Energetic and Entropic Changes in Volume Work and Chemical Reactions

Abstract

In the present study, energetic and entropic changes are investigated on a comparative basis, as they occur in the volume changes of an ideal gas in the Carnot cycle and in the course of the chemical reaction in a lead-acid battery. Differences between reversible and irreversible processes have been worked out, in particular between reversibly exchanged entropy ( ∆ e S ) and irreversibly produced entropy (∆ i S ). In the partially irreversible case, ∆ e S and ∆ i S add up to the sum ∆ S for the volume changes of a gas, and only this function has an exact differential. In a chemical reaction, however, ∆ e S is independent on reversibility. It arises from the different intramolecular energy contents between products and reactants. Entropy production in a partially irreversible Carnot cycle is brought about through work-free expansions, whereas in the irreversible battery reaction entropy is produced via activated co mplexes, whereby a certain, variable fraction of the available chemical ene rgy becomes transformed into electrical energy and the remaining fraction dissipated into heat. The irreversible reaction process via activated complexes has been explained phenomenologically. For a sufficiently high power output of coupled reactions, it is essential that the input energy is not completely reversibly transformed, but rather partially dissipated, because this can increase the process velocity and consequently its power output. A reduction of the counter potential is necessary for this purpose. This is not only important for man-made machines, but also for the viability of cells.