Homogeneous reactions, intrinsic

It is a good idea to run the laboratory reactor without catalyst to check for homogeneous reactions. However, this method does not work when the homogeneous reaction involves reactants that do not occur in the feed but are created by a heterogeneous reaction. It then becomes important to maintain the same ratio of free volume to catalyst volume in the laboratory reactor used for intrinsic kinetic studies as in the pilot or production reactors. 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

In the following sections, we shall explore the applicability of such relationships to experimental data for some simple outer-sphere reactions involving transition-metal complexes. In keeping with the distinction between intrinsic and thermodynamic barriers [eq 7], exchange reactions will be considered first, followed by a comparison of driving force effects for related electrochemical and homogeneous reactions. 

The fact that the reaction rates in solid phase synthesis are not drastically reduced, compared to the homogeneous reactions, indicates that the diffusion of the reagent into the polymeric matrix is not a limiting factor for the method. This has been confirmed by Andreatta and Rinkll9) in kinetic studies on both cross-linked and linear polystyrenes. This means that the intrinsic problems of solid phase synthesis arise from deviations in the linear kinetic course in the final stages of reaction due to non-equivalence of functional groups. 

First, the variation in the intrinsic barriers, AG, for related electrochemical reactions can be expected to be closely similar to those for the same series of homogeneous reactions using a fixed coreactant. If the comparison is made at a fixed electrode potential, E, the (often unknown) driving-force terms cancel provided that the free-energy profiles are symmetrical (the symmetry factor a. = 0.5) so that ... 

This result shows that the distinguishing feature of a zero-order reaction is that the concentration of reactant decreases linearly with time. It is difficult to cite a homogeneous reaction that is intrinsically zero order, although many reactions have apparent zero-order characteristics when the concentration of the species is large. However, in some heterogeneous reactions where the solid phase acts as a catalyst the rate is zero order. An example is the decomposition of NH3 on platinum and tungsten surfaces. ... 

By carrying out a homogeneous reaction in the organic phase with the organic substrate and a known quantity of QY (instead of using MY), the intrinsic rate constant of the organic reaction can be found. 

At this point, distinguishing among the various possibilities is impossible. Instead, one purpose of this paper is to discuss some of the consequences of assuming that the observed decay of P is an intrinsic bi-exponential process arising from homogeneous reaction centers, i.e., case (3) above. Another purpose of this report is to explore the correlation between reduction potential of the primary donor and the reaction rate of the primary event. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

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