oxidation
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Redox (reduction–oxidation, , ) is a type of
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which the
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to different atoms were fully Ionic bond, ionic. It describes the degree of oxidation (loss of elec ...
s of substrate change. Oxidation is the loss of
electrons The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. There are two classes of redox reactions: * ''Electron-transfer'' – Only one (usually) electron flows from the reducing agent to the oxidant. This type of redox reaction is often discussed in terms of redox couples and electrode potentials. * ''Atom transfer'' – An atom transfers from one substrate to another. For example, in the
rusting Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous ferric oxides, hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxi ...
of iron, the oxidation state of iron atoms increases as the iron converts to an oxide, and simultaneously the oxidation state of oxygen decreases as it accepts electrons released by the iron. Although oxidation reactions are commonly associated with the formation of oxides, other chemical species can serve the same function. In
hydrogenation Hydrogenation is a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions e ...
, C=C (and other) bonds are reduced by transfer of hydrogen atoms.


Terminology

"Redox" is a
combination In mathematics, a combination is a selection of items from a set that has distinct members, such that the order of selection does not matter (unlike permutations). For example, given three fruits, say an apple, an orange and a pear, there are th ...
of the words "reduction" and "oxidation". The term "redox" was first used in 1928. The processes of oxidation and reduction occur simultaneously and cannot occur independently. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or ''reducing agent'' loses electrons and is oxidized, and the oxidant or ''oxidizing agent'' gains electrons and is reduced. The pair of an oxidizing and reducing agent that is involved in a particular reaction is called a ''redox pair''. A ''redox couple'' is a reducing species and its corresponding oxidizing form, e.g., / .The oxidation alone and the reduction alone are each called a ''
half-reaction A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. ...
'' because two half-reactions always occur together to form a whole reaction.


Oxidants

''Oxidation'' originally implied a reaction with oxygen to form an oxide. Later, the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. Ultimately, the meaning was generalized to include all processes involving the loss of electrons. Substances that have the ability to ''oxidize'' other substances (cause them to lose electrons) are said to be ''oxidative'' or ''oxidizing'', and are known as
oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
s, oxidants, or oxidizers. The oxidant (oxidizing agent) removes electrons from another substance, and is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is also called an
electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself redox, reduced in the process. Electron acceptors are sometimes ...
. Oxidants are usually chemical substances with elements in high oxidation states (e.g., , , , , ), or else highly
electronegative Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
elements (e.g. O2, F2, Cl2, Br2, I2) that can gain extra electrons by oxidizing another substance. Oxidizers are oxidants, but the term is mainly reserved for sources of oxygen, particularly in the context of explosions.
Nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
is an oxidizer.
Oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
is the quintessential oxidizer.


Reducers

Substances that have the ability to ''reduce'' other substances (cause them to gain electrons) are said to be ''reductive'' or ''reducing'' and are known as
reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...
s, reductants, or reducers. The reductant (reducing agent) transfers electrons to another substance and is thus itself oxidized. And, because it donates electrons, the reducing agent is also called an electron donor. Electron donors can also form
charge transfer complex In chemistry, a charge-transfer (CT) complex or electron-donor-acceptor complex describes a type of supramolecular assembly of two or more molecules or ions. The assembly consists of two molecules that self-attract through Electrostatics, elect ...
es with electron acceptors. The word ''reduction'' originally referred to the loss in weight upon heating a metallic
ore Ore is natural Rock (geology), rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Ret ...
such as a
metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other chemical element, element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of ...
to extract the metal. In other words, ore was "reduced" to metal.
Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794), When reduced without charcoal, it gave off an air which supported respiration and combustion in an enhanced way. He concluded that this was just a pure form of common air and th ...
demonstrated that this loss of weight was due to the loss of oxygen as a gas. Later, scientists realized that the metal atom gains electrons in this process. The meaning of ''reduction'' then became generalized to include all processes involving a gain of electrons. Reducing equivalent refers to
chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entity, molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels ...
which transfer the equivalent of one
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
in redox reactions. The term is common in biochemistry. A reducing equivalent can be an electron, a hydrogen atom, as a hydride ion. Reductants in chemistry are very diverse.
Electropositive Electronegativity, symbolized as '' χ'', is the tendency for an atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. ...
elemental
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
s, such as
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the Symbol (chemistry), symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it ...
,
sodium Sodium is a chemical element with the Symbol (chemistry), symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the ...
,
magnesium Magnesium is a chemical element with the Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group ...
,
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
,
zinc Zinc is a chemical element with the Symbol (chemistry), symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 eleme ...
, and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substan ...
, are good reducing agents. These metals donate or ''give away'' electrons relatively readily. They transfer electrons. ''Hydride transfer reagents'', such as NaBH4 and LiAlH4, reduce by atom transfer: they transfer the equivalent of hydride or H. These reagents widely used in the reduction of
carbonyl In organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that c ...
compounds to
alcohols In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, st ...
. A related method of reduction involves the use of hydrogen gas (H2) as sources of H atoms.


