Overview of Chapter 13
• Intermolecular Forces
• Liquids and their properties
Intermolecular Forces
Strongest to weakest
Na+ Cl-
• Ion-Ion
• Ion- Dipole
• Dipole-Dipole
• Solids and their properties
• Dipole-Induced dipole
• Phase diagrams
• Induced dipole-Induced dipole
• Hydrogen Bonding (Dipole-Dipole)
Dipole-Induced Dipole interactions
Dipole-Dipole Forces
Influence of dipole-dipole forces is seen in the boiling
points of simple molecules.
Compound
N2
NH3
HF
Mol. Wt.
28
14
20
Boiling Point
-196°C
-33°C
20°C
N2
NH3
HF
0D
1.42 D
1.8 D
Okay, so polar molecules can coalesce to
form liquids, but what’s up with non-polar
molecules?
How can non-polar molecules such as O2 and I2
dissolve in water?
The water dipole INDUCES a
dipole in the O2 electric cloud.
Induced dipole - Induced dipole
Formation of a correlated dipole in two nonpolar molecules.
i.e.
Why is gasoline a liquid?
Induced Dipole-Induced Dipole interactions!
aka
London dispersion forces
Occurs in ALL molecules
(all you need is electrons)
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Boiling Points of Hydrocarbons
Induced Dipole-Induced Dipole
• Works with all molecule types
• Works better with larger atoms and molecule
(those are more polarizable)
• These forces are weak, but they add up
C4 H10
C3 H8
C2 H6
CH4
in vacuo
Bulk H2O
1.85 D
2.9 D
In general, BP increases with molecular weight
The larger the molecule the more electron density
More electron density yields a more polarizable molecule
Polarity and Solubility
Polarity and Solubility
“like dissolves like”
“like dissolves like”
i.e.
Polar substances mix well with other polar material
Non-polar with nonpolar
Oil and water don’t mix
1st: dissolve I2 in H2 O (dipole-induced dipole interaction)
2nd: layer I2 in H2O over CCl4 (polar - non-polar do not mix
3rd: shaking allows non-polar I2 to move to CCl4
Boiling Points of Simple
Hydrogen-Containing
Compounds
Hydrogen Bonding
A special form of dipole-dipole attraction,
which enhances dipole-dipole attractions.
6A
5A
7A
4A
H-bonding ONLY when X and Y are N, O, or F
2
Hydrogen Bonding
Between Methanol and
Water
-δ
Hydrogen Bonding in H2O
H-bonding is especially
strong in water because
• the O—
O— H bond is very
polar
• there are 2 lone pairs on
the O atom
• there are 2 H’
H’s available to
bond
H-bond
+δ
H-bonding accounts for many
of water’
water’s unique
properties.
-δ
Hydrogen Bonding in H2O
Ice has open
lattice-like
structure.
Hydrogen Bonding in H2O
Ice has open lattice-like structure.
Ice density is < liquid, so ice floats on water.
One of the VERY few
substances where
solid is LESS DENSE
than the liquid.
What is this?
What is this?
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