
1. water
2. dichloromethane
98%
di
ch
lo
ro
m
w
et
ha
n
...
at
er
2%
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Dielectric constant is another measure of polarity. The value is
proportional to that solvent’s ability to stabilize dissolved ions.
Dipole moment
(D)
Dielectric
constant
Density (g/mL)
water
1.85
80.0
1.00
dichloromethane
1.60
9.1
1.33
Iodine (I2)
0.00
-
-
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Liquids
Section 12.4
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In a liquid
• molecules are in constant
motion
• IMF are moderate
• molecules close together
• Liquids are almost
incompressible
• Liquids do not fill the
container
7
Evaporation
Molecules must have sufficient energy to break IMF. This requires
energy, process is endothermic.
Molar Enthalpy of Vaporization, DvapHo (kJ/mol)
LIQUID
break IMF bonds
Add energy,
DvapHo
VAPOR
make IMF bonds
Remove energy
<---condensation
Molecules lose energy, reform IMF, requires no energy, process is exothermic
Molar Enthalpy of Condensation = - Molar Enthalpy of Vaporization
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Liquids—Distribution of Energies
Number of molecules
lower T
higher T
See Figure 12.13
0
Energy is proportional to T
At higher T’s, larger number
of molecules with enough
energy to break IMF go
from liquid -> vapor.
Molecular energy
Minimum energy required to break
IMF and evaporate
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During evaporation, high energy molecules
carry away energy.
(So you cool down when sweating or after
swimming.)
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Heat of Vaporization ∆vapH
= heat required (kJ/mol) to vaporize the liquid at
constant P.
LIQUID + heat ---> VAPOR
Compd.
H2O
SO2
Xe
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∆vapH (kJ/mol)
40.7
26.8
12.6
BP (oC)
100
-47
-107
∆vapH
IM Force
H-bonds
dipole
induced dipole
11
Liquids
Molecules of liquid are in the vapor state,
they exert a VAPOR PRESSURE
EQUILIBRIUM VAPOR PRESSURE
is the pressure exerted by a vapor
over a liquid in a closed container
rate of evaporation = the rate of condensation
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Equilibrium Vapor Pressure
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Equilibrium Vapor Pressure
17,200 ft High Camp - Denali
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Pressure cookers are used in Himalaya and other ranges
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Clausius-Clapeyron Equation
ln P  5
Water
P  eln P  e5
 148.4 mmHg
Natural log of vapor pressure P is
proportional to 1/T (K-1).
-∆vapH˚/R is SLOPE
ln P = –∆vapH˚/R(1/T) + C
y =
1
 333.3 K
0.0030 K 1
o
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mx
+ b
This is a mathematical description of the
vapor pressure–temperature curve.
Originally, the VP-T data was replotted
different ways – this one is LINEAR,
and the slope value is significant.
C  K  273.15  333.3  273.15  60. oC
15
Equilibrium Vapor Pressure
Same point as
on the lnP vs
1/T plot!
~148 mmHg
60 oC
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Can remove the
troublesome “C” by
taking values at 2
temps, and subtracting.
 D vap H o  1 
   C
ln P1 
R
 T1 
 D vap H o  1 
   C
ln P2 
R
 T2 
Subtract two C-C
equations, one at P1,T1
and other at P2,T2.
  D vap H o  1 

 D vap H o  1 
   C  
   C 
ln P2  ln P1 
R
T
R
 2
 T1 


 D vap H o  1    D vap H o  1 
   
 
ln P2  ln P1 
R
R
 T2  
 T1 
 D vap H o  1 1 
  
ln P2  ln P1 
R
 T2 T1 
o
P2  D vap H  1 1 
  
ln 
P1
R
 T2 T1 
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DHv ap  1
P2
1 
ln
=


P1
R
T
T
 1
2 
Can use this in different ways:
- Given (P1,T1) and (P2,T2), we can calculate ΔvapHo
- Or, look up ΔvapHo in a table. If you know three of four
variables P1,T1, P2,T2 , you can calculate the fourth.
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Section 12.4g Vapor Pressure Calculations (this is like the one I did on the
board, but in this one, the temperature and vapor pressure increase.)
The vapor pressure of liquid cadmium is 400 mm Hg at 983 K. Assuming that
its molar heat of vaporization is constant at 99.0 kJ/mol, the vapor pressure of
liquid Cd is______________ mm Hg at a temperature of 1.02E+3 K.
P1  400 mmHg
T1  983 K
P2  ?
T2  1020 K
D vap H o  99.0 kJ / mol
R  0.0083144 kJ / mol  K
o
P2  D vap H  1 1 
  
ln 
P1
R
 T2 T1 
 D vap H o  1 1 
  
ln P2  ln P1 
R
 T2 T1 
 D vap H o  1 1 
    ln P1
ln P2 
R
 T2 T1 
Isolate P2 variable, by first
taking ln of P2/P1.
Now add lnP2 to both sides.
 99.0 kJ
 1
1 
mol

  ln 400
ln P2 

 0.0083144 kJ   1020 K 983 K 


 mol  K

ln P2 
 99.0 K

 3.6902 x 105 K 1   ln 400
0.0083144
ln P2  0.43939  5.9914  6.4309
P2  e6.4309  620.7 mmHg
(Hint: Always check to make sure the vapor pressure changes in the
correct direction! If the liquid warms up, P should INCREASE.
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Boiling Liquids
Liquid boils when its vapor pressure = ospheric pressure.
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Liquids: VP versus T
If external P = 760 mm Hg, T of boiling is the Normal Boiling
Point
VP of a given molecule at a given T depends on IMF.
Here the VP’s are in the order
BP:
37 °C
ether
O
C2H5
H5C2
dipoledipole
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75 °C
alcohol
O
H5C2
H
H-bonds
100 °C
water
O
H
H
extensive
H-bonds
increasing strength of IMF interactions
22
Critical T and P
Above Critical Temp,
no liquid exists
no matter how high
the pressure
water
Fig. 12-20, p. 577
Table 12-7, p. 577
Helium
Hydrogen
Neon
Nitrogen
Argon
Oxygen
Krypton
Xenon
CO2
Ammonia
Chlorine
Water
H2SO4
Mercury
Gold
Sulfur
Lithium
Iron
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Tc degC
−267 °C
−239 °C
−228 °C
−146 °C
−122 °C
−118 °C
−63 °C
16 °C
31 °C
132 °C
143 °C
373 °C
654 °C
1,476 °C
6,977 °C
1,040 °C
2,950 °C
8,227 °C
Pc (atm)
2.24
12.8
27.2
33.5
48.1
49.8
54.3
57.6
72.8
111.3
76.0
217.7
45.4
1,720.
5,000 .
25
Liquid Surface Tension
Molecules at surface behave differently than those in the interior.
Molecules at surface experience net INWARD FORCE of
attraction.
Results in SURFACE TENSION — the energy required to break
the surface.
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Liquids
IMF also lead to CAPILLARY action and to the existence
of a concave meniscus for a water column.
concave
meniscus
H2 O in
glass
tube
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Stronger ADHESIVE FORCES
between water
and glass
COHESIVE FORCES
between water
molecules
27
Liquids
IMF also leads existence of a convex meniscus for
a mercury column.
CONVEX
meniscus
Mercury
in tube
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Weak ADHESIVE FORCES
between mercury
and glass
COHESIVE FORCES
between mercury atoms
molecules
28