Electronation and deelectronation

The electrochemist John Bockris proposed the words ''electronation'' and ''deelectronation'' to describe reduction and oxidation processes, respectively, when they occur at
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of Electric battery, batteries that can ...
s. These words are analogous to
protonation In chemistry, protonation (or hydronation) is the adding of a proton#Hydrogen ion, proton (or hydron (chemistry), hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is ...
and
deprotonation Deprotonation (or dehydronation) is the removal (transfer) of a proton#Hydrogen ion, proton (or hydron (chemistry), hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online C ...
. They have not been widely adopted by chemists worldwide, although
IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
has recognized the term electronation.


Rates, mechanisms, and energies

Redox reactions can occur slowly, as in the formation of
rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous ferric oxides, hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxi ...
, or rapidly, as in the case of burning fuel. Electron transfer reactions are generally fast, occurring within the time of mixing. The mechanisms of atom-transfer reactions are highly variable because many kinds of atoms can be transferred. Such reactions can also be quite complex, i.e. involve many steps. The mechanisms of electron-transfer reactions occur by two distinct pathways,
inner sphere electron transfer Inner sphere electron transfer (IS ET) or bonded electron transfer is a redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss ...
and
outer sphere electron transfer Outer sphere refers to an electron transfer (ET) event that occurs between chemical species that remain separate and intact before, during, and after the ET event. In contrast, for inner sphere electron transfer the participating redox sites underg ...
. Analysis of bond energies and ionization energies in water allow calculation of the thermodynamic aspects of redox reactions.


Standard electrode potentials (reduction potentials)

Each half-reaction has a ''standard
electrode potential In electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between Electric potential, electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, w ...
'' (''E''), which is equal to the potential difference or
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), w ...
at equilibrium under
standard conditions Standard temperature and pressure (STP) are Technical standard, standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the I ...
of an
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
in which the
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in ...
reaction is the
half-reaction A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. ...
considered, and the
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
is a standard hydrogen electrode where hydrogen is oxidized: : H2 → H+ + e The electrode potential of each half-reaction is also known as its ''reduction potential'' ''E'', or potential when the half-reaction takes place at a cathode. The reduction potential is a measure of the tendency of the oxidizing agent to be reduced. Its value is zero for H+ + e →  H2 by definition, positive for oxidizing agents stronger than H+ (e.g., +2.866 V for F2) and negative for oxidizing agents that are weaker than H+ (e.g., −0.763 V for Zn2+). For a redox reaction that takes place in a cell, the potential difference is: :''E'' = ''E'' – ''E'' However, the potential of the reaction at the anode is sometimes expressed as an ''oxidation potential'': :''E'' = –''E'' The oxidation potential is a measure of the tendency of the reducing agent to be oxidized but does not represent the physical potential at an electrode. With this notation, the cell voltage equation is written with a plus sign :''E'' = ''E'' + ''E''


Examples of redox reactions

In the reaction between
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the chemical ...
and
fluorine Fluorine is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical c ...
, hydrogen is being oxidized and fluorine is being reduced: : This reaction is spontaneous and releases 542 kJ per 2 g of hydrogen because the H-F bond is much stronger than the F-F bond. This reaction can be analyzed as two
half-reaction A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. ...
s. The oxidation reaction converts hydrogen to protons: : The reduction reaction converts fluorine to the fluoride anion: : The half reactions are combined so that the electrons cancel: : The protons and fluoride combine to form
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an imp ...
in a non-redox reaction: :2 H+ + 2 F → 2 HF The overall reaction is: :


