Page 24 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

8 Chapter I
molecules of the air, colliding with the particles of the smoke. This
is Brownian motion.
The kinetic theory of gases is a model proposed to account for the
observed properties of gases. In order that the model is applicable,
certain assumptions are made. For this reason, gases are categorised
into two types: (a) ideal gases (as defined previously) and (b) real gases
(defined as non-ideal gases).

Assumptions of the Kinetic Theory for an Ideal Gas
1. Gases consist of tiny molecules, which are so small and so far
apart that the actual size of the molecules is negligible in
comparison to the large distance between them.
2. The molecules are totally independent of each other, i.e. there are
no attractive or repulsive forces between the molecules.
3. The molecules are in constant random motion. They collide with
each other and with the walls or sides of the container, which
changes the direction of linear motion.
4. For each elastic collision, there is no net loss of kinetic energy.
However, there may be transfer of energy between the particles
in such a collision.
5. The average kinetic energy, of all the molecules is assumed to be
proportional to the absolute temperature T (measured in K) i.e.
kinetic energy oc T.

Validity of the Assumptions of the Kinetic Theory for an Ideal Gas
Assumption I This is largely true, since the compressibility of gases is
very high. However, at high pressures, when a gas is highly com-
pressed, it is not valid to state that the physical size of gas molecules is
practically negligible compared with the distances between the mole-
cules.
Assumption 2 This assumption is approximate, since gases diffuse or
spread out to occupy all space available to them, i.e. there must be no
appreciable binding force between the molecules. However, van der
Waals forces exist (intermolecular forces of attraction and repulsion),
and with polar molecules, other attractive forces exist.
Assumption 3 This assumption is valid, as shown by Brownian
motion.
Assumptions 4 and 5 These assumptions are true, since in any elastic
collision, kinetic energy is not lost.