Irreversible process entropy criteria

A chemical reaction is an irreversible process that produces entropy. The general criterion of irreversibility is d S > 0. Criteria applicable under particular conditions are readily obtained from the Gibbs equation. The changes in thermodynamic potentials for chemical reactions yield the affinity A. All four potentials U, H, A, and G decrease as a chemical reaction proceeds. The rate of reaction, which is the change of the extent of the reaction with time, has the same sign as the affinity. The reaction system is in equilibrium state when the affinity is zero. 

Rather lately, an attempt to generalize this principle to a nonlinear irreversible process has been made by Klein, based on a statistical method for a simplified model. He demonstrated that the minimum production properties are also a useful approximation criterion for the stationary state even when the latter is very far from equilibrium. According to a simple example of an irreversible process given in his calculation, it has been found that the entropy production does not decrease monotonically, but passes through its minimum en route to the stationary state. We have already mentioned these results in connection with the variational principle in nonlinear irreversible processes. 

The method developed here is in many ways analogous to that employed by Schottky, Ulich and Wagner. Both methods emphasize the criterion for establishing the irreversibility of a chemical reaction and for deciding whether the reaction will proceed spontaneously in a particular direction. In De Bonder s method this criterion appears immediately the production of entropy must be positive. On the other hand Schottky, Ulich and Wagner employ as the criterion of irreversibility the loss of useful work associated with the real process when compared with a hypothetical reversible process. As is shown in chap. V, these criteria are equivalent for isothermal changes. For non-isothermal changes, however, the concept of loss of useful work... 

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