Thermodynamics and molecular reality

The whole of classical thermodynamics is based on the empirical laws which lead to the notions of temperature, internal energy and entropy as functions of state. At no point in the development of these laws, or of their consequences, does any reference need to be made to the idea that matter consists of ultimate particles, i.e. the atomic theory. But thermodynamics gives an insight neither into the possible origin of its laws nor any means of making a direct calculation of the thermodynamic properties of a substance. It is the purpose of statistical mechanics to advance knowledge in both of these directions.

First of all it is shown that the laws of thermodynamics are a consequence of the postulates of the quantum theory, together with one other postulate which is of a statistical character. Secondly statistical mechanics makes it possible to obtain important new theorems not known in pure thermodynamics, including methods of calculating heat capacities, free energies, etc., from spectroscopic data.

Statistical mechanics is a large subject, and it is not possible to put forward an entirely rigorous treatment in a short compass. In the present chapter the main ideas will be developed in a fairly simple manner, and the formulae obtained will be applied in subsequent chapters to perfect gases, crystals and reaction kinetics.

The quantum states of macroscopic systems

The whole of this chapter is concerned with the quantum states of macroscopic systems.