12.6 Kinetic Theory of Ideal Gases
The concept of atoms and molecules being the ultimate building
blocks of bulk matter was revived by Dalton in order to explain
the three laws of chemical combinations. In the mid 19th century,
these concepts, together with certain assumptions regarding the
nature, size, distribution and motions of the molecules and atoms,
were synthesized with Newtonian mechanics, in order to explain the
observed behavior of ideal gases. This work by Maxwell, Boltzman
and others, led to the development of what is known as the Kinetic
Theory of gases.
The assumptions of the Kinetic Theory are :
i) Any gas is made up of a very large number of molecules.
ii) Molecules of a gas are identical to each other, with respect to shape, size and mass.
iii) The molecules are perfectly rigid spheres of negligible radius.
iv) The molecules are in an incessant state of chaotic motion in all possible speeds and directions.
v) The distribution of molecules is homogenous and isotropic in any container enclosing the gas.
vi) The molecules exert force on each other only when they collide with each other or with
the walls of the container.
vii) The collision between molecules or molecules and the walls of the container are perfectly
elastic collisions, i.e., only such collisions can change the directions of motion but not their speeds.
viii) Between successive collisions the molecules travel freely with constant velocities; the distance
travelled freely is called free path. On an average, the mean free path of all molecules is the same.
ix) The average Kinetic energy of a molecule is proportional to the absolute temperature of the gas.