The Kinetic Molecular Theory Postulates
The experimental observations about the behavior of gases discussed so far can be explained with a
simple theoretical model known as the kinetic molecular theory. This theory is based on the following postulates,
- Gases are composed of a large number of particles that behave like hard, spherical objects in a state
of constant, random motion.
- These particles move in a straight line until they collide with another particle or the walls of the
- These particles are much smaller than the distance between particles. Most of the volume of a gas
is therefore empty space.
- There is no force of attraction between gas particles or between the particles and the walls of the
- Collisions between gas particles or collisions with the walls of the container are perfectly elastic.
None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container.
- The average kinetic energy of a collection of gas particles depends on the temperature of the gas
and nothing else.
The assumptions behind the kinetic molecular theory can be illustrated with the apparatus shown in
the figure below, which consists of a glass plate surrounded by walls mounted on top of three vibrating motors. A handful
of steel ball bearings are placed on top of the glass plate to represent the gas particles.
When the motors are turned on, the glass plate vibrates, which makes the ball bearings move in a constant,
random fashion (postulate 1). Each ball moves in a straight line until it collides with another ball or with the walls of
the container (postulate 2). Although collisions are frequent, the average distance between the ball bearings is much larger
than the diameter of the balls (postulate 3). There is no force of attraction between the individual ball bearings or between
the ball bearings and the walls of the container (postulate 4).
The collisions that occur in this apparatus are very different from those that occur when a rubber
ball is dropped on the floor. Collisions between the rubber ball and the floor are inelastic, as shown in the figure
below. A portion of the energy of the ball is lost each time it hits the floor, until it eventually rolls to a stop. In this
apparatus, the collisions are perfectly elastic. The balls have just as much energy after a collision as before (postulate
Any object in motion has a kinetic energy that is defined as one-half of the product
of its mass times its velocity squared.
KE = 1/2 mv2
At any time, some of the ball bearings on this apparatus are moving faster than others, but the system
can be described by an average kinetic energy. When we increase the "temperature" of the system by increasing the
voltage to the motors, we find that the average kinetic energy of the ball bearings increases (postulate 6).
How the Kinetic Molecular Theory Explains the Gas Laws
The kinetic molecular theory can be used to explain each of the experimentally determined gas laws.
The Link Between P and n
The pressure of a gas results from collisions between the gas particles and the walls of the container.
Each time a gas particle hits the wall, it exerts a force on the wall. An increase in the number of gas particles in the container
increases the frequency of collisions with the walls and therefore the pressure of the gas.
Amontons' Law (PT)
The last postulate of the kinetic molecular theory states that the average kinetic energy of a gas
particle depends only on the temperature of the gas. Thus, the average kinetic energy of the gas particles increases as the
gas becomes warmer. Because the mass of these particles is constant, their kinetic energy can only increase if the average
velocity of the particles increases. The faster these particles are moving when they hit the wall, the greater the force they
exert on the wall. Since the force per collision becomes larger as the temperature increases, the pressure of the gas must
increase as well.
Boyle's Law (P = 1/v)
Gases can be compressed because most of the volume of a gas is empty space. If we compress a gas without
changing its temperature, the average kinetic energy of the gas particles stays the same. There is no change in the speed
with which the particles move, but the container is smaller. Thus, the particles travel from one end of the container to the
other in a shorter period of time. This means that they hit the walls more often. Any increase in the frequency of collisions
with the walls must lead to an increase in the pressure of the gas. Thus, the pressure of a gas becomes larger as the volume
of the gas becomes smaller.
Charles' Law (V T)
The average kinetic energy of the particles in a gas is proportional to the temperature of the gas.
Because the mass of these particles is constant, the particles must move faster as the gas becomes warmer. If they move faster,
the particles will exert a greater force on the container each time they hit the walls, which leads to an increase in the
pressure of the gas. If the walls of the container are flexible, it will expand until the pressure of the gas once more balances
the pressure of the atmosphere. The volume of the gas therefore becomes larger as the temperature of the gas increases.
Avogadro's Hypothesis (V N)
As the number of gas particles increases, the frequency of collisions with the walls of the container
must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible containers, such as a balloon, will
expand until the pressure of the gas inside the balloon once again balances the pressure of the gas outside. Thus, the volume
of the gas is proportional to the number of gas particles.
Ideal vs. Real Gases
Kinetic Molecular Theory explains the behavior of what is called "Ideal Gases"; real gases behavior
only like real gases under two conditions
1. AT HIGH TEPERATURES gas particles do not attract each other, however, at extremely low temperatures
they do in some cases. For example, O2 combines with a free oxygen atom in the atmosphere
to make O3 (ozone)
2. At LOW PRESSURES gas particle volume does not matter becuase their is a great deal of distance seperating
them and most likely they will not combine. At high pressure, the gas particle volume is important and becomes a factor.