Spontaneous Processes.
The second law of thermodynamics has led to results which are of considerable importance to chemistry, physics and engineering, but to the chemist its greatest value probably lies in the fact that it provides a means of foretelling whether a particular reaction can occur, and if so to what extent. However, thermodynamics can only indicate if the reaction is possible or not; other considerations which lie outside thermodynamics are necessary to determine whether the process will take place slowly or rapidly. Even with this limitation in mind, it must be admitted that information concerning the fundamental possibility of a reaction, apart from its speed, would be of great interest to the chemist. At one time it was believed that chemical changes always occurred spontaneously in the direction of heat evolution that is, in the direction leading to a decrease in the heat content. This conclusion is, however, manifestly incorrect, as is evident from the fact that many reactions which take place spontaneously are known to involve an absorption of heat.
The question being considered resolves itself into the problem of understanding the conditions under which spontaneous processes in general take place. It is convenient in this connection to examine some physical processes that are of spontaneous occurrence; the conclusions drawn are found to be applicable to all changes that tend to take place without external influence. Consider, for example, a bar of metal that is hot at one end and cold at the other; heat will be conducted spontaneously along the bar from the hot end to the cold end until the temperature is uniform. It is important to note, however, that this process is not found to reverse itself spontaneously; it has not been observed that a metal bar of uniform temperature spontaneously becomes hotter at one end and colder at the other.