Abstract:

Current criteria for spontaneity are used with the confined conditions. The various definitions of spontaneous change show the lack of universality. Here, the first law of thermodynamics was put in the total entropy criterion without any constraints. Then another form of entropy criterion was obtained and discussed. The nature of energy change in thermodynamics processes was summarized based on the characteristics of spontaneous process. As a result, the universal definition of spontaneous process was put forward. The original total entropy criterion can distinguish between the reversible and irreversible processes, but not the spontaneous and non-spontaneous processes. Another form of the entropy criterion, i.e., the exergy criterion for the arbitrary process in the closed system, was proposed. This criterion can distinguish between the spontaneous and non-spontaneous processes. Through the theoretical derivation and the actual examples, we found that the exergy criterion is equivalent to the total entropy criterion. The exergy criterion can be returned to all kinds of direction criteria when using the corresponding limited conditions. In the paper, the spontaneity criteria for the processes with variable temperature or pressure were also given, which have not appeared in the current textbooks. In summary, the universal definition and the exergy criterion can solve the problems arisen from the confusing conception and explanation related to spontaneous processes.

Key words: Closed system, Arbitrary process, Spontaneous change, Non-spontaneous change, Exergy, Criteria for spontaneity