Intermolecular Attractions
-- Liquids and Solids
L. Scheffler
IB Chemistry 1-2
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Intermolecular Forces
 Intermolecular
forces are the forces of
attractions that exist between molecules
 The strength of these forces determine:
• The state of matter: solid, liquid, or gas
• The melting and boiling points of
compounds
• The solubilities of one substance in
another.
2
Types of Intermolecular
Forces
Intermolecular forces include:

Hydrogen bonding

Dipole to dipole interactions

van der Waals forces or
dispersion forces
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Hydrogen Bonding
Hydrogen bonding occurs between polar covalent
molecules that possess a hydrogen atom that is
bonded to an extremely electronegative element;
specifically - N, O, and F.
Weak attractions occur
between the hydrogen
atoms of one molecule
and the oxygen atom of
another.
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Hydrogen Bonding
The weak attractions that result form hydrogen
bonding cause molecules to stick together.
As a result molecules
with significant
hydrogen bonding
have higher melting
points and boiling
points than they
would otherwise
have.
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Hydrogen Bonding
Hydrogen bonds are the strongest of all of the
intermolecular forces. They are about onetenth the strength of a covalent bond .
Because hydrogen bonds must be overcome
for a substance to melt or evaporate,
substances that have significant hydrogen
bonding have higher than normal melting and
boiling temperatures
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Dipole-Dipole Attractions
o
o
If the permanent net dipole within the polar
molecules results from a covalent bond
between a hydrogen atom and either fluorine,
oxygen or nitrogen, the resulting
intermolecular force is referred to as a
hydrogen bond
If this attraction occurs between other polar
molecules it is referred to as a dipole to
dipole interaction
.
7
Dipole-Dipole Attractions
o
Dipole-Dipole attractions occur between
o
molecules that have permanent net dipoles.
(polar molecules),
The partial positive charge on one molecule
is electrostatically attracted to the partial
negative charge on a neighboring molecule.
.
8
Dipole-Dipole Attractions
o
Some examples of molecules with dipoledipole interactions include:
•
•
•
SCl2
PCl3
CH3Cl
.
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van der Waals Forces
van der Waals or dispersion forces are very
weak forces of attraction between molecules
They result from:
1.
2.
momentary dipoles occurring due to uneven electron
distributions in neighboring molecules as they
approach one another
the weak residual attraction of the nuclei in one
molecule for the electrons in a neighboring molecule.
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Dispersion Forces
..
van der Waal's Forces are named after the person who
contributed to our understanding of non-ideal gas
behavior). They are also as known dispersion forces
or as London Forces (named after Fritz London who first
described these forces theoretically in 1930)
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Dispersion Forces



The more electrons that are present in the molecule,
the stronger the dispersion forces will be.
Dispersion forces are the only type of intermolecular
force that operates between non-polar molecules
Dispersion forces exist between non-polar
molecules such as
 hydrogen (H2)
 chlorine (Cl2)
 carbon dioxide (CO2)
 methane (CH4)
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Dispersion Forces
van der Waals or dispersion forces are the weakest
of the intermolecular forces
They are typically only 0.1% to 1% as strong as
covalent bonds between atoms in a molecule
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London Dispersion Forces
The van der Waals or
London dispersion
force is a temporary
attractive force that
occurs when the
electrons in two
adjacent atoms
occupy positions that
make the atoms form
temporary dipoles.
This force is sometimes called
an induced dipole-induced
dipole attraction.
1
Effects of London
Dispersion Forces


