Heterogeneous Chemical Reactions in Solutions

R.R. Dogonadze and A.M. Kuznetsov, Kinetics of Heterogeneous Chemical Reactions in Solutions, VINITI, Moscow, 1978 (in Russian). 

Dogonadze RR, Kuznetsov AM (1978) Kinetics of heterogeneous chemical reactions in solutions. In Kinetics and catalysis. VINITI, Moscow, pp 223-254... 

Electrode reactions are heterogeneous chemical reactions in solution and may include elementary electron-transfer steps, ion transfer, potential-independent, or chemical steps, and so on. 

Heterogeneous chemical reactions in which adsorbed species participate are not pure chemical reactions, as the surface concentrations of these substances depend on the electrode potential (see Section 4.3.3), and thus the reaction rates are also functions of the potential. Formulation of the relationship between the current density in the stationary state and the concentrations of the adsorbing species in solution is very simple for a linear adsorption isotherm. Assume that the adsorbed substance B undergoes an... 

This chapter is meant to serve both as a guide for the beginner and as an overview for the nonelectrochemist with a need to know the methods available. Approximately half of the chapter is concerned with various aspects of linear sweep and cyclic voltammetry in view of the importance and widespread use of these techniques. Some general aspects of the heterogeneous electron transfer process, and the chemical reactions associated with it, are introduced in this part. Electrochemical reactions in which the electroactive substrate is formed in a chemical reaction in solution prior to the electron transfer [1-5] and catalysis of chemical reactions by electron transfer [6] are not included in this chapter. The reader interested in the details of such reactions should consult the presentations referred to. The reader is encouraged also to consult Chapter 1, where a number of basic electrochemical concepts are discussed in detail. 

Electrochemical reactions at an electrode snrface differ from normal heterogeneous chemical reactions in that they involve the participation of one or more electrons that are either added to (reduction) or removed from (oxidation) the reactant species. The explicit inclusion of electrons as reactants or products means that the reaction rate depends on the electric potential. Electron transfer processes occur within a small portion of the double layer immediately adjacent to the electrode surface (10 to 50 mn in thickness) where solution-phase electroneutrality does not hold and where very strong electric fields (on the order of 10 V/cm) exist during a charge transfer reaction. We begin the analysis of electrochemical kinetics by defining a generic electrode reaction ... 

Etching in solution comprises a heterogeneous chemical reaction in which an etchant reacts with a solid and oxidizes it to produce a soluble reaction product. It is important to note that if the reaction product were to be insoluble in the etchant, then the reaction would stop and etching would cease. 

The first reaction mechanism we are going to consider is the ECjrre mechanism, where E refers to a heterogeneous electron transfer reaction and Cirre to an irreversible chemical reaction in solution. The ECirre case enables... 

Electrode processes are a class of heterogeneous chemical reaction that involves the transfer of charge across the interface between a solid and an adjacent solution phase, either in equilibrium or under partial or total kinetic control. A simple type of electrode reaction involves electron transfer between an inert metal electrode and an ion or molecule in solution. Oxidation of an electroactive species corresponds to the transfer of electrons from the solution phase to the electrode (anodic), whereas electron transfer in the opposite direction results in the reduction of the species (cathodic). Electron transfer is only possible when the electroactive material is within molecular distances of the electrode surface thus for a simple electrode reaction involving solution species of the fonn... 

An electrode reaction is always a heterogeneous chemical process, in that it involves the passage of an electron from an electrode (metal or semiconductor) to a chemical species in solution, or vice versa. 

It is possible that the species Red generated at the electrode surface may be unstable and tend to decompose. It may also be involved in chemical reactions with other species present in solution while it is moving towards the mass of the solution (homogeneous chemical reactions) or while it is still adsorbed on the electrode surface (heterogeneous chemical reactions). Furthermore, the new species formed during such reactions may be electroactive. These kind of reactions are called following chemical reactions (following, obviously, the electron transfer). 

One of the simplest electrode reactions is the EC mechanism (also called a following chemical reaction) in which the electrogenerated species (R) rearranges or reacts with some other solution component (Z) at a rate characterized by the rate constant k. The EC mechanism is summarized by the following reaction sequence, in which the labels E and C identify the heterogeneous electron-transfer reaction (electrode reaction) and the subsequent homogeneous solution reaction (chemical reaction), respectively ... 

The selection of diffusion equation solutions included here are diffusion from films or sheets (hollow bodies) into liquids and solids as well as diffusion in the reverse direction, diffusion controlled evaporation from a surface, influence of barrier layers and diffusion through laminates, influence of swelling and heterogeneity of packaging materials, coupling of diffusion and chemical reactions in filled products as well as permeation through packaging. 

If one were to describe the essence of electrode kinetics in one short phrase, it would be the transition from electronic to ionic conduction, and the phenomena associated with and controlling this process. Conduction in the solution is ionic, whereas in the electrodes and the connecting wires it is electronic. The transition from one mode of conduction to the other requires charge transfer across the interfaces. This is a kinetic process. Its rate is controlled by the catalytic properties of the surface, the chemisorption of species, the concentration and the nature of the reacting species and all other parameters that control the rate of heterogeneous chemical reactions. 

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