The Need for a Second Law

For a scientist, the primary interest in thermodynamics is in predicting the spontaneous direction of natural processes, chemical or physical, in which by spontaneous we mean those changes that occur irreversibly in the absence of restraining forces—for example, the free expansion of a gas or the vaporization of a hquid above its boiling point. The first law of thermodynamics, which is useful in keeping account of heat and energy balances, makes no distinction between reversible and irreversible processes and makes no statement about the natural direction of a chemical or physical transformation. 

As we noticed in Table 5.1, AC/ = 0 both for the free expansion and for the reversible expansion of an ideal gas. We used an ideal gas as a convenient example because we could calculate easily the heat and work exchanged. Actually, for any gas, AC/has the same value for a free and a reversible expansion between the corresponding initial and final states. Furthermore, AC/ for a compression is equal in magnitude and opposite in sign to AC/ for an expansion no indication occurs from the first law of which process is the spontaneous one. 

A clue to the direction that needs to be followed to reach a criterion of spontaneity can be obtained by noticing in Table 5.1 that Q and Ware equal to zero for the reversible cycle but are not zero for the irreversible cycle. In other words, it is changes in the surroundings as well as changes in the system that must be considered in distinguishing a reversible from an irreversible transformation. Evidently, then, we need to find a 

Chemical Thermodynamics Basic Concepts and Methods, Seventh Edition. By Irving M. Klotz and Robert M. Rosenberg 

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But these observations, and other similar ones, could still hardly suggest the need for a second law. One could argue that experience suffices to predict these phenomena. 

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