8.3 Intermolecular Forces
Why do
molecules stick
together in
liquids and
solids?
Ionic substances stay together through the attraction between positive and negative ions.
Network covalent substances stay together because all the molecules or atoms are
interconnected. What about water? What force attracts water molecules to other water
molecules in a liquid or solid?
Intermolecular
and van der
Waals
attractions
We learned in an earlier chapter that all atoms and
molecules are in continual motion, so why don’t the
individual molecules, which are moving at extremely high
velocity at room temperature, bounce off of each other
forming a gas? Why are there liquid and solid molecular
substances? The answer must be that the molecules stick to
each other. These attractions between molecules are called
intermolecular attractions, and are sometimes referred
to as van der Waals attractions. These attractions are
much weaker than ionic or covalent bonds which tend to be
100 to 1000 times stronger than intermolecular attractions.
Dipole-dipole
and London
dispersion
attractions
There are two broad categories of intermolecular attractions: dipole-dipole attractions
and London dispersion attractions. All molecules attract to each other using the London
dispersion type attraction. However, for polar molecules the dipole-dipole attraction
tends to be the more important of the intermolecular forces due to higher strength of the
dipole-dipole attraction. There is one kind of dipole-dipole attraction that is significantly
stronger and that has been given a special name - hydrogen bonding. We will look at each
of these kinds of attractions in more detail on the following pages.
intermolecular attractions - the attractions between molecules.
van der Waals attractions - another term used to describe the attractions between
molecules. Most sources consider these identical to the broad term “intermolecular
attractions”. However, some people only associate the term van der Waals attractions
with the London dispersion type of intermolecular attraction.
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Section 8.3 Intermolecular Forces
Dipole-dipole attractions
Some
molecules are
polar and form
dipole-dipole
attractions
Some molecules are polar because they have some polar
covalent bonds as part of their structure. This means that at
least one part of the molecule will be partially positive and
another part partially negative. This allows for the attractions
between the oppositely charged parts of different molecules.
The attraction between two polar molecules is call a dipoledipole attraction. In the case of very long polymers like
proteins, different parts of the same molecule can attract to
itself, stabilizing the overall structure of a single molecule.
Formaldehyde is a substance
used as a preservative, and is
a polar molecule. Because
the molecule is polar, dipoledipole attractions will form
between the molecules.
The more polar
a molecule is,
the stronger it
will attract to
other
molecules
The strength of the attraction between polar molecules can be measured by their boiling
points. The higher the boiling point, the stronger the individual molecules cluster
together, preventing them from separating and forming a gas. Below you can see that
more polar molecules have higher boiling points. Usually, when we talk about a molecule
being more polar than another, we mean that it has more polar covalent bonds. Oxygen
has a high electronegativity, so by adding oxygen to a molecule, you typically form polar
covalent bonds and cause a molecule to become polar (or more polar). Anytime atoms
with high electronegativity are part of a molecule you usually have a polar molecule.
dipole-dipole attraction - the attractions between the positive part of one polar
molecule and the negative part of another polar molecule.
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Hydrogen bonding and the special role of water
Hydrogen
bonding
In many cases, a polar covalent bond is formed between
hydrogen and nitrogen or oxygen. Since there is a
moderate difference in electronegativity between
hydrogen and either nitrogen or oxygen, you get a polar
covalent bond with hydrogen becoming partially
positive. When two molecules form a dipole-dipole
attraction between hydrogen on one molecule and
nitrogen or oxygen on another molecule, the attraction
is stronger than other dipole-dipole attractions, so we
give it a special name: hydrogen bonding.
Water has
special
properties
Water is composed of only oxygen and
hydrogen, so hydrogen bonds form between
water molecules and between water and other
polar molecules with similar kinds of bonds.The
unique physical and chemical properties of water
explain why it is so important. For example,
when water freezes, it becomes LESS dense!
Most materials are more dense as solids than as
liquids. Ice is less dense than liquid water
because hydrogen bonds force water molecules
to align in a crystal structure where molecules
are farther apart than they are in a liquid.
Surface tension
Iceburg photo curtesy of: NOAA
Another feature of water and other
polar liquids is surface tension.
As water molecules pull together
they tend to form spherical drops.
That’s why water drops are round.
At the surface of liquid water the
molecules pull together, forming a
“skin.” This is how water striders
are able to walk across the water
instead of sinking into it.
hydrogen bonding - the attractions between the partially positive hydrogen from
one polar molecule to the partially negative oxygen or nitrogen on another polar
molecule.
surface tension - a force that acts to pull a liquid surface into the smallest possible
area, for example, pulling a droplet of water into a sphere.
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Section 8.3 Intermolecular Forces
Other examples of hydrogen bonding
DNA is held
together with
hydrogen
bonds
When we refer to DNA, we usually mean the
spiralling polymer that encodes all the genetic
information needed by an organism to function
and reproduce. In fact, DNA is really two
molecules spiraling around and attracted to
each other. In order to read the code, the two
strands of DNA must be separated from each
other and then go back together. It is hydrogen
bonding which holds the DNA strands
together, but not so strongly that they can’t be
separated and have the genetic code do its job.
Hydrogen
bonds
influence
protein shapes
In proteins, the shape of the molecule is one of its most important features, giving the
molecule a specific function. Many forces help to shape the molecule into its final form,
and hydrogen bonding plays a key role. Two common structures that are found in
proteins are the alpha helix and the beta sheet. Both of these structures are stabilized by
forming hydrogen bonds across different parts of the same molecule.
PVA
Paper glue is a mixture of the
polymer
polyvinyl
acetate
(PVA) and water. When water is
present the glue is a liquid,
allowing the long polymer
molecules to flow by each other.
As the water evaporates, the
long PVA molecules attract each
other directly forming many
hydrogen bonds and locking the
molecules in place, forming a
solid.
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Normal “wet”
glue. Polymer
molecules
“lubricated” by
water.
As glue dries, many
more H-bonds form
between the
polymer molecules
so the glue hardens
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London dispersion attraction
Non-polar
molecules can
change
Even non-polar molecules cluster together into
liquids and solids, so there must be an attraction
even between them even if it is usually much
weaker than the dipole-dipole type attractions.
In fact, when two non-polar molecules come
very close to each other they can become very
slightly polar and stabilize themselves. This
polarization is temporary and easily disrupted
by the motions of molecules, yet it is enough to
allow them to condense together. This
attraction between non-polar molecules is
called London dispersion attraction.
Large surface
area molecules
However, if a molecule is large enough, then there is more surface area over which it can
form these temporary attractions, so even non-polar molecules can be solids at room
temperature. However, they tend to be soft solids like wax, or butter.
Getting close
counts
The key to stronger overall attractions between non-polar molecules is not just pure size
though. All intermolecular attractions are only felt when very close, so molecules that
can pack together more tightly also have an advantage.
London dispersion attraction - the attraction that occurs between non-polar
molecules due to temporary slight polarizations that occur when the normally equal
distribution of electrons is shifted.
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