The Nature of Gases: Part
1
Kinetic Theory and a Model for 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 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
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
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.