London forces are the attractive
forces that cause non-polar
substances to condense to
liquids and to freeze into solids
when the temperature is
lowered sufficiently.
Phase changes occur when
molecules are sufficiently close
and dispersion forces are
sufficiently strong to hold
molecules together
The Liquid State
The liquid state of a material
has a definite volume, but it
does not have a definite
shape and takes the shape
of the container, unlike that
of the solid state.
Unlike the gas state, a liquid
does not occupy the entire
volume of the container if
the container volume is
larger than the volume of
the liquid.
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The Liquid State
At the molecular level, the
arrangement of the molecules
is random, unlike that of the
solid state in which the
molecules are regular and
periodic.
Molecules are still closely
packed but they can slip past
each other and move around
the body of the liquid.
There may be some short order
intermolecular ordering or
structure, however.
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Solids, Liquids and Gases
The intermolecular
forces between particles
become stronger as
particles are packed
closer together and
move less rapidly
Energy is required to
convert from solid to
liquid to gas
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Vapor Pressure and
Boiling
o
o
o
Energy is required for a
liquid to evaporate
The vapor pressure of a
liquid depends on the
degree to which it will
evaporate at a given
temperature
Liquids evaporate at the
surface as long as the vapor
pressure of the liquid is less
than the pressure of the
atmosphere above the liquid
In order to evaporate, a water
molecule must have enough
energy
To overcome the hydrogen
bonds that hold it in place
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Vapor Pressure curves
for various liquids
Pressure
in torr
v it in place
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Vapor Pressure and
Boiling
o
o
o
The weaker the intermolecular forces in a
substance, the higher its vapor pressure
will be at any given temperature
Volatile liquids have relatively high vapor
pressures and hence they also have low
boiling temperatures
If the vapor pressure of a liquid is equal
to the atmospheric pressure the
substance will boil.
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Boiling
The boiling temperature depends on the pressure above the liquid.
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Boiling point of water and
elevation
The boiling temperature depends on the pressure above the liquid.
Atmospheric pressure decreases with increasing elevation.
23
Hydrogen bonding & the
boiling point
Molecules that undergo significant hydrogen bonding tend to have
much higher boiling points than they would otherwise have.
24
Characteristics of the
Liquid State
The most familiar liquid states at room
temperature are water, alcohol, benzene,
carbon tetrachloride, corn oil, and
gasoline.
Two elements, Bromine and Mercury are
liquids at room temperature. A third
element Gallium has a melting point
slightly above room temperature
Br
Ga
Glasses, although solids, are often called
frozen liquids, because the arrangements
of molecules in glasses are very similar to
those in liquid states.
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Properties of liquidsViscosity


Viscosity of a liquid is a measure of
the resistance of a liquid to flow,
Viscosity is measured in N s m-2 (SI
Units) or poise (P) or centipoise (cP).
1 P = 0.1 N s m-2
1 cP = 0.001 N s m-2
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Surface Tension




Surface tension is the energy required to
stretch a unit change of the surface area.
Thus its units are N * m m-2 = N/m.
There is no direct correlation between
viscosity and surface tension. These two
properties are independent of each other.
The surface tension is due to the
unbalanced force experience by molecules
at the surface of a liquid.
As a result of surface tension, a drop of
liquid tends to form a sphere, because a
sphere offers the smallest area for a
definite volume.
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Surface Tension



Substances with low surface
tension have a tendency to form
thin films.
When detergent is added to
water, it lowers the surface
tension.
Blowing soap water with a straw
forms bubbles, due to the low
surface tension.
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Cohesion and Adhesion




Cohesion is the intermolecular
attraction between like molecules,
Adhesion is the intermolecular
attraction between unlike
molecules.
Liquids with high surface tensions
have strong cohesion forces, and
they are poor wetting liquid due to
low adhesion forces.
A detergent or wetting agent is
a substance that increases the
adhesion force between two
different materials.
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Soaps and Detergents
Molecules of soaps and
detergents have both a polar
and an non-polar portion.
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Soaps and Detergents



For this reason soaps and
detergents are referred to as
wetting agents.
The wetting agent increases
the wetting action of water
with the non-polar material.
By this action, dirt is removed
when washed with water.
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Capillary Action




When a small tube is dipped into a liquid,
the level in the tube is usually higher or
lower than that of the bulk liquid.
If adhesion force between the tube
material and the liquid is stronger than the
cohesion force, the level is higher.
Otherwise, the level is lower.
Such phenomena are called capillary
action.
Capillary action is one of the factors
responsible for transport of liquid and
nutrients in plants, and sometimes in
animals.
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