Intermolecular Forces
Solids, Liquids, and Gases
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Intermolecular Attractions
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Intermolecular Vs Intramolecular
forces
• Intermolecular forces are much weaker
than intramolecular forces.
• Example:
• We need about 41kJ to vaporize 1 mole of
water at its boiling point (to overcome
intermolecular forces), but about 930 kJ
are needed to break the two O-H bonds.
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Ion – Dipole interaction
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Dipole – Dipole Interaction
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London Dispersion Forces
(instantaneous dipoles)
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Polarizability: the ease with which the electron distribution
around an atom or molecule can be distorted.
Dispersion forces increases with:
1. polarizability
2. atomic or molecular mass
3. number of electrons
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Van der Waals Forces
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Dipole – dipole forces
Dipole – induced dipole forces
Dispersion forces (London forces)
But not ion-dipole forces or hydrogen
bonding
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Example: Correlate intermolecular
forces to mp
Compound
CH4
CF4
CCl4
CBr4
CI4
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Melting point, oC
-182.5
-150.0
-23.0
90.0
171.0
Which is greater: dipole dipole or
dispersion forces?
• CH3F has a dipole moment of 1.8 Debye,
while CCl4 has no dipole moment. The
boiling point of the two compounds are 78.4 and 76.5 oC. Although CH3F has both
dipole – dipole forces and dispersion
forces, it has a much lower BP (smaller
intramolecular forces) than CCl4 which has
dispersion forces only. However this is not
always true.
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Example: What types of intermolecular
forces exists between the following pairs:
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•
•
•
•
•
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a. HBr and H2S
Cl2 and CBr4
I2 and NO3NH3 and C6H6
CH3Cl and CCl4
KBr and H2O
Polarity of molecules
Intermolecular forces
Polar - polar
Dipole – dipole (+Dispersion)
Polar - nonpolar
Dipole – induced dipole
(+Dispersion)
Ion – dipole (+Dispersion)
Polar + ion
Nonpolar + nonpolar
Ion – induced dipole
(+Dispersion)
Dispersion
Ionic compounds
Ionic bonds (Strongest)
Nonpolar + ion
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Hydrogen Bonding
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A hydrogen bond can form when:
• We have molecules containing one of the
following groups:
• N-H
• O-H
• H-F
• And O, N, or F atoms in the same
molecule or other molecules
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Density of water
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• Highest density occurs at 4oC.
• From 0 – 4oC, trapping prevails which
results in an increase in density
• Above 4oC, thermal expansion
predominates resulting in a decrease in
density
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