Chemistry - A Molecular Science

Figure 7.6 Dipolar interactions in a polar molecule Dipolar interactions in HCl, which are represented with dashed lines, are between the Cl atom of one mo

lecule (negative end of dipole)

and the H atom of another molecu

le (positive end of dipole).

O

HH

O

HH

Figure 7.7 Dipole of water Regions of charge are due to electronegativity differences between the bound atoms. The c

enter of positive charge (blue region) lies

between the two hydrogen atoms, while the center of negative charge (red region) resides on the oxygen atom. The molecular dipole is represented by an arrow

pointing from the center of

positive charge toward the ce

nter of negative charge.

of attraction of the opposite charges created by the induced dipoles is called a

dispersion

force

. Dispersion forces are the most common forces between molecules, because
dispersion forces exist in all molecular substances
. Dispersion forces are most important

in molecules with large electron clouds, where electrons have more freedom of motion. Thus, dispersion forces typically increase with molecular size or molar mass.

DIPOLE-DIPOLE (DIPOLAR) FORCES Most molecules contain atoms with partial pos

itive charge and atoms with partial negative

charge as a result of the electronegativity differences that produce polar bonds. If the centers of positive and negative charge do not coincide, two poles are produced and the molecule has a

permanent dipole

. The magnitude of the dipole depends upon both the

size and the separation of the two centers of charge; if the centers of charge coincide, their separation is zero and they have no permanen

t dipole. Molecules with permanent dipoles

are said to be a

polar

, while those that do not have permanent dipoles are said to be

nonpolar

. Interactions between the centers of opposite charge of the permanent dipoles of

different molecules are called

dipolar forces

or

dipole-dipole forces

. Dipolar forces act

in addition to dispersion forces.

For example, the dispersion forces in HCl and F

are 2

expected to be nearly the sa

me because they have similar molar masses (38 and 37 g/mol,

respectively). However, HCl is polar, and as shown in Figure 7.6, the partial positive charge on the hydrogen atom of one HCl mole

cule interacts with the partial negative

charge on the chlorine atom of an adjacent molecule to produce a dipolar force between the molecules. This force is in addition to

the dispersion forces, so the intermolecular

forces are greater in HCl than in F

, which makes HCl gas much easier to liquefy. 2

While electronegativity differences are important in determining the charge on the two

centers, geometry determines their separa

tion. For example, hydrogen is less

electronegative than oxygen, so each hydrog

en atom in a water molecule carries some

positive charge, while the oxygen atom carries

some negative charge. The magnitude of

the charge on the oxygen must be twice that on each hydrogen atom because a water molecule is neutral, so the partial charges must sum to zero. The center of positive charge lies midway between the two H atoms, and because water is bent (Figure 7.7), it does not coincide with the center of negative charge

on the oxygen atom. The combination of large

electronegativity differences and a ~104

o bond angle makes water a very polar molecule.

As shown in Figure 7.7, the molecular dipole is often shown as an arrow pointing from the center of positive charge toward the center of negative charge.

Chapter 7 States of Matter and Changes in State

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