The Kinetic-Molecular Theory
Of
Matter
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The Kinetic-Molecular Theory was developed
to explain the observed properties of matter.
Since matter can be found in three common
states (?), the theory has been tailored to fit
these states.
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The basic assumptions of kineticmolecular theory are:
Postulate 1
◦ Gases consist of discrete molecules that are
relatively far apart.
◦ Gases have few intermolecular attractions.
◦ The volume of individual molecules is very
small compared to the gas’s volume.
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Proof - Gases are easily
compressible.
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Postulate 2
◦ Gas molecules are in constant, random, straight line
motion with varying velocities.
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Proof - Brownian motion displays molecular
motion.
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Postulate 3
◦ Gas molecules have elastic collisions with
themselves and the container.
◦ Total energy is conserved during a collision.
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Proof - A sealed, confined gas exhibits no
pressure drop over time.
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Postulate 4
◦ The kinetic energy of the molecules is
proportional to the absolute temperature.
◦ The average kinetic energies of molecules of
different gases are equal at a given
temperature.
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Proof - Brownian motion increases as
temperature increases.
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The basis of the theory is that as the
temperature increases the velocity of the gas
molecules increase. This is because there
kinetic energy is increasing.
Some of the properties of gases explained by
the theory include:
Expansion: Gases will fill their container
since they are moving rapidly and they do not
attract each other.
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Fluidity: Since there is little attraction
between the molecules they easily flow past
one another. Liquids can also flow and both
are referred to as fluids.
Low Density: Since there is a lot of space
between gas molecules, gases have a low
density.
The density of gases is much less than
that of solids or liquids.
Densities
Solid
Liquid
Gas
(g/mL)
H2O
0.917
0.998
0.000588
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CCl4
1.70
1.59
0.00503
Gas molecules must be very far apart
compared to liquids and solids.
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Compressibility: Since the molecules are far
apart they can be forced closer together.
Diffusion and effusion:
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Diffusion is the intermingling of gases.
Effusion is the escape of gases through
tiny holes.
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Gases that obey all the postulates of the
Kinetic Molecular Theory are called Ideal
gases. Most of the gases we are familiar with
behave ideally at normal temperatures and
presures.
Composition of Dry Air
Gas
N2
O2
Ar
CO2
He, Ne, Kr, Xe
CH4
H2
% by Volume
78.09
20.94
0.93
0.03
0.002
0.00015
0.00005
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The main difference between a liquid and a
gas is that the molecules in a liquid do attract
one another. This restricts their motion so
they stay in the liquid state. The molecules in
a liquid have enough motion for them to flow
past one another (fluid).
Some properties of liquids include:
Higher density than gases. There are more
molecules (mass) in a given volume so the
density goes up.
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Relatively incompressible: The molecules in the
liquid are already close together so increasing
pressure will not compress the liquid.
Liquids can mix with one another, they diffuse
into one another. See p334.
Liquids have the property of surface tension. The
molecules at the surface of a liquid are attracted
less that the molecules within the liquid. This
creates a tension at the surface.
See p 335.
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Capillary action refers to the attraction of a
liquid to a solid surface. (fig. 7, p335)
Boiling and evaporation: If the molecules in a
liquid possess sufficient energy it is possible
for them to overcome the attraction between
them and move into the gaseous state. If we
supply the energy in the form of heat we can
speed up the process and we say the liquid is
boiling. If the attractive forces in the liquid
are weak the liquid will evaporate rapidly.
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The molecules in a solid are closely packed
together giving solids a definite shape and
volume. The attractive forces between
molecules are slightly stronger in the solid
state than in the liquid state.
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Amorphous solids do not have a well
ordered molecular structure.
◦ Examples of amorphous solids include waxes,
glasses, asphalt.
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Crystalline solids have well defined
structures that consist of extended array
of repeating units called unit cells.
◦ Crystalline solids display X-ray diffraction
patterns which reflect the molecular structure.
◦ The Bragg equation, detailed in the textbook,
describes how an X-ray diffraction pattern can
be used to determine the interatomic distances
in crystals.
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Unit cells are the smallest repeating unit
of a crystal.
◦ As an analogy, bricks are repeating units for
buildings.
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There are seven basic crystal systems.
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We can think of
solids as falling
into two groups:
◦ Crystalline—
particles are in
highly ordered
arrangement.
◦ Amorphous—no
particular order in
the arrangement of
particles.
In ionic crystals, ions
pack themselves so
as to maximize the
attractions and
minimize repulsions
between the ions.
Because of the
order in a crystal,
we can focus on
the repeating
pattern of
arrangement
called the unit
cell.
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Diamonds are an example of a covalentnetwork solid in which atoms are
covalently bonded to each other.
◦ They tend to be hard and have high melting
points.
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Graphite is an example of a molecular
solid in which atoms are held together
with van der Waals forces.
◦ They tend to be softer and have lower melting
points.
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Metals are not
covalently bonded,
but the attractions
between atoms are
too strong to be van
der Waals forces.
In metals, valence
electrons are
delocalized
throughout the solid.
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As we have mentioned all phase changes also
involve changes in energy.
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Solids
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Solids
Liquids
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Solids
Liquids
Gases
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Change States
◦ heating
◦ cooling
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Illustration of changes in state
◦ requires energy
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Vapor Pressure
Vapor pressure is the pressure exerted by
a liquid’s vapor on its surface at
equilibrium.
Vapor Pressure (torr) and boiling point for
three liquids at different temperatures.
0 oC
20oC
diethyl ether185 442
ethanol
12
44
Water
5
18
30oC normal boiling point
647
74
32
36oC
78oC
100oC
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Vapor Pressure as a function of
temperature.
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Boiling Points and Distillation
The boiling point is the temperature at which
the liquid’s vapor pressure is equal to the
applied pressure.
The normal boiling point is the boiling point
when the pressure is exactly 1 atm.
Distillation is a method we use to separate
mixtures of liquids based on their differences
in boiling points.
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Phase diagrams are a convenient way to
display all of the different phase transitions
of a substance.
This is the phase diagram for water.
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Compare water’s phase diagram to carbon
dioxide’s phase diagram.
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