electrochemistry(redirected from Electrochemical reduction)
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electrochemistry,science dealing with the relationship between electricity and chemical changes. Of principal interest are the reactions that take place between electrodeselectrode,
terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In most familiar circuits current is carried by metallic conductors, but in some circuits the current passes for some distance through a nonmetallic conductor.
..... Click the link for more information. and the electrolyteselectrolyte
, electrical conductor in which current is carried by ions rather than by free electrons (as in a metal). Electrolytes include water solutions of acids, bases, or salts; certain pure liquids; and molten salts.
..... Click the link for more information. in electric and electrolytic cells (see electrolysiselectrolysis
, passage of an electric current through a conducting solution or molten salt that is decomposed in the process. The Electrolytic Process
The electrolytic process requires that an electrolyte, an ionized solution or molten metallic salt, complete an
..... Click the link for more information. ), as well as the reactions that take place in an electrolyte as electricity passes through it. The principles of electrochemistry are applied in a variety of ways, e.g., in electroplating and in the generation of electricity by magnetohydrodynamicsmagnetohydrodynamics
, study of the motions of electrically conducting fluids and their interactions with magnetic fields. The principles of magnetohydrodynamics are of particular importance in plasma physics. See nuclear energy.
..... Click the link for more information. . See batterybattery, electric,
device that converts chemical energy into electrical energy, consisting of a group of electric cells that are connected to act as a source of direct current.
..... Click the link for more information. ; voltaic cellvoltaic cell,
a simple device with which chemical energy is converted into electrical energy. Two dissimilar metals (e.g., copper and zinc) are immersed in an electrolyte (e.g., a dissolved sulfate). If the metals are connected by an external circuit, one metal is reduced (i.e.
..... Click the link for more information. .
the branch of physical chemistry that studies the bulk and surface properties of solids and liquids containing mobile ions and the mechanisms of the processes involving the participation of the ions at phase boundaries and within the bulk. The practical importance of electrochemical processes, the role of the processes in living organisms, and the unique features of their experimental study have led to the establishment of electrochemistry as an independent scientific discipline.
Origin of basic electrochemical concepts. In 1800, A. Volta constructed the first source of continuous current (voltaic pile). He related the generation of an electromotive force (emf) to contact with different metals (contact theory). The British scientists W. Nicholson and A. Carlisle, using a voltaic pile, decomposed (1800) water into hydrogen and oxygen by electrolysis. In 1807, H. Davy produced metallic potassium by the electrolysis of moist potassium hydroxide, which was the first application of the electrochemical method to the production of a new substance. V. V. Petrov, who studied (1803) the electroreduction of metals from their oxides, laid the foundations for the study of electrochemistry in Russia. In 1833–34, M. Faraday established the most important quantitative laws of electrochemistry (seeFARADAYS LAWS). He also introduced the terms “electrolysis,” “electrolyte,” “electrode,” “cathode,” “anode,” “cation,” “anion,” and “ion,” although he still did not regard ions as separate entities. Faraday showed that the generation of electric energy by a galvanic cell is always accompanied by a chemical reaction. During this period (1838), the British scientist J. Daniell discovered the first galvanic cell and B. S. Iakobi discovered electroplating, the first widespread application of electrochemistry (seeELECTROPLATING TECHNOLOGY).
Study of current flow. R. Clausius demonstrated (1857) that free charged particles must exist in solutions that transmit current and that it is precisely the motion of these particles that produces the electric current. The development of a method for determining the transport number (seeTRANSPORT NUMBER) by J. Hittorf (1853–59) and the precise measurements of electrical conductivity by F. Kohlrausch (1874) proved the independent motion of ions and paved the way for S. Arrhenius’ theory of electrolytic dissociation (1887; see). The regularities of dissociation of weak electrolytes was established by W. Ostwald (1888). The possibility of the existence of free ions became clear after the introduction of the concept of the energy of solvation (for aqueous solutions, the energy of hydration). I. A. Kablukov was the first to note the necessity of accounting for such a chemical interaction. P. Debye and the German scientist E. Hiickel discovered (1923) that the properties of dilute solutions of strong electrolytes in solvents with high dielectric constant may be explained quantitatively upon the assumption of their complete dissociation by taking into account the electrostatic interaction between the charged particles. A theory was subsequently developed that was applicable to higher concentrations, and nonaqueous solutions and molten electrolytes were studied. Special attention was directed in subsequent decades to solid electrolytes with high ionic conductivity. Modern physical methods are used to study the interaction of ions with one another and with the solvent (seeCHEMICAL PHYSICS).
