Chemistry - A Molecular Science

Chapter 7 States of Matter and Changes in State

Intra

molecular

forces

exist

within

a molecule. They are the bonds between atoms that

hold the atoms together in the molecule. Inter

molecular forces

exist

between

molecules. They are the forces between different

molecules that keep the molecules in the liquid and solid states.

The N

≡N bond is a very strong bond, so the

intra

molecular force between the nitrogen

atoms in an N

molecule is very strong. However, N 2

is a gas at room conditions because 2

the

inter

molecular forces between different N

molecules are very weak. 2

Intermolecular forces, like all interactions

in chemistry, are electrostatic. Thus,

condensed phases in molecular substances result because there is an attraction between regions of opposite charge on the molecules. However, these charges are smaller, more diffuse, and farther apart than those in bonds,

so the force of attraction between different

molecules is much less than that between the atoms in a bond. Bond energies lie between 100 and 1000 kJ/mol, but the strengths of most

intermolecular interactions are less than 10

kJ/mol. Consider that the bond formed with Super Glue

® is an intramolecular (bonding)

interaction, while

static cling

is an intermolecular interaction.

Although molecules are electrically neutral,

many have regions of nonzero charge due

to asymmetric distributions of their valence

electrons that produce regions with above

normal electron density that

are slightly negative (

• δ) and regions of depleted electron

density that are slightly positive (

+δ

). Such molecules contain two poles, one positive (

+δ

)

and one negative (

• δ). The presence of two poles in the molecule results in a

molecular

dipole

, much like the bond dipole discussed in S

ection 5.2. In this section, we discuss the

origin of these dipoles and the forces they create.

(a)

dispersion forces

(b) (c)

XYZ

d+ d+

d+

d+

d- d-

d-

d-

Figure 7.5 Dispersion forces Regions of negative charge are shown

in red, while regions of positive

charge are shown in blue. (a) Molecules X, Y and Z have symmetrical charge distributions and are not interacting. (b) Random electron movement produces a temporary dipole in Y. (c) The temporary dipole in Y induces dipoles in X and Z. The attraction between induced dipoles is called a dispersion force.

DISPERSION FORCES Consider the three molecules X, Y, and Z in Figure 7.5. In Figure 7.5a, they have a symmetric charge distribution, so they have no dipoles. However, random electron motion in molecule Y (Figure 7.5b) results in a temporary dipole

because, for an instant, there are

more electrons on the left side producing a pa

rtial negative charge. Movement of electrons

to the left leaves the right side slightly electron deficient producing a partial positive charge there. The negative end (red) has increased electron density, which repels electrons in molecule X. The positive end (blue) is electron deficient, so it attracts the electrons in molecule Z. In Figure 7.5c, dipoles in molecules X and Z have been induced

by the

random dipole generated in molecule Y. The presence of these

induced dipoles

results in

intermolecular interactions between the molecu

les (dotted lines in Figure 7.5c). The force

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State

University