Metal displacement

In this type of reaction, a metal atom in a compound (or in a solution) is replaced by an atom of another metal. For example,
copper Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and electrical conductivity. A fre ...
is deposited when
zinc Zinc is a chemical element with the Symbol (chemistry), symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 eleme ...
metal is placed in a
copper(II) sulfate Copper(II) sulfate, also known as copper sulphate, is an inorganic compound with the chemical formula . It forms hydrates , where ''n'' can range from 1 to 7. The pentahydrate (''n'' = 5), a bright blue crystal, is the most commonly encountered hy ...
solution: : In the above reaction, zinc metal displaces the copper(II) ion from copper sulfate solution and thus liberates free copper metal. The reaction is spontaneous and releases 213 kJ per 65 g of zinc. The ionic equation for this reaction is: : As two
half-reaction A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. ...
s, it is seen that the zinc is oxidized: : And the copper is reduced: :


Other examples

* The reduction of
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wat ...
to
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
in the presence of an acid (
denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denitr ...
): :: * The
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
of
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic, and their odors are usuall ...
s, such as in an
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combust ...
, produces
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
,
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
, some partially oxidized forms such as
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, and heat
energy In physics, energy (from Ancient Greek: wikt:ἐνέργεια#Ancient_Greek, ἐνέργεια, ''enérgeia'', “activity”) is the physical quantity, quantitative physical property, property that is #Energy transfer, transferred to a phy ...
. Complete oxidation of materials containing
carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electrons available to form covalent bond, covalent chemical bonds. It belongs to gro ...
produces carbon dioxide. * The stepwise oxidation of a hydrocarbon by oxygen, in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
, produces water and, successively: an
alcohol Alcohol most commonly refers to: * Alcohol (chemistry) In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. The ...
, an
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl group ...
or a
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...
, a
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with substituent, R referring to the alkyl, alkenyl, aryl, or other group. ...
, and then a
peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where R = any Chemical element, element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The ...
.


Corrosion and rusting

* The term
corrosion Corrosion is a Erosion, natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosio ...
refers to the electrochemical oxidation of metals in reaction with an oxidant such as oxygen.
Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous ferric oxides, hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxi ...
ing, the formation of
iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are nonstoichiometric, non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the ...
s, is a well-known example of electrochemical corrosion; it forms as a result of the oxidation of
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundance ...
metal. Common rust often refers to
iron(III) oxide Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally a ...
, formed in the following chemical reaction: :: * The oxidation of iron(II) to iron(III) by
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...
in the presence of an acid: :: :: :Here the overall equation involves adding the reduction equation to twice the oxidation equation, so that the electrons cancel: ::


Disproportionation

A
disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term ca ...
reaction is one in which a single substance is both oxidized and reduced. For example, thiosulfate ion with sulfur in oxidation state +2 can react in the presence of acid to form elemental sulfur (oxidation state 0) and
sulfur dioxide Sulfur dioxide ( IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcani ...
(oxidation state +4). : Thus one sulfur atom is reduced from +2 to 0, while the other is oxidized from +2 to +4.


Redox reactions in industry

Cathodic protection Cathodic protection (CP; ) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrific ...
is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects protected metal to a more easily corroded "
sacrificial anode A galvanic anode, or sacrificial anode, is the main component of a galvanic cathodic protection system used to protect buried or submerged metal structures from corrosion. They are made from a metal alloy with a more "active" voltage (more n ...
" to act as the anode. The sacrificial metal instead of the protected metal, then, corrodes. A common application of cathodic protection is in galvanized steel, in which a sacrificial coating of zinc on steel parts protects them from rust. Oxidation is used in a wide variety of industries such as in the production of
cleaning products Cleaning agents or hard-surface cleaners are substances (usually liquids, powders, sprays, or granules) used to remove dirt, including dust, stains, bad smells, and clutter on surfaces. Purposes of cleaning agents include health, beauty, removing ...
and oxidizing
ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
to produce
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
. Redox reactions are the foundation of
electrochemical cell An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
s, which can generate electrical energy or support electrosynthesis. Metal
ore Ore is natural Rock (geology), rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Ret ...
s often contain metals in oxidized states such as oxides or sulfides, from which the pure metals are extracted by
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including Silver mining#Ore processing, silver, iron-making, iron, copper extracti ...
at high temperature in the presence of a reducing agent. The process of
electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...
uses redox reactions to coat objects with a thin layer of a material, as in chrome-plated automotive parts, silver plating
cutlery Cutlery (also referred to as silverware, flatware, or tableware), includes any hand Tool, implement used in preparing, serving, and especially eating food in Western culture. A person who makes or sells cutlery is called a cutler. The city of Sh ...
,
galvanization Galvanization or galvanizing (American and British English spelling differences, also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot ...
and gold-plated
jewelry Jewellery (British English, UK) or jewelry (American English, U.S.) consists of decorative items worn for personal adornment, such as brooches, ring (jewellery), rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be at ...
.


