LECTURE 9
Intermolecular Forces
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INTERMOLECULAR FORCES:
Bonding forces are relatively strong because they involve larger charges that are
closer together.
Intermolecular forces are relatively weak because they typically involve smaller
charges that are farther apart.
Figure 12.9
Covalent and van der Waals radii.
Figure 12.10
Periodic trends in covalent and van der Waals radii (in pm).
Figure 12.11
Polar molecules and dipole-dipole forces.
THEHydrogen
HYDROGEN
BOND
The
Bond
A dipole-dipole intermolecular force
A hydrogen bond may occur when an H atom in a molecule, bound to small highly
electronegative atom with lone pairs of electrons, is attracted to the lone pairs in
another molecule.
The atoms which are small and highly electronegative are N, O, and F.
H
hydrogen bond
acceptor
..
O
..
O
..
hydrogen bond
acceptor
H
H
..
N
hydrogen bond
donor
..
hydrogen bond
acceptor
..
N
..
F
..
..
..
..
F
..
hydrogen bond
donor
hydrogen bond
donor
Figure 12.12
Dipole moment and boiling point.
Drawing Hydrogen Bonds Between Molecules of a
Substance
SAMPLE PROBLEM 12.3
PROBLEM:
Which of the following substances exhibits H bonding? For those that do,
draw two molecules of the substance with the H bond(s) between them.
O
C2H6
(a)
PLAN:
(c)
CH3C
NH2
Find molecules in which H is bonded to N, O or F. Draw H bonds in the
format
B:
H A
SOLUTION:
(b)
CH3OH
(b)
H
(a) C2H6 has no H bonding sites.
H
H
C O H
H
H
(c)
H
O C
H
O
C H 3C
H
H
N
H
H
N
C CH3
H
O
H
O
N
C CH3
C H 3C
N
H
H
O
Figure 12.13
Hydrogen bonding and boiling point.
Polarizability and Charge-Induced Dipole Forces
Distortion of an electron cloud
•Polarizability increases down a group.
Size increases and the larger electron clouds are further
from the nucleus.
•Polarizability decreases left to right across a period.
Increasing Zeff shrinks atomic size and holds the electrons
more tightly.
•Cations are less polarizable than their parent atom because they are
smaller.
•Anions are more polarizable than their parent atom because they are
larger.
Figure 12.14
Dispersion forces among nonpolar particles.
separated Cl2
molecules
instantaneous
dipoles
Figure 12.15
Molar mass and boiling point.
Figure 12.16
Molecular shape and boiling point.
SAMPLE PROBLEM 12.4
PROBLEM:
Predicting the Types Intermolecular Force
For each pair of substances, identify the key intermolecular force(s) in
each substance, and select the substance with the higher boiling point:
(a) MgCl2 or PCl3
(b) CH3NH2 or CH3F
(c) CH3OH or CH3CH2OH
CH3
(d) Hexane (CH3CH2CH2CH2CH2CH3)
CH3CCH2CH3
or 2,2-dimethylbutane
PLAN:
Use the formula, structure, and Table 2.2.
CH3
•Bonding forces are stronger than nonbonding (intermolecular) forces.
•Hydrogen bonding is a strong type of dipole-dipole force.
•Dispersion forces are always present, but decisive when the difference is molar
mass or molecular shape.
SAMPLE PROBLEM 12.4
Predicting the Types Intermolecular Force
continued
SOLUTION:
(a) Mg2+ and Cl- are held together by ionic bonds while PCl3 is covalently bonded and the
molecules are held together by dipole-dipole interactions. Ionic bonds are stronger than
dipole interactions and so MgCl2 has the higher boiling point.
(b) CH3NH2 and CH3F are both covalent compounds and have bonds which are polar.
CH3NH2 can H bond while that in CH3F cannot. Therefore, CH3NH2 has the stronger
interactions and the higher boiling point.
(c) Both CH3OH and CH3CH2OH can H bond but CH3CH2OH has more CH for more
dispersion force interaction. Therefore, CH3CH2OH has the higher boiling point.
(d) Hexane and 2,2-dimethylbutane are both nonpolar with only dispersion forces to hold the
molecules together. Hexane has the larger surface area and therefore, the greater
dispersion forces and higher boiling point.
Figure 12.17
Summary diagram for analyzing the intermolecular forces in a sample.