Chemical and Biomolecular Engineering, Department of


Date of this Version



Chemical Engineering Education (Virginia Polytechnic Institute and State University)


Copyright © 2004, ChE Division of ASEE. Used by permission


Coupling refers to a flux occurring without its primary thermodynamic driving force; for example, mass flux without a concentration gradient called the thermal diffusion is a well-known coupled process. Coupling also refers to a flux occurring in a direction opposite to the direction imposed by its driving force; for example, a mass flux can occur from a low to a high concentration region and is called the active transport or uphill transport, such as potassium and sodium pumps coupled to chemical energy released by the hydrolysis of adenosine triphosphate (ATP) in biological systems. Although the coupled processes seem to be in conflict with the principles of second law of thermodynamics, interestingly, the second law allows the progress of a process against its driving force and hence with a decrease in entropy DSj < 0, but only if it is coupled with another process with larger positive entropy change, i.e. DSk >> 0, thus producing a positive total entropy change (DSj + DSk) > 0. This is consistent with the second-law statement that a finite amount of organization may be obtained at the expense of a greater amount of disorganization in a series of coupled spontaneous processes. This can have important implications in describing the coupled phenomena and organized structures in complex systems, such as biological energy conversion cycles