The Nature of Gases
Kinetic Theory and a Model for Gases
Objectives
 Upon completion of this presentation, you
will be able to …
 state the Kinetic Theory of Gases.
 state the assumptions of the Kinetic Theory of
Gases.
 apply the assumptions of the Kinetic Theory
to ideal and real gases.
Introduction
 Kinetic means motion.

Kinetic energy, KE, is the energy of motion.
 The kinetic theory is a model of matter where ...

... all matter is composed of tiny particles ...

... in constant motion.
 The kinetic theory applies to all states of matter.
 We will apply it first to gases and then extend
the theory to liquids and solids.
Introduction
 The kinetic theory describes the behavior of
matter in its various states.
 There are a series of assumptions we make
about matter in order to use the theory.
 These assumptions help us to understand
how to apply the theory to ...

... gases ...

... liquids ...

... and solids.
First Assumption
 The particles in a gas are considered to be very
small, very hard spheres with an insignificant
volume.
 This means that the individual gas particles ...

are atoms or small molecules

are very far apart in relation to the size of the
particles

have very little attraction or repulsion towards
one another

move independently of each other
Second Assumption
 The motion of the particles in a gas is rapid,
constant, and random.
 This means that the individual gas particles
...
 spread out to fill any volume or shape of
container
 travel in straight lines until they encounter
another particle or another object
 change direction only after a collision
Second Assumption
 Experimental measurements of gas
molecules show that they move quite
rapidly, even at room temperature.
 O2 molecules have an average speed of
1,700 km/hr (1,060 mph).
 However, they only travel about 70 nm until
they encounter another particle.

This is about 500 times their diameter
 Each gas molecule travels in a very crooked
path called a random walk.
Third Assumption
 All collisions between gas particles are
perfectly elastic.
 This means that the individual gas particles
...
 transfer kinetic energy during a collision
 collide without a loss of kinetic energy
 have a total kinetic energy that remains
constant
Applications
 We can use these assumptions to
understand the behavior of real gases.
 Compressibility:

Gases are compressible.

This can be explained by the first assumption:

the small size of gas particles

the large distance between gas particles
Applications
 We can use these assumptions to
understand the behavior of real gases.
 Expansion:

Gases expand to fill all available space of a
container.

This can be explained by the second
assumption:

gas particles move rapidly, constantly, and
randomly

this movement will allow the gas particles to
move to the limits of the container
Applications
 We can use these assumptions to
understand the behavior of real gases.
 Density:

Gases are by far the least dense of the states
of matter.

This can be explained by the first assumption:

the combination of the small size of gas
particles and the large distance between gas
particles leads to a very low density.
Note:
 The textbook lists only 3 assumptions of the
kinetic theory of matter.
 There are, in fact, many more assumptions in
a complete treatment of the kinetic theory.
 All of these assumptions help us to better
understand the nature of matter in all of its
states.
Note:
 Some examples of additional assumptions
include:
 The number of molecules is so large that
statistical treatment can be applied.
 The average kinetic energy of the gas
particles depends only on the temperature
of the system.
 The time during collision of molecule with the
container's wall is negligible as comparable
to the time between successive collisions.
 The equations of motion of the molecules are
time-reversible.
Summary
 First Assumption:
 The particles in a gas are considered to be
very small, very hard spheres with an
insignificant volume.
 Second Assumption:
 The motion of the particles in a gas is rapid,
constant, and random.
 Third Assumption:
 All collisions between gas particles are
perfectly elastic.