The application of thermodynamic laws to electrochemistry; study of electromotive forces. The quantitative examination of any electrolytic system, independent of molecular-statistical concepts, is based on thermodynamics. In 1851, W. Thomson (Lord Kelvin) concluded on the basis of the first law of thermodynamics that the emf of a galvanic cell E is determined by the enthalpy of the reaction that occurs in it. A thermodynamic explanation for the emf was given by J. W. Gibbs (1875) and H. L. F. von Helmholtz (1882). It follows from the second law of thermodynamics that the emf is determined not by the change in the total energy but by the free energy in the chemical reaction:
(1) E = –ΔG/nF
where ΔG is the difference in the Gibbs free energy of the products and the starting compounds, n is the number of electrons that participate in the reaction, and F is the faraday. A galvanic cell may yield electric energy only as a result of the loss of the free energy of the reactants. Equation (1) assumes the reversibility of all the processes in the cell, that is, satisfaction of the equilibrium conditions, and determines the maximum value of the electric energy that may be produced as a consequence of a given reaction. The relationship between E, the reaction enthalpy ΔH, and the absolute temperature T is given by the Gibbs-Helmholtz equation:
(2) E = –ΔH/nf + T∂E/∂T
W. Nernst provided (1889) a convenient form for the thermodynamic relations of electrochemistry. The emf E may be represented as the difference in the electrode potentials of the two electrodes, each of which gives the emf of a circuit consisting of the given electrode and some reference electrode, for example, the standard hydrogen electrode. In the simplest case of a metal in equilibrium with a dilute solution containing ions of the metal in concentration c,
(3) E = E0 + (RT/nF) In c (the Nernst equation)
where R is the gas constant and E0 is the standard electrode potential of the given electrode. In the general case, the value of c must be replaced by the ion activity. The general condition of equilibrium is determined by the need for the invariance of the electrochemical potential of any particle in the system in any part of the system.
Electrochemical kinetics. Modern electrochemistry focuses on electrochemical kinetics, that is, the study of the mechanism and the laws of the behavior of electrochemical reactions. Under real conditions, for example, in electrolysis, in metal corrosion, in chemical sources of current, and in living organisms, electrochemical equilibrium, as a rule, is not attained and the understanding of electrochemical processes requires knowledge of the kinetic regularities. Since the electron, which is regarded as the simplest stable chemical particle, is a necessary participant in processes on the boundary of a metal (or semiconductor) and an electrolyte, the study of the nature of electrochemical reaction steps is important for determining the chemical kinetics. The modern theory of reaction steps is based on quantum mechanics. A prerequisite for its development was the concept that charge transfer may determine the measured rate of an electrochemical process as a whole (slow discharge theory), a concept advanced (1930) by the German scientist M. Volmer and by T. Erdey-Grús. A. N. Frumkin established (1933) the quantitative relationship between the rate of an electrochemical reaction and the structure of the electrical double layer on the metal-electrolyte interface (seeDOUBLE LAYER). R. Gurney (Great Britain) first applied (1931) quantum mechanics to electrochemistry. In 1935, M. Polanyi (Hungary) and J. Horiuchi (Japan) laid the basis for the transition state theory, or the theory of the activated complex, later developed by H. Eyring (USA). According to the modern quantum theory, any charge transfer either on a phase boundary or within the bulk of the solution is related to a change in the structure of the polar solvent and the reorientation of its dipoles. The nature of the change in degrees of freedom of the classical and quantum system differs significantly. Particles that are strongly bound to the solvent, such as electrons and protons, have an inherent quantum nature of movement. Tunnel effects are probable for these particles. Quantum theory has provided a rational explanation for the empirically established relation of the rate of an irreversible process given in terms of the current density i and the electrochemical overvoltage η, or electrode potential, in the equation of the German scientist J. Tafel (1905) η = a + b log; i (where a and b are constants and log is the natural logarithm), and has indicated the limits of its applicability. The energy characteristics of the transition state and, thus, the rate of the process, depend on the nature of the metal and the presence of adsorbed foreign particles. These effects, which may lead to a significant acceleration of the process, are combined under the name electrocatalysis (see). In the case of electrochemical processes that are accompanied by the formation of a new phase, for example, in the electrodeposition of metals, the probability of the formation of nucleation centers and the conditions for crystal growth must also be taken into account.
