Intermolecular Forces

Intermolecular Forces


Intermolecular Forces

Intermolecular forces are generally much weaker than covalent bonds

Thus, when a molecular substance changes states the atoms within the molecule are unchanged

The temperature at which a liquid boils reflects the kinetic energy needed to overcome the attractive intermolecular forces (likewise, the temperature at which a solid melts).

Thus, the strength of the intermolecular forces determines the physical properties of the substance

Attractive forces between neutral molecules

Typically, dipole-dipole and dispersion forces are grouped together and termed van der Waals forces (sometimes the hydrogen bonding forces are also included with this group)

Attractive forces between neutral and charged (ionic) molecules

Note that all of these forces will be electrostatic in nature

Ion-dipole

Dipole-Dipole Forces

A dipole-dipole force exists between neutral polar molecules


Boiling points increase for polar molecules of similar mass, but increasing dipole:

Substance

Molecular Mass (amu)

Dipole moment, u (D)

Boiling Point (K)

Propane

44

0.1

231

Dimethyl ether

46

1.3

248

Methyl chloride

50

2.0

249

Acetaldehyde

44

2.7

294

Acetonitrile

41

3.9

355


London Dispersion Forces

Nonpolar molecules would not seem to have any basis for attractive interactions.

A Model To Explain London Dispersion Forces:

Helium atoms (2 electrons)

Due to electron repulsion, a temporary dipole on one atom can induce a similar dipole on a neighboring atom

The ease with which an external electric field can induce a dipole (alter the electron distribution) with a molecule is referred to as the "polarizability" of that molecule

thus, dispersion forces tend to increase with increasing molecular mass

Hydrogen Bonding

A hydrogen atom in a polar bond (e.g. H-F, H-O or H-N) can experience an attractive force with a neighboring electronegative molecule or ion which has an unshared pair of electrons (usually an F, O or N atom on another molecule)

Hydrogen bonds are considered to be dipole-dipole type interactions

Water is unusual in its ability to form an extensive hydrogen bonding network


1996 Michael Blaber