Discussion on definition and understanding of the thermodynamic spontaneous process

Abstract The spontaneous process is one of the most important concepts in thermodynamics. However, there are widespread confusions about it because of markedly different definitions in many textbooks of physical chemistry as well as its daily and nonscientific usage. To clarify the confusions, definitions of the spontaneous process in a number of widely used textbooks are reviewed and analyzed. Then, key points to correctly understand the thermodynamic spontaneous process are examined, and two modified definitions based on definitions in textbooks of physical chemistry are proposed. The recommended definition is that any closed system that is in equilibrium with its surroundings but not at equilibrium internally has the tendency to change to an equilibrium state, and this tendency of change is called the spontaneous process or the spontaneous direction. It emphasizes that the foremost precondition to define the spontaneous process is the equilibrium between the closed system and its surroundings. Accordingly, the “external influence” in several common definitions of the spontaneous process is revealed to be the energy irreversibly exchanged between the system and its surroundings, rather than energy or work in many definitions. Besides, typical criteria of the spontaneous process are discussed to reveal their necessary conditions based on proposed definitions, and some unanswered questions in textbooks of physical chemistry are resolved. The whole discussion would be helpful for chemistry lecturers and students to precisely understand the spontaneous process.