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electrolysis(ĭlĕktrŏl`əsĭs), 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 electrolyteelectrolyte
, 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. , an ionized solution or molten metallic salt, complete an electric circuit between two electrodes. When the electrodes are connected to a source of direct current one, called the cathode, becomes negatively (−) charged while the other, called the anode, becomes positively (+) charged. The positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode, through the wiring and an electric generator, and then back to the cathode constitutes the current in the external circuit.
For example, when electrodes are dipped into a solution of hydrogen chloride (a compound of hydrogen and chlorine) and a current is passed through it, hydrogen gas bubbles off at the cathode and chlorine at the anode. This occurs because hydrogen chloride dissociates (see dissociationdissociation,
in chemistry, separation of a substance into atoms or ions. Thermal dissociation occurs at high temperatures. For example, hydrogen molecules (H2
..... Click the link for more information. ) into hydrogen ions (hydrogen atoms that have lost an electron) and chloride ions (chlorine atoms that have gained an electron) when dissolved in water. When the electrodes are connected to a source of direct current, the hydrogen ions are attracted to the cathode, where they each gain an electron, becoming hydrogen atoms again. Hydrogen atoms pair off into hydrogen molecules that bubble off as hydrogen gas. Similarly, chlorine ions are attracted to the anode, where they each give up an electron, become chlorine atoms, join in pairs, and bubble off as chlorine gas.
Commercial Applications of Electrolysis
Various substances are prepared commercially by electrolysis, e.g., chlorine by the electrolysis of a solution of common salt; hydrogen by the electrolysis of water; heavy water (deuterium oxide) for use in nuclear reactors, also by electrolysis of water. A metal such as aluminum is refined by electrolysis. A solution of aluminum oxide in a molten mineral decomposes into pure aluminum at the cathode and into oxygen at the anode. In these examples the electrodes are inert.
In electroplating, the plating metal is generally the anode, and the object to be plated is the cathode. A solution of a salt of the plating metal is the electrolyte. The plating metal is deposited on the cathode, and the anode replenishes the supply of positive ions, thus gradually being dissolved. Electrotype printing plates, silverware, and chrome automobile trim are plated by electrolysis.
The English scientist Michael Faraday discovered that the amount of a material deposited on an electrode is proportional to the amount of electricity used. The ratio of the amount of material deposited in grams to the amount of electricity used is the electrochemical equivalent of the material. Actual electric consumption may be as high as four times the theoretical consumption because of such factors as heat loss and undesirable side reactions.
An electric cell is an electrolytic system in which a chemical reaction causes a current to flow in an external circuit; it essentially reverses electrolysis. A battery is a single electric cell (or two or more such cells linked together for additional power) used as a source of electrical energy. Metal corrosion can take place by electrolysis in an unintentionally created electric cell. The Italian physicist Alessandro VoltaVolta, Alessandro, Conte
, 1745–1827, Italian physicist. He was professor of physics at the Univ. of Pavia from 1779 and became famous for his work in electricity. Napoleon I made him a count and a senator of the kingdom of Lombardy.
..... Click the link for more information. discovered the principle of the electric cell (see 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. ) in 1800. Within a few weeks William Nicholson and Sir Anthony Carlisle, English scientists, performed the first electrolysis, breaking water down into oxygen and hydrogen.
the aggregate of electrochemical oxidation-reduction processes occurring at electrodes immersed in an electrolyte upon the passage of an electric current through the electrolyte. Electrolysis forms the basis of the electrochemical method of the laboratory and industrial production of various substances, both simple substances (electrolysis in the narrow sense of the word) and complex ones (see).
The first studies and applications of electrolysis date to the end of the 18th and beginning of the 19th centuries, when electrochemistry was becoming established as a science (see). Of great importance in the development of the theoretical basis of electrolysis was M. Faraday’s discovery (1833–34) of the exact relationship between the amount of electric current consumed during electrolysis and the amount of substance produced at each electrode (seeFARADAYS LAWS.) The industrial use of electrolysis became possible after the development of powerful direct-current generators in the 1870’s.
A unique feature of electrolysis is the spatial separation of the oxidation and reduction processes. The electrochemical oxidation occurs at the anode, and the reduction occurs at the cathode. Electrolysis is performed in special apparatus called electrolyzers. Electrolysis is induced by the energy of direct current supply and the energy released during chemical transformations at the electrodes. The power consumed in electrolysis is expended on raising the Gibbs free energy of the system in the course of the formation of the desired products and is partially dissipated in the form of heat in the course of overcoming the resistance in the electrolyzer and in other parts of the electric circuit (seeGIBBS FREE ENERGY).
As a result of electrolysis, the reduction of the ions or molecules of the electrolyte with the formation of new products occurs at the cathode. The cations accept electrons and are transformed into ions of lower oxidation state or into atoms, for example, in the reduction of iron ions (Fe3+ + e– → Fe2+) and the electrodeposition of copper (Cu2+ + 2e– → Cu). Neutral molecules may participate in transformations at the cathode directly, or they may react with the intermediate products of the cathodic process. The oxidation of the ions or molecules found in the electrolyte or within the anode material (the anode dissolves or is oxidized) occurs at the anode, for example, the liberation of oxygen (4OH– → 4e– + 2H2O + O2), the liberation of chlorine (2Cl– → 2e– + Cl2), the formation of chromate (Cr3+ + 3OH– + H2O→CrO42– + 5H+ + 3e–), the dissolution of copper (Cu → Cu2+ + 2e–), and the oxidation of aluminum (2A1 + 3H2O → Al2O3 + 6H+ + 6e–). The electrochemical production of a substance (in the atomic, molecular, or ionic state) involves the transfer of one or several charges from the electrode to the electrolyte (or vice versa) according to the chemical reaction equation. In the latter case, this is achieved, as a rule, through a sequence of one-electron steps, with the formation at the electrode of intermediate ions or radical species, which frequently remain at the electrode in the adsorbed state.
The rates of the electrode reactions depend on the composition and concentration of the electrolyte, on the electrode material, on the electrode potential, and on temperature, among other factors. The rate of each reaction is determined by the rate of electric charge transfer per unit of electrode surface per unit time. Thus, the current density serves as a measure of the rate.
The amount of product formed in electrolysis is determined by Faraday’s laws. If a number of products are formed concurrently at each electrode as a result of several electrochemical reactions, the amount of current (given in percent) consumed in the formation of the product of each of these reactions is called the current efficiency of the given product.
The advantages of electrolysis over chemical methods for the production of various products lie in the relative ease of control of the rate and selectivity of reaction by regulating the current. Electrolysis conditions are easily controlled. As a consequence, processes may be performed in either the “mildest” or “most vigorous” oxidation or reduction conditions and produce the strongest oxidizing and reducing agents used in science and industry. Electrolysis is the principal method for the industrial production of aluminum, chlorine, and sodium hydroxide and an important method for the production of fluorine, alkali metals, and alkaline-earth metals. It is an efficient method of refining metals. Hydrogen and oxygen are produced by the electrolysis of water. The electrochemical method is used for the synthesis of various classes of organic compounds and many oxidizing agents (persulfates, permanganates, perchlorates, and perfluoro-organic compounds). The use of electrolysis for the treatment of surfaces includes cathodic electroplating processes in machine building, instrument-making, and the aviation, electrical engineering, and electronics industries, as well as in anodic polishing and etching processes, anodic mechanical sizing, and the oxidation (anodizing) of metal articles. Metal installations and structures are protected from corrosion by electrolysis under controlled conditions (anodic and cathodic protection).
REFERENCESSee references under .
E. V. KASATKIN