Lecture 44 Intermolecular Forces, Liquids II

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Lecture 44 - Intermolecular Forces, Liquids II
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In Summary...

• ion - ion
• ion - dipole
• dipole - dipole
• H-bond
• ion - induced dipole
• dipole - induced dipole
• dispersion

stronger forces
van der Waals forces
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for example...

• Which has the higher boiling temperature?
• MgCl2(l) or PCl3(l)

polar covalent bonds
ionic bonds
1412oC
76oC
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for example...

• Which has the higher boiling temperature?
• CH3NH2 or CH3F

polar covalent bonds
polar covalent bonds
can form H-bonds!
no H-bonds
-6oC
-78oC
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for example...

• Which has the higher boiling temperature?
• CH3OH or CH3CH2OH

larger, more polarizable
78oC
66oC
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Properties of Liquids

• Surface Tension
• Capillarity
• Viscosity
• Vapor Pressure

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Surface Tension
interior of liquid
surface of liquidhigher energy
low energy
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Surface Tension

• expanding a surface requires energy
• Surface
• Tension, J/m2
• CH3(CH2)2CH3(l) 0.0184 (dispersion)
• H2O(l) 0.0728 (H-bonds)NaCl(l)
  0.100 (ionic bonds)
• Hg(l) 0.472 (metallic bonds)

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Why Wax Your Car?
Water wets the surface
Water
Paint
Metal
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Why Wax Your Car?
Water Dirt (rolls off)
Wax
Paint
Metal
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Capillarity
Si
-
-
O
O
strong
stronger !
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Capillarity
Air
SiO2(s)
Water wets SiO2
H2O(l)
H2O(l)
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Capillarity
Si
-
-
O
O
Hg Hg
Hg
strong
weaker !
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Capillarity
Air
SiO2(s)
Hg does not wet SiO2
Hg(l)
Hg(l)
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Viscosity
Intermolecular forces hinder flow !
Flow of Liquid
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Viscosity vs. Temperature

• Temp., oC Viscosity of
• water, N s m-2
• 20 0.001
• 40 0.00065
• 60 0.00047
• 80 0.00035

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Motor Oil
heat
ordinarily, oil
lower viscosity
heat
polymeric viscosity index improver
oil molecule
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Maple Syrup


sugar
sugar
sugar
Heat
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Changes of State

• States S, L, G

DHgt0, DSgt0
vaporization
L
G
condensation
DHlt0, DSlt0
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for example,

• H2O(l) ? H2O(g) DHvap 44.0 kJ/mol
• (at 25oC)
• Get out of the pool - you feel cool!
• H2O(g) ? H2O(l) DHcond - 44.0 kJ/mol
• (at 25oC)

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Vaporization
Liquid phase
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Vaporization
energy diff DHvap
Liquid phase
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DHvap and Intermolecular Forces

• Liquid Dominant DHvap,kJ/mol
• Force
• CS2 dispersion 27.4
• H2O H-bond 44.0
• NaCl ionic 787 (U)

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Vapor Pressure
Temp T
at equilibrium, rate of evaporation rate of
condensation
p
G
p vapor pressure of liquid, pvap (at T)
L
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Vapor Pressure
normal boiling point
1 atm
pvap
Tb
T
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Vapor Pressure
(Clausius-Clapeyron equation)
-DHvap
1
ln(pvap)
C
R
T
m x b
y
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Vapor Pressure
-DHvap
slope
R
ln(pvap)
1 / T