Redox reactions in biology

Top:
ascorbic acid Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a Topical medication, topical 'serum' ingredient to treat melasma (dar ...
(
reduced form In statistics, and particularly in econometrics, the reduced form of a simultaneous equations model, system of equations is the result of solving the system for the endogenous variables. This gives the latter as functions of the exogenous variables, ...
of
Vitamin C Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a Topical medication, topical 'serum' ingredient to treat melasma (dar ...
)
Bottom: dehydroascorbic acid ( oxidized form of
Vitamin C Vitamin C (also known as ascorbic acid and ascorbate) is a water-soluble vitamin found in citrus and other fruits and vegetables, also sold as a dietary supplement and as a Topical medication, topical 'serum' ingredient to treat melasma (dar ...
)
Many important
biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
processes involve redox reactions. Before some of these processes can begin iron must be assimilated from the environment.
Cellular respiration Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
, for instance, is the oxidation of
glucose Glucose is a simple sugar with the Chemical formula#Molecular formula, molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis f ...
(C6H12O6) to CO2 and the reduction of
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
to
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
. The summary equation for cell respiration is: : The process of cell respiration also depends heavily on the reduction of NAD+ to NADH and the reverse reaction (the oxidation of NADH to NAD+).
Photosynthesis Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemica ...
and cellular respiration are complementary, but photosynthesis is not the reverse of the redox reaction in cell respiration: : Biological energy is frequently stored and released by means of redox reactions. Photosynthesis involves the reduction of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
into
sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
s and the oxidation of
water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
into molecular oxygen. The reverse reaction, respiration, oxidizes sugars to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce
nicotinamide adenine dinucleotide Nicotinamide adenine dinucleotide (NAD) is a Cofactor (biochemistry), coenzyme central to metabolism. Found in all living cell (biology), cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphat ...
(NAD+) to NADH, which then contributes to the creation of a
proton gradient An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts, the chemical gradient, or difference in solute concentration across a membrane, and ...
, which drives the synthesis of
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all ...
(ATP) and is maintained by the reduction of oxygen. In animal cells,
mitochondria A mitochondrion (; ) is an organelle found in the cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which i ...
perform similar functions. See the ''
Membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological Cell (biology), cell. That is, there is a difference in the energy required for e ...
'' article.
Free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Geron ...
reactions are redox reactions that occur as a part of
homeostasis In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism ...
and killing microorganisms, where an electron detaches from a molecule and then reattaches almost instantaneously. Free radicals are a part of redox molecules and can become harmful to the human body if they do not reattach to the redox molecule or an
antioxidant Antioxidants are compounds that inhibit oxidation, a chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to anothe ...
. Unsatisfied free radicals can spur the mutation of cells they encounter and are, thus, causes of cancer. The term ''redox state'' is often used to describe the balance of GSH/GSSG, NAD+/NADH and NADP+/NADPH in a biological system such as a cell or organ. The redox state is reflected in the balance of several sets of metabolites (e.g., lactate and
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the keto acids, alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate acid, conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throug ...
, beta-hydroxybutyrate, and
acetoacetate Acetoacetic acid (also acetoacetate and diacetic acid) is the organic compound with the formula CH3COCH2COOH. It is the simplest beta-keto acid, and like other members of this class, it is unstable. The methyl and ethyl esters, which are quite st ...
), whose interconversion is dependent on these ratios. An abnormal redox state can develop in a variety of deleterious situations, such as hypoxia, shock, and
sepsis Sepsis, formerly known as septicemia (septicaemia in British English) or blood poisoning, is a life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs. This initial stage is follo ...
. Redox mechanism also control some cellular processes. Redox proteins and their genes must be co-located for redox regulation according to the
CoRR hypothesis The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("redox") state of its gene products. CoRR is short for "co-location for redox regulation ...
for the function of DNA in mitochondria and chloroplasts.


