KINETIC MOLECULAR THEORY OF GASESKinetic molecular theory of gases
Assumptions of the kinetic molecular theory of gases
The kinetic theory of gases is a physical model that may be expressed in mathematical
form. It assumes:
1.Gases consist of tiny particles (molecules or atoms) which are in chaotic and
random motion.
2.The particles do notattract each other.
3.The collision of two molecules does not alter the overall energy of both molecules.
4.The volume of the particles is negligible compared with the volume of the container.
As we have already noted, assumption 1 immediately provides us with an explana-
tion of gas pressure. The particles frequently collide with the molecules which make
up the container walls: it is the force of these collisions which (on a gigantic scale) is
responsible for the pressure of a gas.
Assumption 2 is a simplifying feature which applies onlyto gases at low pressures
where the particles are so far apart from each other that they behave as independent
‘mathematical points’. Attractive forces cannotbe ignored in the solid and liquid state
nor in explaining why gases condense to form liquids. This is one reason why solids and
liquids are theoretically more complicated to deal with than gases, and why there is no
simple equivalent of the ideal gas equation (discussed below) for these states of matter.
Assumption 3 is another way of stating that the average kinetic energy of particles
in a gas remains fixed at a particular temperature. If the energy of particles were lost
in collisions (either wall–gas particle or gas particle–gas particle collisions) the
total energy of the gas would be continuously draining away, and (contrary to experi-
mental observation) the pressure of a gas would fall with time.
Kinetic energy of gas molecules
Since the molecules of a gas are in constant motion, they all possess kinetic energy.
The greater the kinetic energy of a gas molecule, the greater is its speed. Experiments
show that molecules in a sample of gas are not all travelling at the same speed.
Figure 10.9 shows the spread of molecular speeds of nitrogen at 300 K and 3000 K,
and of chlorine gas at 300 K.
10.4
161Air
Use the data in Table 10.1 to calculate
the volume of argon in 1.0 dm^3 of air.
What mass of argon is contained in
1.0 kg of air? (Assume the air is dry and
at sea level.)Exercise 10H