ELECTROCHEMICAL REACTION

any process either caused or accompanied by the passage of an electric current and involving in most cases the transfer of electrons between two substancesone a solid and the other a liquid. Under ordinary conditions, the occurrence of a chemical reaction is accompanied by the liberation or absorption of heat and not of any other form of energy; but there are many chemical reactions thatwhen allowed to proceed in contact with two electronic conductors, separated by conducting wiresliberate what is called electrical energy, and an electric current is generated. Conversely, the energy of an electric current can be used to bring about many chemical reactions that do not occur spontaneously. A process involving the direct conversion of chemical energy when suitably organized constitutes an electrical cell. A process whereby electrical energy is converted directly into chemical energy is one of electrolysis; i.e., an electrolytic process. By virtue of their combined chemical energy, the products of an electrolytic process have a tendency to react spontaneously with one another, reproducing the substances that were reactants and were therefore consumed during the electrolysis. If this reverse reaction is allowed to occur under proper conditions, a large proportion of the electrical energy used in the electrolysis may be regenerated. This possibility is made use of in accumulators or storage cells, sets of which are known as storage batteries. The charging of an accumulator is a process of electrolysis; a chemical change is produced by the electric current passing through it. In the discharge of the cell, the reverse chemical change occurs, the accumulator acting as a cell that produces an electric current. Finally, the passage of electricity through gases generally causes chemical changes, and this kind of reaction forms a separate branch of electrochemistry that will not be treated here. any process either caused or accompanied by the passage of an electric current and involving in most cases the transfer of electrons between two substancesone a solid and the other a liquid. The breadth of occurrence and application of electrochemical reactions is vast. Included are such major categories of activity as electrolysis, metallurgy, battery and fuel cell technology, electroplating, analytical chemistry, and biological research. There are many spontaneously occurring chemical reactions which, when allowed to proceed under special circumstances, liberate electrical energy in the form of a current. A process of this type involves the direct conversion of the chemical energy, which is liberated in a reaction, to electrical energy. (An apparatus whereby such a process is brought about constitutes an electrical cell.) Conversely, the energy of an electric current can be utilized to produce many chemical reactions that do not occur spontaneously. In processes of this kind, electrical energy is directly converted into chemical energy, which is stored up in the products of the reactions. Electrolysis is such an electrochemical process. Because of their chemical energy, the products of electrolysis have a tendency to react spontaneously with one another, reproducing the substances that were consumed during the electrolytic process. If this (reverse) reaction is allowed to occur under proper conditions a large proportion of the electrical energy used in electrolysis may be regenerated. This possibility is utilized in accumulators or storage cells, sets of which are known as storage batteries. Storage cells are often referred to as secondary cells to distinguish them from primary cells, which are not designed or constructed for recharging after their original supply of energy has been consumed. The charging of an accumulator is a process of electrolysis; a definite chemical change is produced by the electric current passing through it. In the discharge of the cell, the reverse chemical change occurs spontaneously; the accumulator is acting now as a cell that produces an electric current. The storage of electrical energy in a secondary cell thus involves its conversion into chemical energy, which can be reconverted into electrical energy when desired. Substances that are reasonably good conductors of electricity may be divided into two groups: metallic, or electronic, conductors and electrolytic conductors. The metals and a few substances such as graphite, manganese dioxide, and lead sulfide exhibit metallic conductivity; the passage of an electric current through them produces heating and magnetic effects but no chemical changes. Electrolytic conductors or electrolytes comprise most acids, bases, and salts, either in the molten condition or in solution in water or other solvents. Plates or rods composed of a suitable metallic conductor dipping into the fluid electrolyte are employed to conduct the current into and out of the liquid; i.e., to act as electrodes. When a current is passed through an electrolyte between suitable electrodes, not only are heating and magnetic effects produced, but also definite chemical changes occur at or in the neighbourhood of the electrodes, the process being one of electrolysis. At the negative electrode, or cathode, the chemical change may be either the deposition of a metal or the liberation of hydrogen and formation of a basic substance or some other chemical reduction process; whereas at the positive electrode, or anode, it may be either the dissolution of the anode itself, the liberation of a nonmetal, the production of oxygen and an acidic substance, or some other chemical oxidation process. In some cases, these primary products of electrolysis then react with the electrolyte or with the material of which the electrodes are composed, yielding secondary products. Additional reading Engineering aspects are presented by C.L. Mantell, Electrochemical Engineering, 4th ed. (1960); and Geoffry Prentice, Electrochemical Engineering Principles (1991). John O'M. Bockris and Amulya K.N. Reddy, Modern Electrochemistry, 2 vol. (1970), is a textbook, relatively easy to understand, that is slanted toward readers in other fields who want to use electrochemical reactions. John O'M. Bockris and D.M. Drazic, Electro-Chemical Science (1972), is another simple text. Additional introductory works are Philip H. Rieger, Electrochemistry (1987); Bryant W. Rossiter and John F. Hamilton (eds.), Physical Methods of Chemistry, 2nd ed., vol. 2, Electrochemical Methods (1986); and Allen Bard and Larry R. Faulkner, Electrochemical Methods: Fundamentals and Applications (1980). More advanced works include K.J. Vetter, Electrochemical Kinetics, rev. ed. (1967), the first textbook on the fundamentals and the standard work; Paul Delahay, Double Layer and Electrode Kinetics (1965); B.E. Conway, Theory and Principles of Electrode Processes (1965); Ilana Fried, The Chemistry of Electrode Processes (1973); Electrochemistry (irregular), part of the Topics in Current Chemistry series; and Allen J. Bard, Encyclopedia of Electrochemistry of the Elements (1973 ). John O'M. Bockris Aleksandar R. Despic The Editors of the Encyclopdia Britannica