Gases

Kinetic-Molecular Theory


Kinetic-Molecular Theory

The ideal gas equation

PV = nRT

describes how gases behave.

But, why do gases behave this way?

What happens to gas particles when conditions such as pressure and temperature change?

The Kinetic-Molecular Theory ("the theory of moving molecules"; Rudolf Clausius, 1857)

  1. Gases consist of large numbers of molecules (or atoms, in the case of the noble gases) that are in continuous, random motion
  2. The volume of all the molecules of the gas is negligible compared to the total volume in which the gas is contained
  3. Attractive and repulsive forces between gas molecules is negligible
  4. The average kinetic energy of the molecules does not change with time (as long as the temperature of the gas remains constant). Energy can be transferred between molecules during collisions (but the collisions are perfectly elastic)
  5. The average kinetic energy of the molecules is proportional to absolute temperature. At any given temperature, the molecules of all gases have the same average kinetic energy. In other words, if I have two gas samples, both at the same temperature, then the average kinetic energy for the collection of gas molecules in one sample is equal to the average kinetic energy for the collection of gas molecules in the other sample.


Pressure

 


Absolute Temperature


Molecular Speed

What is the speed (velocity) of a molecule possessing average kinetic energy?

Example:

Suppose we have four molecules in our gas sample. Their speeds are 3.0, 4.5, 5.2 and 8.3 m/s.


Application of the "Kinetic Molecular Theory" to the Gas Laws

Effect of a volume increase at a constant temperature

Effect of a temperature increase at constant volume


1996 Michael Blaber