Chemistry - A Molecular Science

Chapter 7 States of Matter and Changes in State

144

Example 7.6

CO

is linear, so the centers of 2

positive (blue) and negative (red)

charge coincide (both are on the carbon atom), which makes CO

nonpolar. The centers of positiv

e and negative charge do not

coincide in SO

, so it is a polar molecule. The resulting dipole is 2

represented by an arrow pointing from the center of positive charge to the center of negative charge. OC O

OO

S

Figure 7.9 Molecular dipoles are the sums of bond dipoles CO

is linear, so the bond dipoles (blue arrows) cancel, making it 2

nonpolar, but SO

is bent, so the bond dipoles add to produce a 2

OCO permanent dipole (red arrow).

S

OO

d+ d-

Figure 7.8 Charge distribution in CO

and SO 2

(^2)
Which has the greater dipole, CO
or SO 2
? 2
The electronegativities of both sulfur and carbon
are 2.5, while that of oxygen is 3.5. Thus,
the carbon and sulfur atoms carry similar posi
tive charges, as do the oxygen atoms in the
two molecules. The centers of positive charge
lie on the carbon and sulfur atoms, while
the centers of negative
charge lie on the centers of the
lines that connect the two oxygen
atoms. The relative positions
of the centers of
positive and negative charge depend upon
the molecular shapes, which are shown in Figure 7.8. CO
is linear, so the center of 2
negative charge (the midpoint of
the O-O line) lies on top of
the center of positive charge. Since the centers of positive and negative charge coincide, CO
has no permanent dipole and is a nonpolar molecule. Sulfur dioxide is bent, so the 2
line connecting the two oxygen atoms no longer passes through the sulfur. Consequently, the center of negative
charge does not coincide with the
center of positive charge, and
SO
has a permanent dipole, 2
i.e
., it is a polar molecule.
Molecular polarity can also be viewed as the vector sum of the bond dipoles as shown
in Figure 7.9. The two bond dipoles in CO
are of the same magnitude, but they cancel 2
because they point in opposite direction. In SO
, the two bond dipole vectors are not 2
opposed and add to give the resultant molecular dipole.
Intermolecular
dipole-dipole (dipolar) forces
result from the attraction between the
oppositely charged poles of polar molecules:
the more polar the molecule, the stronger the
force

. The difference between dispersion forces and dipolar forces is that the dipole in
dipolar interactions is permanent rather th

an induced or temporary. Molecules with

permanent dipoles also exert dispersion forces. Thus, SO

molecules experience both 2

dipole-dipole and dispersion interactions, while CO

molecules interact only through 2

dispersion forces. HYDROGEN BONDING The

hydrogen bond

is an especially strong case of a dipolar interaction in compounds

with N-H, O-H or F-H bonds. The strength of this force results from two factors:

1. The large electronegativity differences

between electropositive H and the very

electronegative N, O, and F atoms result in large partial charges on the atoms.

2. The small size of the elements allows the H atom on one molecule to get very close to the

N, O, or F atom of an adjacent molecule.

The close proximity of relatively large charges produces a strong intermolecular interaction.

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