Thermodynamics and Kinetics of Redox Reactions

The thermodynamics and kinetics of redox reactions with the participation of biologically active compounds. 

For first-year university students. This book bases itself initially on redox reactions as an introduction, covering the amounts and E/pH diagrams, before moving on to present the basics of electrochemistry (the thermodynamics and kinetics of electrode reactions), adapting its approach to the target reader. It includes a large number of basic exercises. 

The results previously discussed clearly demonstrate that electrochemical techniques are powerful tools to study compoimds of interest to human and animal health. Particularly, linear, cyclic, convolution, square wave voltammetries, and controlled potential bulk electrolysis allow inferring the reaction mechanism and perform a full thermodynamic and kinetics of redox couples controlled by diffusion, adsorption as well as those which show a mixed control diffusion/adsorption. On the other hand, the square wave voltammetry coupled to adsorptive accumulation of redox couples which are both electroactive, and show specific interactions with the electrode smface allows detecting and quantifying substrates at trace levels. 

Gray HB, Winkler JR (1996) Electron transfer in proteins. Annu Rev Biochem 65 537 Fedurco M (2000) Redox reactions of heme-containing metalloproteins dynamic effects of self-assembled monolayers on thermodynamics and kinetics of cytochrome c electron-transfer reactions. Coord Chem Rev 209 263... 

Both the thermodynamics and kinetics of electron transfer reactions (redox potentials and electron transfer rates) have steric contributions, and molecular mechanics calculations have been used to identity them. A large amount of data have been assembled on Co3+/Co2+ couples, and the majority of the molecular mechanics calculations reported so far have dealt with hexaaminecobalt (III/II) complexes. 

Before we generalize, we should discuss the question Can we estimate redox reactivity with the help of thermodynamics A simple affirmative answer would be wrong. But it is often useful to consider thermodynamics, in order to gain some insight into reaction mechanisms and kinetics. Most redox reactions are, from a stoichiometric point of view, processes where more than one electron is transferred. But most of these processes occur in a series of one-electron... 

Solvation of HO" (and other oxy anions, YO ) affects to a major degree the thermodynamics and kinetic for its reaction with electron-acceptor molecules. Solvation energies determine the ionization energy of HO (and its redox potential as an electron donor) and the electron affinity of the electron-acceptor molecule. 

The versatility of this mode of operation has made it extremely powerful for fabrication of microstructures. In the feedback mode an ultramicroelectrode is held close above a substrate in a solution containing one form of electroactive species, either reduced or oxidized, that serves as a mediator (Fig. 1). The latter is usually used both as a means of controlling the distance between the UME and the surface and to drive the microelectrochemical process on the surface. This poses a number of requirements that must be taken into account when configuring the system. The basic limitation stems from the requirement that the electrochemical reaction be confined only to the surface. This means that the electroactive species generated at the UME will react with the surface or with other species attached to it. In addition, it is preferable in most cases that the redox couple used should exhibit chemical and electrochemical reversibility, so that it is effectively regenerated on the surface. The regeneration of the redox couple on the surface is required for controlling the UME-substrate distance. Finally, the thermodynamics and kinetics of the electrochemical process on the surface will dictate the choice of the redox couple introduced. 

The last chapter in this introductory part covers the basic physical chemistry that is required for using the rest of the book. The main ideas of this chapter relate to basic thermodynamics and kinetics. The thermodynamic conditions determine whether a reaction will occur spontaneously, and if so whether the reaction releases energy and how much of the products are produced compared to the amount of reactants once the system reaches thermodynamic equilibrium. Kinetics, on the other hand, determine how fast a reaction occurs if it is thermodynamically favorable. In the natural environment, we have systems for which reactions would be thermodynamically favorable, but the kinetics are so slow that the system remains in a state of perpetual disequilibrium. A good example of one such system is our atmosphere, as is also covered later in Chapter 7. As part of the presentation of thermodynamics, a section on oxidation-reduction (redox) is included in this chapter. This is meant primarily as preparation for Chapter 16, but it is important to keep this material in mind for the rest of the book as well, since redox reactions are responsible for many of the elemental transitions in biogeochemical cycles. 

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

---