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Tro Chpt. 11
Tro 11.2
Liquids, solids and intermolecular forces
•  Solids, liquids and gases - A Molecular Comparison
•  Intermolecular forces
•  Intermolecular forces in action: surface tension, viscosity and capillary action
•  Vaporization and vapor pressure
•  Sublimation and fusion
•  Quantitative aspects of phase changes
•  Phase Diagrams
•  Skip sections 11.10-11.13
Tro 11.3
INTERMOLECULAR vs. INTRAMOLECULAR INTERACTIONS
TYPES OF INTERMOLECULAR
INTERACTIONS
C + 4H
4 mol C-H bonds x 414 kJmol-1
or 1656 kJ per mol of methane!
8.9 kJ
+
Non-covalent, intermolecular interactions between
covalent molecules
(1) London (dispersion) forces
(2) Dipole-dipole forces
(3) Hydrogen bonding
1 joule:
* the energy required to lift a small apple (102 g) one meter against Earth's gravity.
* one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer.
* The amount of energy released if a can of beer is dropped from wait height
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ION–ION INTERACTION
+
Eatt ∝
–
ION–DIPOLE INTERACTION
+
d
d δ–
electrostatic interaction
Q1Q2
d
Q is charge
Eatt ∝
Qµ
d2
Opposite charges attract
δ-
Eatt ∝
µ4
d6
δ–
δ+
δ–
δ+
repulsion
net effect, averaged
over time
More polar molecules have
larger attractive force
EXAMPLE:
H
Cl
H
ions in solution
181 pm
DIPOLE–DIPOLE INTERACTION
δ–
δ+
attraction
:O
Cl–
102 pm
Q charge
µ dipole moment
:
Na+
neutral polar
molecule with
dipole moment
H
Na+
EXAMPLE:
δ+
δ+
Cl
H
DISPERSION INTERACTION
LONDON INTERACTION
• Nonpolar molecules will liquify, so there must be attractive
intermolecular forces
• Electrons are always moving in molecules
• At some times, there is an instantaneous dipole moment
• When this occurs, a dipole is induced in the adjacent atom and an
attractive force is the result
Eatt ∝
1
d6
• Highly polarizable molecules are more subject to dispersion forces
• LDF increases with molecular size
• Molecular shape is also involved
• LDFs occur for all molecules
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DISPERSION INTERACTION
SIZE
Ar
Ar
H
Cl
Cl
H
Ar
Molar Mass and Boiling Point
LDF only intermolecular force for noble gases
LDF in addition to dipole-dipole
BP = – 185.7 °C = 87.5 K
Xe BP = – 107.1 °C = 166.1 K
atomic size related to polarizability
Dipole Moment and Boiling Point
Molecular Shape and Boiling Point
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Practice – Choose the Substance in Each Pair with
the Highest Boiling Point
a) 
CH4
b.p. = -164 ˚C
b) 
CH3CH2CH2CH3
a) 
CH2FCH2F
CH3CHF2
b.p. = 30.7 ˚C
b.p. = -0.5 ˚C
CH3CH2CH=CHCH2CH3
Practice – Choose the Substance in Each Pair with
the Highest Boiling Point
cyclohexane
b.p. = -24.9 ˚C
b)
or
b.p.
trans: 47.5 °C
cis: 60.3 °C
b.p. = 66.4 ˚C
b.p. = 80.7 ˚C
• A ‘H’ bonded to a very electronegative atom (O, N, F)
can interact with lone pair electrons on (O, N, F) on
another molecule
• H-bonds are directional
• H-bond strength related to dipole moment of bond
• Properties of H2O are related to H-bonding in liquid
and solid
• H-bond energies usually 4-25 kJ/mol
• H bonds weak compared to covalent bonds
• H bonds strong compared to intermolecular forces
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Practice – Choose the substance in each pair that is a
liquid at room temperature (the other is a gas)
a)  CH3OH
CH3CHF2
b)  CH3-O-CH2CH3 CH3CH2CH2NH2 Practice – Choose the substance in each pair
that is more soluble in water
a)  CH3OH
CH3CHF2
b)  CH3CH2CH2CH3 CH3NH2 5
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Tro 11.4
Surface tension:
Molecules at the
liquid surface
have a higher
potential energy
than those in the
interior. As a
result, liquids
tend to minimize
their surface area
and the surface
behaves like a
membrane or
“skin”. Surface tension allows a paper click to float on water!
