Chapter 12
Intermolecular Forces
Van der Waals Forces
A collection of weak, short
range, and attractive forces
between groups of atoms or
molecules arising from electric
dipole moments.
Instantaneous and Induced Dipoles
Displacement of electrons cause
polarization giving rise to an
instantaneous dipole.
This dipole can affect
neighboring molecules causing
induced dipoles.
Dispersion (London forces)
Instantaneous dipole – induced dipole
attraction.
Related to polarizability which is the relative
tendency for a charge distribution to distort
from its normal shape in an atom or a
molecule.
Dispersion forces ⍺ Polarizability ⍺ atomic or
molecular size (the volume of the electron cloud
around a substance) ⍺ the melting points and
boiling points
Dipole-Dipole Interactions
In a polar substance, the molecules have permanent dipole
moments, so the molecules tend to line up with the positive end of
one dipole directed toward the negative ends of neighboring
dipoles.
Hydrogen Bonding
In a hydrogen bond an H atom is covalently bonded to a highly electronegative
atom, which attracts electron density away from the H nucleus.
Weak hydrogen bonding is occasionally encountered between an H atom of
one molecule and a Cl or S atom in a neighboring molecule.
Compared with other intermolecular forces, hydrogen bonds are relatively
strong.
Other Properties Affected by
Hydrogen Bonding
Dimerization
viscosity
The more –OH groups are
presented, the more H-bonds
are formed and therefore the
viscosity increases.
Intramolecular Hydrogen Bonding
In molecules with an H atom
covalently bonded to one
highly electronegative atom
(for example, O or N) and with
another highly electronegative
atom nearby in the same
molecule.
Intermolecular Hydrogen Bonding The hydrogen bond occur
between two molecules
Some properties of Liquides
Cohesive Forces
Intermolecular forces between
like molecules.
The forces exerted between
molecules holding them
together in the drop
Cohesive forces within the
liquid create an internal
friction, which reduces the rate
of flow.
Adhesive Forces
Intermolecular forces between
unlike molecules.
the forces between liquid
molecules and the surface
Surface Tension
It’s energy or work required to
increase the surface area of a
liquid (joules per square meter-
J/m2)
Viscosity
A liquids resistance to flow.
The stronger the intermolecular
forces of attraction, the greater
the viscosity.
When a liquid flows, one
portion of the liquid moves with
respect to neighboring
portions.
Enthalpy of Vaporization
Check out the slide
Boiling
The pressure exerted by
escaping molecules equals
that exerted by molecules of
the atmosphere, and boiling is
said to occur.
Boiling Point
The temperature at
which the vapor pressure of a
liquid is equal to standard
atmospheric pressure (1 atm)
is the normal boiling point.