Physical Properties

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 The strength of dispersion forces between
molecules can affect physical properties of the
substances they make up.
 In other words… the amount of energy needed
to break an intermolecular bond in a compound,
in a substance can determine such things as
melting and boiling points and solubility!
 Let’s learn more!!...
Melting and Boiling Points

Solids are able to melt, and liquid substances are able to
boil, when there is enough energy present for particles to
overcome their forces of attraction with surrounding
particles to break free and move around more.
 For substances to melt or boil they have to break the
intermolecular bonds between their particles. If the
particles can break free and move around more, then
their state will change.
i.e.: solids  liquids and liquids  gases
 Bond energies tell the amount of energy that is required
to break the intermolecular bonds between particles.
 As you increase the temperature of a substance, you
increase the energy in its ions or molecules. So
depending on how strong the forces of attraction are
between particles, a substance may require more energy
(higher temperatures) in order to break the bonds.
Factors That Cause High Melting
and Boiling Points

Larger molecules that have more electrons have
more vibrations within their molecules. This
means that they can easily cause an uneven
distribution of charge within their molecules.
 This increases the strength of the dispersion
forces between larger molecules, making their
bonds stronger, and a lot more difficult to break.
 Their bonds are stronger, so they require a lot
more energy to break them which means higher
melting and boiling points.
Factors That Cause Low Melting
and Boiling Points



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The shape of molecules can also affect their boiling and
melting point.
Molecules with a spherical shape have a smaller surface
area than a straight chain molecule even if they have the
same number of electrons.
A smaller surface area means there is less chance of
causing an uneven distribution of charge within the
molecule. This also means less opportunities to induce a
charge on a nearby molecule.
Therefore, substances with more spherical shaped
molecules will have weaker dispersion forces.
Weaker dispersion forces mean that there is less energy
(lower temperatures) required to break the bonds holding
molecules together.
Ionic and Molecular Compounds

Ionic compounds generally have extremely high
melting points because their bonds are much
stronger and more difficult to break. It takes a
lot of energy. For example, calcium fluoride,
CaF2.
 Molecular Compounds generally have low
boiling points because they have much weaker
dispersion forces between their particles. So it
takes a lot less energy (lower temperatures) to
break the intermolecular bonds. For example,
bromide, Br2.
Mechanical Properties of Solids
Ionic compounds are brittle; they are not malleable
and ductile like most metals. This is because ionic
compounds consist of positive and negative charges
which attract each other.
If you tried to alter a solid ionic compound by bending
it or hitting it with a hammer, it would cause the
charges to go out of line and suddenly instead of
opposite charges being aligned, like charges become
aligned and repel each other.
This creates a line or plane along which substances
break because there are no intermolecular forces of
attraction. An example is magnesium oxide, MgO.
Mechanical Properties of Solids
With molecular compounds, if they are polar molecules they
are quite brittle because there are positive and negative
charges within the dipoles that can become aligned with their
like charge and repel, causing a plane along which they could
be broken.
They also tend not to be as hard as ionic compounds because
polar molecules are only “slightly” charged and so that allows
for weaker intermolecular forces and more movement and
space between particles. Ex. Ice cubes
If the molecular compounds are non-polar, however, they are
often quite soft even though they are solids at room
temperature. This is because the forces of attraction that may
occur temporarily between their molecules are random and
constantly changing. This allows molecules to be able to slide
all over each other causing a substance to be soft but still
have enough attraction between particles to be solid. A good
example is wax.
Electrical Conductivity


In order to conduct electrical currents,
negative and/ or positive charges have to
be able to move freely and independently
of one another.
As we know metallic elements are the best
conductors of electricity, but what about
ionic and molecular compounds?
Electrical Conductivity


Ionic Compounds are only good conductors of
electricity when they are in a liquid state or dissolved
in water. In a solid state the electrons in ionic
compounds are held too tightly within the individual
ions to be able to move about and generate an
electric current or flow of charge.
However, in the liquid state, or when the compound is
dissolved in water, the electrons are not held as tightly
and are able to move around each other, or in the
case of the dissolved compound, through the water.
Electrical Conductivity


Generally, molecular compounds can’t
conduct electricity at all because the
electrons are all localized within the
molecules, and even though polar
molecules are slightly charged, the charges
can’t leave the molecules and move around.
Non-polar molecular compounds can’t
conduct electricity at all.
Solubility

1.
2.
3.

Solubility depends on the forces between
particles. In order for a solid to dissolve there are
three energy changes that must happen.
The forces between the particles in the solid must
be broken. (Between ions and ions or molecules
and molecules).
The forces between the particles in the liquid
must be broken.
There must be an attraction between the particles
of the solid and the liquid.
If the force of attraction between the particles of
the liquid and solid is much greater than the
attraction between the like particles, then the solid
has a much greater chance of dissolving.
Solubility
 For ionic compounds, they are usually
soluble in a polar substance like water
because there is a force of attraction
between the charged particles of the ionic
compound, and the water, causing the
compound to break down and dissolve in
the water. Ex. KCl
Solubility
 The same goes for polar molecular compounds. They
generally dissolve well in other polar substances like
water.
 This is because there is a force of attraction between
the charged particles in the compound and in the water.
Ex. NH3
 Polar compounds do not however, dissolve in non-polar
solvents because the forces of attraction are much
greater between the charges of the individual
compound.
 The force of attraction between the polar compound
and the non-polar solvent is pretty much non-existent.
Solubility
 Non-polar molecules do not dissolve in polar
solvents. This is because the forces of
attraction between the water molecules are
very strong, so the water molecules remain
attached to each other rather than attracting
the non-polar molecules. An example is iodine
in water.
 Non-polar molecules will, however, dissolve in
non-polar solvents. There is a greater
attraction between the molecules in the solid
and liquid, causing the solid to break down
and dissolve. An example is iodine in
kerosene.
Conclusion
 We just learned how dispersion forces between
molecules or ions in a compound can have an
affect on the physical properties of the related
substance.
 We learned that the strength of the dispersion
forces in compounds affects: the melting and
boiling points, malleability, hardness, solubility, and
electrical conductivity of that substance!
 Now let’s do a Lab!!
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