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Entropy in the Realm of Chemical Reactions
Abstract
This chapter discusses some general properties of affinities that follow from the fact that chemical reactions can be interdependent when a substance is a reactant in more than one reaction. In many chemical reactions, the intermediate reactants are often in a steady state or nearly so. In a series of reactions in which intermediate compounds are produced and consumed, if all the intermediates are in a steady state, then it is possible to define an extent of reaction for the net reaction and write the rate of entropy production in terms of the affinity and the velocity of the net reaction. The concepts of chemical potential and affinity not only describe chemical reactions but also flow of matter from one region of space to another. Entropy, as formulated in the chapter, encompasses all aspects of transformations of matter: changes in energy, volume and composition.
... We choose to work with ideal systems for clarity of presentation, but we show in the Supplemental Materials (SI Appendix Section 3.2 ) that the results presented below remain valid for non-ideal solutions. Then, as in the examples, one can show that the affinity of the network at a macroscopic scale can be written as the logarithm of the ratio of an effective equilibrium constant K and a reaction quotient Q [30]: ...
... Given the form the fluxes of (21), the corresponding elementary affinities are given by [30,38] ...
... as required [30]. It should be emphasized at this stage that because the concentrations of the fuel/food species are absorbed in the kinetic constants, K is an effective equilibrium constant that includes these concentrations as well. ...
Autocatalytic chemical networks play a predominant role in a large number of natural systems such as in metabolic pathways and in ecological networks. In this work, we present a theoretical framework that identifies the thermodynamic conditions under which autocatalytic networks run optimally in a non-equilibrium stationary state. Our theory shows that the overall reaction associated with the network is aligned with the thermodynamic force that drives the system out of equilibrium. We also demonstrate that the thermodynamic force required to operate at a maximal flux obeys universal constraints that are independent of the kinetics, but solely determined by the stoichiometry of the overall process and by the structural properties of the underlying chemical reaction network.
Theoretical principles of chemical thermodynamics and a computer program that quickly provides all the relevant information. Keywords (Audience): Upper-Division Undergraduate
Thermodynamics and Free Energy of Chemical Substances
- G.N. Lewis
- M. Randall