Redox cycling

Wide varieties of
aromatic compounds Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...
are enzymatically reduced to form
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
that contain one more electron than their parent compounds. In general, the electron donor is any of a wide variety of flavoenzymes and their coenzymes. Once formed, these anion free radicals reduce molecular oxygen to
superoxide In chemistry, a superoxide is a Chemical compound, compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen species, reactive oxygen ion superoxide is p ...
and regenerate the unchanged parent compound. The net reaction is the oxidation of the flavoenzyme's coenzymes and the reduction of molecular oxygen to form superoxide. This catalytic behavior has been described as a futile cycle or redox cycling.


Redox reactions in geology

Minerals are generally oxidized derivatives of metals. Iron is mined as its
magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to be ...
(Fe3O4). Titanium is mined as its dioxide, usually in the form of
rutile Rutile is an oxide mineral The oxide mineral class includes those minerals in which the oxide anion (O2−) is bonded to one or more metal alloys. The hydroxide-bearing minerals are typically included in the oxide class. The minerals with compl ...
(TiO2). To obtain the corresponding metals, these oxides must be reduced, which is often achieved by heating these oxides with carbon or carbon monoxide as reducing agents. Blast furnaces are the reactors where iron oxides and coke (a form of carbon) are combined to produce molten iron.The main chemical reaction producing the molten iron is: :


Redox reactions in soils

Electron transfer reactions are central to myriad processes and properties in soils, and electron "activity", quantified as Eh (platinum electrode potential (voltage) relative to the standard hydrogen electrode) or pe (analogous to pH as -log electron activity), is a master variable, along with pH, that controls and is governed by chemical reactions and biological processes. Early theoretical research with applications to flooded soils and paddy rice production was seminal for subsequent work on thermodynamic aspects of redox and plant root growth in soils. Later work built on this foundation, and expanded it for understanding redox reactions related to heavy metal oxidation state changes, pedogenesis and morphology, organic compound degradation and formation, free radical chemistry, wetland delineation, soil remediation, and various methodological approaches for characterizing the redox status of soils.


Mnemonics

The key terms involved in redox can be confusing. For example, a reagent that is oxidized loses electrons; however, that reagent is referred to as the reducing agent. Likewise, a reagent that is reduced gains electrons and is referred to as the oxidizing agent. These mnemonics are commonly used by students to help memorise the terminology: * "Oil rig, OIL RIG" — oxidation is loss of electrons, reduction is gain of electrons * "LEO the lion says GER [grr]" — loss of electrons is oxidation, gain of electrons is reduction * "LEORA says GEROA" — the loss of electrons is called oxidation (reducing agent); the gain of electrons is called reduction (oxidizing agent). * "RED CAT" and "AN OX", or "AnOx RedCat" ("an ox-red cat") — reduction occurs at the cathode and the anode is for oxidation * "RED CAT gains what AN OX loses" – reduction at the cathode gains (electrons) what anode oxidation loses (electrons) * "PANIC" – Positive Anode and Negative is Cathode. This applies to electrolytic cells which release stored electricity, and can be recharged with electricity. PANIC does not apply to cells that can be recharged with redox materials. These galvanic cell, galvanic or voltaic cells, such as fuel cells, produce electricity from internal redox reactions. Here, the positive electrode is the cathode and the negative is the anode.


See also

* Anaerobic respiration * Bessemer process * Bioremediation * Calvin cycle * Chemical equation * Chemical looping combustion * Citric acid cycle * Electrochemical series * Electrochemistry * Electrolysis * Electron equivalent * Electron transport chain * Electrosynthesis * Galvanic cell * Hydrogenation *
Membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological Cell (biology), cell. That is, there is a difference in the energy required for e ...
* Microbial fuel cell * Murburn concept * Nucleophilic abstraction * Organic redox reaction * Oxidative addition, Oxidative addition and reductive elimination * Oxidative phosphorylation * Partial oxidation * Pro-oxidant * Redox gradient * Redox potential * Reducing agent * Reducing atmosphere * Reduction potential * Exothermic, Thermic reaction * Transmetalation * Sulfur cycle


References


Further reading

* *


External links


Chemical Equation Balancer
– An open-source chemical equation balancer that handles redox reactions.




Online redox reaction equation balancer, balances equations of any half-cell and full reactions
{{Authority control Soil chemistry Chemical reactions Articles containing video clips Redox Reaction mechanisms