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Progressive Science Initiative
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AP Chemistry
Unit 4: Presentation D
Intermolecular Forces
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Chemical Bonding
The temperature on Pluto is -230 degrees C,
warm enough to break the weak forces
between hydrogen molecules in ice and cause
them to sublimate to a gas.
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Intermolecular Forces
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Intermolecular forces exist between neighboring molecules as
opposed to intramolecular forces which exist inside molecules.
INTERMOLECULAR
INTRAMOLECULAR
attractive forces between
different molecules
attractive forces within a
molecule
*Note: Intermolecular forces are much weaker than intramolecular forces due to the
greater distance between charges which diminishes the Coulombic attractions.
Intermolecular Forces
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There are essentially two types of intermolecular forces:
London Dispersion Forces and Dipole-Dipole Forces.
London Dispersion Forces
Occur between all molecules and are caused by the temporary
polarization of molecules due to the random positions of electrons
+
eee- e- eee- e-
ee-
Normal
-
e- ee- ee- e-
Polarized
These polarized molecules then form attractions called
London dispersion forces.
+
ee-
+
-
e- ee- ee- e-
ee-
-
e- ee- ee- e-
London Dispersion Force
Intermolecular Forces
The greater the number of electrons in the molecule, the
greater the polarization and the stronger the London
Dispersion Forces will be.
Iodine (I2)
Flourine (F2)
106 electrons
18 electrons
MP = 113.7 C, solid @ 25 C
MP = -219.7 C, gas @ 25 C
The surface area of the molecule also influences the ability of
"LDF's" to form. The greater the surface area, the greater the
"LDF's", all else being equal.
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1 The indicated interaction pointed to by arrow is an
intermolecular force.
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True
False
2 Which of the following is TRUE regarding intra and
inter molecular forces?
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A Intra are stronger due to decreased coulombic
attractions
B Intra are stronger due to increased coulombic
attractions
C Inter are stronger due to decreased coulombic
attractions
D Inter are stronger due to increased coulombic
attractions
3 Which of the following would experience the greatest
london dispersion forces between molecules?
A CO2
B Br2
C H2
D CH4
E SiH4
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4 Which of the following solids would require the least
energy to sublimate?
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A C3H8(s)
B O2(s)
C CH4(s)
D Ar(s)
E I2(s)
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5 Butane (C4H10) can exist as one of two
isomers: isobutane and n-butane
shown to the right. The melting points
of isobutane and n-butane are -160 C
and -140 C respectively. Which of the
following BEST explains why?
isobutane
n-butane
A Isobutane is more polarized due to having fewer
electrons
B n-butane is more polarized due to having more
electrons
C Isobutane and n-butane have equal numbers of
electrons but isobutane has less surface area
D Isobutane and n-butane have equal numbers of
electrons but n-butane has more surface area
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Intermolecular Forces
Unlike "LDF's", Dipole-Dipole forces exist only between
molecules with permanent dipoles and are therefore stronger
than "LDF's" bond for bond.
Dipole-Dipole Forces
Attractions between opposite poles of polar molecules. The more
polar the molecule, the greater the charges, which results in
stronger Coulombic attractions.
+
-
+
-
H
Cl
H
Cl
dipole-dipole force
+
-
+
H
F
H F
dipole-dipole force
MP = -85 C
MP =19 C
less polar
more polar
-
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Intermolecular Forces
Hydrogen bonds are a special type of dipole-dipole force that is
generated between the highly electropositive H on a molecule
and a highly electronegative atom on a neighboring molecule.
Hydrogen Bonds
Only F, O, and N are electronegative enough to create a positive
charge big enough on H to create a hydrogen bond with a
neighboring F, O, or N.
F
F
Hydrogen Bonds
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Intermolecular Forces
Hydrogen bonds create unexpectedly high melting and boiling
points.
Strength of
Boiling Point
H-Bonds?
LDF's
(1 atm)
Substance
# of e-
H2Te
106
High
No
-2.2 C
H2Se
70
Medium
No
-41.3 C
H2S
34
Low
No
-60.0 C
H2O
20
Very low
Yes
100.0 C
Intermolecular Forces
Hydrogen bonds play an important role in polymers both
synthetic (plastics) and natural (proteins).
R
N
Hydrogen bonding stabilizes the
secondary structure of proteins
O
H
O
R
H-Bond
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Intermolecular Forces
When evaluating properties such as boiling point, one must
consider the forces present in each substance. Although LDF's
are in general weaker than dipole-dipole forces (DDF's), large
non-polar molecules can have higher boiling points than smaller
polar molecules, even those with hydrogen bonding.
Substance
LDF
DDF MP @1 BP @ Heat of
H-Bonds
Rank
Rank atm 1 atm Vaporization
H2O
3
Yes
1
0C
100 C 40.6 kJ/mol
I2
1
No
3
114 C 184 C 41.6 kJ/mol
CH3COCH3
2
No
2
-95 C 56 C
29 kJ/mol
*When comparing properties of molecular compounds, first determine if
the molecule is polar. If it is, dipole-dipole forces will be part of the
discussion. If not, LDF's will be the only intermolecular force to consider.
6 Which of the following would have dipole-dipole
intermolecular forces?
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A Br2
B CH4
C CH3F
D CF4
E None of these
7 Which of the following best explains why ammonia
(NH3) has a higher boiling point than methane (CH4)?
A Ammonia is non-polar and methane is polar
B Ammonia can form hydrogen bonds while methane
cannot
C Ammonia has greater london dispersion forces
D Ammonia can form london dispersion forces while
methane cannot
E None of these
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8 Which of the following best explains how carbon
tetrabromide has a higher boiling point than water?
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A CBr4 is more polar and can form stronger dipole dipole forces
B CBr4 can form chlorine bonds which are stronger
than hydrogen bonds
C CBr4 has greater london dispersion forces due to
the larger electronegativity differences
D CBr4 has greater london dispersion forces due to
the greater polarizability of the molecule
E None of these are true
9 Which of the following would be expected to have the
highest heat of vaporization?
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A H2
B Kr
C PH3
D CO2
E He
10 Which of the following would have the lowest boiling
point?
A HI
B I2
C Br2
D F2
E C3H8
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11 Which of the following would correctly rank the
following from highest to lowest boiling point?
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A NH3 > PH3 > CCl4
B CCl4 > NH3 > PH3
C PH3 > NH3 > CCl4
D NH3 > CCl4 > PH3
E CCl4 > PH3 > NH3
12 In which of the following substances would hydrogen
bonding be found between molecules?
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A HCl
B CH3F
C CH3NH2
D CH3COCH3
E None of these
13 The indicated bond is a hydrogen bond.
True
False
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14 Which of the following arrows points to a hydrogen
bond in the molecule below?
D
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E
C
A
B
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Off to Unit 5 where we will examine the various
states of matter in detail.