Conceptual issues related to reversibility and reversible work produced in closed and open flow systems

The concepts of reversibility and reversible work are subtle and illusive. Students often face difficulties in understanding these concepts. The definitions of reversibility given in textbooks on thermodynamics are generally descriptive and qualitative, subject to interpretation. The reversible work often discussed in the textbooks is actually the work produced in a mechanically reversible process and not the work produced in a completely reversible process. Although the P − V work produced in a mechanically reversible process is a path-dependent quantity, the work produced in a completely reversible process (closed or open system) is not a path-dependent quantity. It is a state function provided that the system interacts with a single thermal energy reservoir. In this article, the precise definitions of reversibility which are unambiguous and quantitative are discussed. The work produced in a reversible process is analyzed and discussed in detail. Both closed and open systems are considered. The material described in this article is suitable for third year chemical engineering students who have completed an introductory engineering thermodynamics course in their second year.