Viscosity
Capillary Action
•  Viscosity: the resistance of a liquid to flow
–  1 poise = 1 P = 1 g/cm·s
–  often given in centipoise, cP
•  larger intermolecular attractions = larger viscosity
•  higher temperature = lower viscosity
•  the adhesive forces pull the surface liquid up the side of the tube,
while the cohesive forces pull the interior liquid with it
•  the liquid rises up the tube until the force of gravity counteracts
the capillary action forces
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Tro 11.5
Equilibrium nature of phase changes
Water (dyed red) and mercury in a glass test tube
Dynamic Equilibrium
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Vapor Pressure
Vapor Pressure as a function of temperature and intermolecular forces
Clausius-Clapeyron Equation
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Clausius-Clapeyron Equation
Boiling Point
Clausius-Clapeyron plot for diethyl ether
Determining vapor pressure under different conditions
The vapor pressure of ethanol is 115 torr at 34.9 ˚C. If ΔHvap 40.5
kJmol-1, calculate the temperature (in ˚C) when the vapor pressure
is 760 torr.
T2 = 350 K or 77˚C
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Tro 11.6 Phase Changes - Sublimation and Deposition
•  molecules in the solid have thermal energy that allows them to vibrate
•  surface molecules with sufficient energy may break free from the surface and
become a gas – this process is called sublimation
•  the capturing of vapor molecules into a solid is called deposition
•  the solid and vapor phases exist in dynamic equilibrium in a closed container
–  at temperatures below the melting point
–  therefore, molecular solids have a vapor pressure
solid
sublimation
deposition
Melting = Fusion
gas
Quantitative aspects of phase changes:
Tro 11.7
How much energy is released when 2.5 mol of water vapor at 130˚C is cooled to
ice at -40˚?
Stage 1: H2O (g) [130 ˚C] H2O (g) [100˚C]
q = n x Cwater(g) x ΔT
q = (2.5 moles) x (33.1 Jmol-1˚C-1) x (100-130˚C)
q = -2482 J
Stage 2: H2O (g) [100 ˚C] H2O (l) [100˚C]
q = n x (-ΔHvap)
q = 2.5 x (-Δ40.7 kJmol-1)
q = -102 kJ 10
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Stage 3: H2O (l) [100 ˚C] H2O (l) [0˚C]
q = n x Cwater(l) x ΔT
q = (2.5 moles) x (75.4 Jmol-1˚C-1) x (0-100˚C)
q = -18850 J
Stage 4: H2O (l) [0 ˚C]
H2O (s) [0˚C]
kJmol-1)
q = -15.0 kJ Stage 5: H2O (s) [0 ˚C]
–  and the density of the vapor to increase
–  and the density of the liquid to decrease
•  at some temperature, the meniscus between the liquid and vapor
disappears and the states commingle to form a supercritical
fluid
•  supercritical fluid have properties of both gas and liquid states
q = n x (-ΔHfus)
q = 2.5 x (-Δ6.02
Wicked Weird Phases - Supercritical Fluid
•  as a liquid is heated in a sealed container, more vapor collects
causing the pressure inside the container to rise
H2O (s) [-40˚C]
q = n x Cwater(s) x ΔT
q = (2.5 moles) x (37.6 Jmol-1˚C-1) x (-40 - 0˚C)
q = -3760 J
Using Hess’s law, sum of q for all 5 stages = -142 kJ
Tro 11.8
Typical phase diagram for most substances
Supercritical n-pentane
For most substances, increasing pressure will solidify a liquid (i.e. the
solid is more dense than the liquid). The opposite is true for water
(note the negative slope for the solid-liquid line)
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Information extracted from phase diagrams
Starting at the triple point, what phase exists when the pressure is held constant
and the temperature is increased to 0.5˚C
gas!
Starting at the triple point, what phase exists when the temperature is held
constant and the pressure is increased to to 20 mm Hg?
liquid!
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