Electrochemical kinetics also considers the structure of the phase boundary, especially of the metal-electrolyte interface, at which an electric field arises as a result of the spatial separation of charge, the double layer. G. Lippmann first proposed a method for studying the double layer (see). The theory of the double layer was developed subsequently by G. Gouy (France, 1910), O. Stern (Germany, 1924), Frumkin, and the American scientist D. Grahame. The introduction of the concept of zero-charge potential by Frumkin (1927) eliminated the contradiction between the contact and chemical theories of the emf.
Electrochemical processes consist of a series of steps (see). The prolonged passage of current requires the supply of the reactant from the solution to the electrode surface and the removal of the reaction products, which is achieved through diffusion; the migration of charged particles under the influence of an electric field must also be taken into account. The supply of reactant is accelerated by stirring the liquid, that is, through convective diffusion. The current produces a concentration polarization (seePOLARIZATION, ELECTROCHEMICAL). In addition to the charge-transfer steps and the diffusion steps, the overall process may include purely chemical and other types of steps, for example, the production of nucleation centers, the inclusion of discharged atoms in the crystal lattice, and the release of gas bubbles. The accumulation of intermediate products on the electrode surface above their equilibrium concentration and the slow rates of the diffusion processes and discharge steps lead to the polarization of the electrode and overvoltage. Overvoltage, which is negligibly small for current densities used in practice, indicates reversibility of the process. The degree of reversibility of the process as a whole increases with increasing exchange current between the starting compounds and the final reaction products at the equilibrium potential. The reversibility of a multistep process assumes the reversibility of all its steps. The irreversibility of a process is often a consequence of the slow rate of one of its steps, which determines the rate of the process as a whole. Various types of electrical measurements are used to ascertain the mechanism of electrochemical reactions: (1) the determination of the dependence of the potential on the direct-current density, (2) the measurement of the total electrical resistance, (3) the determination of the dependence of the potential or current on elapsed time with variously preprogrammed changes in a second variable with time, and (4) various nonlinear methods. The state of the electrode surfaces is also studied using optical methods, as is the surface tension.
Electrochemical kinetics is the basis of the modern theory of metal corrosion. In electrolytic solutions, corrosion is the result of two or more concurrent electrochemical processes. The development of accurate and convenient methods for the study of the mechanisms of electrode processes, especially the polarographic method proposed by J. Heyrovsky (seePOLAROGRAPHY), has proved to be of great importance for the development of electrochemical kinetics.
Practical significance. Electrochemical methods are widely used in various branches of industry. The chemical industry makes use of electrolysis, which is the most important method for the production of chlorine, alkalies, many oxidizing agents, fluorine, and organofluorine compounds. The electrosynthesis of the most varied chemical compounds has gained increasing importance. The production of aluminum, magnesium, sodium, lithium, beryllium, tantalum, titanium, and zinc and the refining of copper are based on electrochemical methods (see). Hydrogen is produced by the electrolysis of water on a relatively limited scale, although with the increasing exhaustion of natural fuel reserves and the increasing production of electric power, this method for the production of hydrogen will increase. Protective and decorative electroplating is used in various industries, as well as electroplating with given optical, mechanical, and magnetic properties. The anodic dissolution of metals is successfully replacing the mechanical treatment of hard and superhard metals and alloys. Electrochemical information converters are finding increasing use in industry (seeELECTROCHEMICAL ENGINEERING). Of major importance is the development of the electric automobile. The rapidly increasing demand for independent sources of electric power in industry, space exploration, and household uses has stimulated the search for new electrochemical systems with enhanced efficiency, power capacity, and safety. Various electrochemical methods of analysis and electro-physical and electrochemical methods of treatment are becoming increasingly more common (see and ELECTROPHYSICAL AND ELECTROCHEMICAL METHODS OF TREATMENT).
An understanding of the most important biological processes, such as the assimilation and use of food energy, the transmission of the nervous impulse, and the detection of the visual image is impossible without taking into account electrochemical links, which are related primarily to the functioning of biological membranes. The resolution of these problems sets new tasks for theoretical electrochemistry and should significantly affect future medical practice.
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A. N. FRUMKIN