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Chapter 11
Intermolecular Forces
Covering pages 462 – 472, 490
Intermolecular Forces
• Forces between molecules
• Weaker than covalent bonds within a molecule
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Dipole-Dipole Forces
• Forces between polar molecules
• Opposite ends of polar molecules attract each other
• The least influential of the three types of IM forces
Dispersion Forces
• Due to temporary distortion of charge clouds
• Consider two Xe atoms
– Temporarily, a cloud may have more e- on one side
– Distortion induces similar distortion in neighboring atom
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Dispersion Forces
• Temporary distortions create temporary dipoles
• Temporary dipoles attract each other
– Forces of attraction = Dispersion forces
Dispersion Forces
• Magnitude of dispersion forces depend on flexibility
of charge cloud
– Easily distorted means high dispersion forces
• In general, this depends on number of e- in molecule
– More e- mean larger charge cloud in which outer e- are far
from nuclei
– Larger charge cloud has e- that are not held as tightly
• In general, higher molecular mass means stronger
dispersion forces
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Dispersion Forces
• All molecules have dispersion forces
• Nonpolar molecules: Dispersion forces only IM force
• Polar molecules:
– Dipole-dipole forces most important for small molecules
– Dispersion forces frequently comparable in size or greater
for large molecules
• Rough rule of thumb: “Large” means molecular mass > 100 amu
Hydrogen Bonding
• Particularly strong dipole-dipole forces
• Involves three most electronegative elements
– F, N, O
• One molecule must contain H-F, H-O, or H-N bonds
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Hydrogen Bonding
• Both molecules don’t need F-H, O-H, N-H bonds
IM Forces: Summary
• Nonpolar molecules
– Dispersion forces only possible IM force
– Dispersion forces stronger for larger molecules
• Polar molecules
– Both dispersion and dipole-dipole IM forces
– Dipole-dipole generally only important for small molecules
• Molecules containing F-H, O-H, or N-H bonds
– Both dispersion forces and hydrogen bonding
• Dispersion usually dominant IM force if molar mass difference > 100
– Hydrogen bonding frequently important biologically
• NOT important for ionic compounds
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Physical Properties and IM Forces
• Molecules with strong IM forces have tight hold on
each other in liquids and solids
– Difficult for molecules to leave liquid or solid
– Difficult for molecules to move around within liquid
– Difficult for objects to penetrate liquid surface
• Molecules with different IM forces don’t attract each
other
Phase Changes
• Melting: solid -> liquid
• Boiling: liquid -> gas
• Higher IM forces generally lead to higher MP and BP
– BP
•
•
•
•
•
CH4
C4H10
C2H5OH
H2O
C8H18
-161o
-0.5o
78.1o
100o
125o
Molar mass = 18 g/mole
Molar mass = 114 g/mole
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Phase Changes
• Rank the following in order of increasing MP
– Identify ionic and molecular
• Ionic: forces depend on charge and sizes of ions
• Molecular: intermolecular forces matter
• Ionic forces stronger than intermolecular forces
•
•
•
•
•
NaCl
C3H8
BaO
C2H5OH
H2 O
Vapor Pressures
• Equilibrium Vapor Pressure
– Closed container
– # molecules leaving liquid =
# molecules entering
– Pressure = equilibrium VP
• High IM forces
– Difficult for molecules to leave liquid
– Few molecules in vapor phase
• Higher IM forces mean lower VP
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Vapor Pressures
• In each pair, which will have the highest equil. VP?
• C5H12 or C8H18
• C3H7NH2 or C4H10
Physical Properties and IM Forces
• Viscosity
– Measure of “thickness” of a liquid – its resistance to flow
• High viscosity liquid is “thick” and doesn’t flow
readily
• High IM forces generally lead to high viscosities
– Physical entanglement also matters
– See Table 11.3 in text
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Physical Properties and IM Forces
• Surface tension
– Molecules on surface of liquid
only feel forces from within
– Tightens surface
• Surface tension is measure of the
“tightness”
– Surface tension is reason bugs
can walk on water
• High IM forces lead to high
surface tension
Solubility
• Molecules with similar IM forces attract each other
• Molecules with different IM forces do not
• Solubility of molecular compounds
– Like dissolves like
– Polar compounds tend to dissolve in other polar
compounds
– Nonpolar compounds tend to dissolve in other nonpolar
compounds
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Solubility
• Would you expect Br2 to be more soluble in water or
octane (C8H18)?
• Would you expect CH3OH to be more soluble in
water or octane (C8H18)?
Physical Properties and IM Forces
• IM forces are reason water beads
on waxed surface
– Wax: nonpolar hydrocarbon chain
– Water: polar and H-bonding
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Physical Properties and IM Forces
• IM forces are reason that water adheres to glass
• Water is attracted to glass.
– Negatively charge oxygen
atoms on surface
• Water adheres to glass and is pulled
up a narrow tube
– Capillary action
Hydrogen Bonding in Biology
• Base pairs in DNA are held
together by H-bonds
• Also secondary structures
of proteins
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Hydrogen Bonding in Biology
• Active site of an enzyme: 3-dimensional pocket
structurally tailored to accept a particular substrate
• H-bonds are one way that
substrate is held in active site
Water
• Solid water (ice) floats on liquid water
– Unusual: most solids are more dense than their liquid
• Hydrogen bonding is reason
– Molecules in solid are held apart by H-bonds
– Molecules in liquid are closer to each other – more dense
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