Entropic Mechanism of Coupling Chemical Reactions in Open Systems

Entropic Mechanism of Coupling Chemical Reactions in Open Systems 

If the steady state concentrations of the components are shifted, but not too far from their equilibrium values, the interconnection between the fluxes and chemical forces (chemical affinities, in our case) should satisfy the well-known linear relationships that are usually postulated in the linear thermodynamics of irreversible processes [15-18]. We do not consider here the phenomenological equations of nonequilibrium thermodynamics. For details the reader can refer to numerous excellent monographs and review articles devoted to the applications of nonequilibrium thermodynamics in the description of chemical reactions and biological processes (see, for instance, [22-30]). In many cases, the conventional phenomenological approaches of linear and nonlinear nonequilibrium thermodynamics appear to be useful tools for the 

Being a useful theoretical instrument, the phenomenological approach, however, cannot clarify the molecular mechanisms of chemical processes. The famous Onsager reciprocal relationships were derived in their general form from the principle of the local reversibility of physical processes. For chemical reactions, some useful flux-force relationships can be derived if we know the mechanism of the reactions considered. In this way, in principle, it becomes possible to express the phenomenological Onsager proportionality coefficients through the kinetic constants of elementary steps. 

We want to illustrate here the behavior of the open systems using the simplest examples of monomolecular chemical reactions. For quantitative analysis, in this section we will consider thermodynamic and kinetic relations for coupled monomolecular reactions AoB and BoC), Analysis of the behavior of this reaction in an open system gives explicit and simple relationships, between the kinetic and thermodynamic characteristics of the reaction mixture on one hand, and the flux through the system on the other. However, for didactic reasons we start our analysis with a consideration of the simplest monomolecular reaction, the transformation of S into P. 

As noted above, a simple injection of A5 molecules of S into the equilibrium mixture of S and P will lead to producing AP molecules of P with the stoichiometry ratio rj= AP/AS = K/ 1 + K where K = is the equilib- 

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