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CHAPTER 13

Kinetic Molecular

Theory

13.1. INTRODUCTION
In Chap. 12 laws governing the behavior of gases were presented. The fact that gases exert pressure was
stated, but no reasons why gases should exhibit such behavior were given. The kinetic molecular theory (KMT)
explains all the gas laws that we have studied and some additional ones. It describes gases in terms of the behavior
of the molecules that make them up. (The noble gases exist as individual atoms, but for purposes of explaining
the KMT they will be included and treated as molecules. They may be thought of as monatomic molecules.)

13.2. POSTULATES OF THE KINETIC MOLECULAR THEORY
All gases, under ordinary conditions of temperature and pressure, are made of molecules (including one-
atom molecules such as are present in samples of the noble gases). That is, ionic substances do not form gases
under conditions prevalent on earth. The molecules of a gas act according to the following postulates:
1. Molecules are in constant random motion.
2. The molecules exhibit negligible intermolecular attractions or repulsions except when they collide.
3. Molecular collisions are elastic, which means that although the molecules transfer energy from one to another,
as a whole they do not lose kinetic energy when they collide with one another or with the walls of their
container.
4. The molecules occupy a negligible fraction of the volume occupied by the gas as a whole.
5. The average kinetic energy of the gas molecules is directly proportional to the absolute temperature of the
gas.
3
1
KE = kT = mv 2
2 2
The overbars mean “average.” The k in the proportionality constant is called the Boltzmann constant.Itis
equal to R, the ideal gas law constant (with unfamiliar units), divided by Avogadro’s number. Note that this
k is the same for all gases.
Postulate 1 means that molecules move in any direction whatsoever until they collide with another molecule
or a wall, whereupon they bounce off and move in another direction until their next collision. Postulate 2 means
that the molecules move in a straight line at constant speed between collisions. Postulate 3 means that there is no
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