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HY DO GASES BEHAVE THE WAY they do? In some of the previous

chapters, we have used gases as examples with an implicit under-

standing that we are modeling their behavior, but we never really got into a

discussion of why they have that behavior.

The basic understanding of how gases behave is called the kinetic theory of

gases. It is based on several assumptions, mostly related to classical mechanics.

In part because of this, most of the basics of the kinetic theory were worked

out in the 1860s by Ludwig Boltzmann and James Clerk Maxwell (two names

that should be familiar by now). It was the culmination of over two centuries

of investigations into the nature of the gas phase, starting with Boyle and con-

tinuing through Gay-Lussac, Charles, Amonton, Dalton, and Graham, among

others.

In this chapter, we will review the kinetic theory of gases, focusing on ideal

gases. This review allows us to revisit some of the topics from Chapter 1 (in

which we discussed the nature of gases in a more phenomenological perspec-

tive) so that we can better apply these ideas when we consider chemical reac-

tions in the gas phase in the next chapter.

19.1 Synopsis

What we will find in this chapter is that the physical behavior of gases can be

understood if some simple assumptions are made. Suppose we treat an indi-

vidual gas particle as a hard piece of matter: What are the properties of a col-

lection of “hard pieces of matter”? It turns out that we can predict some prop-

erties by applying classical, rather than quantum, mechanics. The physical

behavior of gases can be considered as a statistical average of all of the indi-

vidual gas particles, so some of the ideas in this chapter are reminiscent of sta-

tistical thermodynamics. In addition, we will be focusing on the physical be-

havior of gases, not their chemical behavior. Because chemistry depends on

electrons, it is vital to understand how electrons behave in order to understand

how chemicals behave; that is, we need quantum mechanics. But in order to

understand the physical behavior of matter, we are able to use more simple

physical theories of nature. Some of these physical behaviors are relevant to

chemical behavior, as we will see in chemical kinetics.

19.1 Synopsis

19.2 Postulates and Pressure

19.3 Definitions and Distributions
of Velocities of Gas Particles

19.4 Collisions of Gas Particles

19.5 Effusion and Diffusion

19.6 Summary

The Kinetic Theory of Gases