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oxidation and reduction
oxidation and reduction, complementary chemical reactions characterized by the loss or gain, respectively, of one or more electrons by an atom or molecule. Originally the term oxidation was used to refer to a reaction in which oxygen combined with an element or compound, e.g., the reaction of magnesium with oxygen to form magnesium oxide or the combination of carbon monoxide with oxygen to form carbon dioxide. Similarly, reduction referred to a decrease in the amount of oxygen in a substance or its complete removal, e.g., the reaction of cupric oxide and hydrogen to form copper and water.
When an atom or molecule combines with oxygen, it tends to give up electrons to the oxygen in forming a chemical bond. Similarly, when it loses oxygen, it tends to gain electrons. Such changes are now described in terms of changes in the oxidation number, or oxidation state, of the atom or molecule (see valence). Thus oxidation has come to be defined as a loss of electrons or an increase in oxidation number, while reduction is defined as a gain of electrons or a decrease in oxidation number, whether or not oxygen itself is actually involved in the reaction.
In the formation of magnesium oxide from magnesium and oxygen, the magnesium atoms have lost two electrons, or the oxidation number has increased from zero to +2. This is also true when magnesium reacts with chlorine to form magnesium chloride. In solution, ferrous iron (oxidation number +2) may be oxidized to ferric iron (oxidation number +3) by the loss of an electron. In the reduction of cupric oxide the oxidation number of copper has changed from +2 to zero by the gain of two electrons. The two processes, oxidation and reduction, occur simultaneously and in chemically equivalent quantities. In the formation of magnesium chloride, for every magnesium atom oxidized by a loss of two electrons, two chlorine atoms are reduced by a gain of one electron each.
Oxidation-reduction reactions, called also redox reactions, are most simply balanced in the form of chemical equations by arranging the quantities of the substances involved so that the number of electrons lost by one substance is equaled by the number gained by another substance. In such reactions, the substance losing electrons (undergoing oxidation) is said to be an electron donor, or reductant, since its lost electrons are given to and reduce the other substance. Conversely, the substance that is gaining electrons (undergoing reduction) is said to be an electron acceptor, or oxidant.
Common reductants (substances readily oxidized) are the active metals, hydrogen, hydrogen sulfide, carbon, carbon monoxide, and sulfurous acid. Common oxidants (substances readily reduced) include the halogens (especially fluorine and chlorine), oxygen, ozone, potassium permanganate, potassium dichromate, nitric acid, and concentrated sulfuric acid. Some substances are capable of acting either as reductants or as oxidants, e.g., hydrogen peroxide and nitrous acid.
The corrosion of metals is a naturally occurring redox reaction. Industrially, many redox reactions are of great importance: combustion of fuels; electrolysis (oxidation occurs at the anode and reduction at the cathode); and metallurgical processes in which free metals are obtained from their ores.
(1) A supplemental musical notation for piano, first violin, or accordion that gives the primary melodies played by other instruments, showing the entrances of whole sections or solo instruments. Reductions are used extensively in adaptations or transcriptions of orchestral works for smaller combinations of instruments. They allow such orchestral works to be performed without a score or a conductor, since several instruments’ parts can be played on one instrument.
(2) An abridged score of shorter works (marches, dances, song transcriptions) for bands. All the parts are written on three or four musical staffs and are left untransposed, even for those instruments in keys other than C.
the restoration of a previous state or the breakdown of the complex into a simpler form. In various branches of science and technology, “reduction” refers to processes that result in a decrease in the dimensions or a simplification of the structure of an object. It can also refer to a weakening of the tension or force and sometimes to the complete disappearance of an object.
in biology, a decrease in the size or a simplification of the structure of an organ; reduction often refers to a loss of inherent organic functions in the course of the individual development of an organism (ontogeny) or the historical development of an organism (phylogeny). The complete disappearance of an organ or of tissue is sometimes called reduction.
the confiscation of crown lands that had been acquired by the feudal aristocracy, carried out by the monarchy in a number of European states in the 16th and 17th centuries.
Reduction was carried out on the largest scale and acquired its greatest significance in the second half of the 17th century in Sweden, where it was done to strengthen absolutism. Partial reduction was decided on by the Riksdag in 1655, during the reign of Charles X. Extensive reduction was carried out under Charles XI, whose policies were supported by the nobility with small landholdings, the burghers, and the richer peasants; the process was initiated in 1680 and was basically concluded by 1700. As a result of this policy, the extent of large-scale land-holding by the Swedish nobility, both in Sweden and in its dependent territories, was reduced approximately by one-half as compared to the first half of the 17th century.
In Poland, the decision to carry out a reduction (egzekucja, rewindykacja) of crown lands distributed after 1504 was taken at the sejm of 1562–63 at the insistence of the nobility. Reduction in Poland represented one stage in the struggle within the ruling class for the redistribution of landholdings.
in linguistics, a weakening of vowel sounds in unstressed position. In quantitative reduction, vowel length is decreased. In qualitative reduction, distinct articulation is lost, resulting in a change in the way the vowel is formed and a neutralization of phonemic opposition. In Russian, the degree of reduction depends on the position of the vowel relative to word stress, as in zolotoi (“golden”), which can be represented phonetically as [zəlʌtój]. Sometimes the term “reduction” is used to refer to any change from a fuller form of a linguistic element (sound, word, sentence) to a shorter form. The term “reduced” sometimes refers to all reduced sounds, positional variants and independent phonemes alike.
a methodological tool that plays an important role in logic, mathematics, and other deductive sciences.
Reduction involves a transformation of given data (problems, propositions) into the form most convenient from a certain standpoint, for example, a form that is logically simpler and more easily analyzed. Reduction of one problem to another plays a dual role: on the one hand, the solution of the second problem is applicable to the first problem, and on the other hand, the impossibility of solving the first problem—at least with certain specified methods—means that the second problem is also unsolvable by the same methods. Reduction can thus be used to derive from a positive (negative) solution of one problem a positive (negative) solution of an entire class of problems. The term “reduction” is also used in a natural sense to refer to inferences, methods of proof (such as reductio ad absurdum), concepts, conceptions, and theories.
In astronomy and geodesy, the term “reduction” is used to refer to the reduction of observations and measurements from one frame of reference to another through the introduction of corrections needed to account for the influence of various factors; these corrections are themselves often referred to as reductions. For example, the position of a star observed at a given moment differs from the position as given in star catalogs because of precession, nutation, and the star’s proper motion. Astronomical observations made at different moments of time are therefore corrected, or reduced, to a single epoch. Geodetic measurements made at some point of the earth’s surface are reduced by means of computations to a neighboring point or the corresponding point of the reference ellipsoid.
the relative magnitude of deformation imparted to a billet during forging operations, such as drawing, upsetting, or extrusion. Reduction is characterized by the reduction coefficient, which is defined as the ratio of the initial and final crosssectional area of the deformed billet or as the corresponding ratio of the billet’s initial and final length and height. The coefficient also depends on the shape of the die faces (flat, notched, or composite), the temperature, and the rate of deformation. The reduction coefficient may be 10 or more, in which case the coarsegrained cast structure of the original ingot acquires a clearly expressed fibrous structure. This ensures that the metal has uniform properties and high-quality mechanical characteristics in the longitudinal direction.
An evaluation strategy (or reduction strategy), determines which part of an expression (which redex) to reduce first. There are many such strategies.
See graph reduction, string reduction, normal order reduction, applicative order reduction, parallel reduction, alpha conversion, beta conversion, delta conversion, eta conversion.