CHEM 110 Chapter 12 Smart Notes October, 2011
CHEM 110 Chapter 12 Smart Notes
12.1: Intermolecular Forces
o Van Der Waals Forces
The term Van der Waals forces is used to describe,
collectively intermolecular forces of the London type and
interactions between permanent dipoles.
o Polarizability is the term to describe the relative
tendency for a charge distribution to distort from its
normal shape in an atom or a molecule.
Polarizability increases as electrons increase
Polarizability increases as molar mass increases
Polarizability increases as dispersion forces
Elongated less compact molecules have greater
Polarization due to more intermolecular
interactions. CHEM 110 Chapter 12 Smart Notes October, 2011
o 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 neighbouring dipoles.
o A polar with a non-polar. The polar molecule induces
a temporary dipole in the non-polar molecule.
Induced Dipole-Induced Dipole or London Dispersion
o Dispersion forces exist between all molecules.
o Dispersion (London) Forces are intermolecular forces
associated with instantaneous and induced dipoles.
An instantaneous dipole is when a non-polar
species become momentarily polar when the
electrons are concentrated in one region of an
atom or a molecule. CHEM 110 Chapter 12 Smart Notes October, 2011
An induced dipole occurs when a molecule that
has an instantaneous dipole moment induces a
neighbouring atom or molecule to produce a
Hydrogen Bonding occurs when Hydrogen
covalently bonds with Fluorine, Oxygen, or
Nitrogen (intramolecular hydrogen bonding),
which are highly electronegative atoms.
o Hydrogen Bonding in Water (Intermolecular Hydrogen
Ice has a low density. When ice melts, some
of the hydrogen bonds are broken, this allows
molecules to be more compactly arranged,
accounting for the increase in density when ice
melts. CHEM 110 Chapter 12 Smart Notes October, 2011
Water’s density decreases as temperature
Without hydrogen bonding, all lakes would
freeze from the bottom up. This is because
the denser water sinks to the bottom of the
lake and the colder surface water freezes.
Furthermore, the ice over the top of the lake
then tends to insulate the water below from
further heat loss. CHEM 110 Chapter 12 Smart Notes October, 2011 CHEM 110 Chapter 12 Smart Notes October, 2011
12.2: Some Properties of Liquids
o Surface Tension
Surface tension is the energy, or work, required to increase
the surface area of a liquid.
Energy per Unit Area (typically in Joules per
As temperature decreases, surface tension increases.
o The forces exerted between molecules holding them
together in the drop are cohesive forces
o The forces between liquid molecules and the surface
are adhesive forces.
When cohesive > adhesive = water drop forms
When adhesive > cohesive = water drop collapses CHEM 110 Chapter 12 Smart Notes October, 2011 CHEM 110 Chapter 12 Smart Notes October, 2011
The water is drawn slightly up the walls of the tube by
adhesive forces between water and glass. The interface
between the water and the air above it, called a meniscus,
is concave, or curved in. With liquid mercury, the meniscus
is convex, or curved out.
A liquid’s resistance to flow
o Stronger intermolecular forces = Greater Viscosity.
o Viscosity generally decreases with increased