oxidation-reduction reaction
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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 charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s of substrate change. Oxidation is the loss of
electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
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 iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
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 between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organ ...
, 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 " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
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. Oxidants are usually chemical substances with elements in high oxidation states (e.g., , , , , ), or else highly
electronegative Electronegativity, symbolized as , 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 atomic number and the d ...
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 nitri ...
is an oxidizer.
Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
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 me ...
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 In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. Typical reducing agents undergo permanent chemi ...
. 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 electrostatic forc ...
es with electron acceptors. The word ''reduction'' originally referred to the loss in weight upon heating a metallic
ore Ore is natural 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. Retrieved 7 Apr ...
such as a
metal oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
to extract the metal. In other words, ore was "reduced" to metal.
Antoine Lavoisier Antoine-Laurent de Lavoisier ( , ; ; 26 August 17438 May 1794),
CNRS (
chemical species A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the wa ...
which transfer the equivalent of one
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
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 , 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 atomic number and the ...
elemental
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s, such as
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
,
sodium Sodium is a chemical element with the 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 of the periodic table. Its only stable ...
,
magnesium Magnesium is a chemical element with the 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 2 of the periodic ta ...
,
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, Abundanc ...
,
zinc Zinc is a chemical element with the 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 (IIB) of the periodi ...
, and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
, 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, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
compounds to alcohols. A related method of reduction involves the use of hydrogen gas (H2) as sources of H atoms.


Electronation and deelectronation

The electrochemist
John Bockris Bernhardt Patrick John O’Mara Bockris (5 January 1923 – 7 July 2013) was a South African professor of chemistry, latterly at Texas A&M University. During his long and prolific career he published some 700 papers and two dozen books. His ...
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 circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s. These words are analogous to
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...
and
deprotonation Deprotonation (or dehydronation) is the removal (transfer) of a proton (or hydron, or hydrogen cation), (H+) from a Brønsted–Lowry acid in an acid–base reaction.Henry Jakubowski, Biochemistry Online Chapter 2A3, https://employees.csbsju. ...
. 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 iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
, 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 chemical reaction that proceeds via a covalent linkage—a strong electronic interaction—between the oxidant and the reductant reactants. In inner sphere electron tran ...
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, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrode ( ...
'' (''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 static electric field, it corresponds to the work needed per unit of charge to ...
at equilibrium under
standard conditions Standard temperature and pressure (STP) are 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 International Union ...
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 wh ...
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 a
standard hydrogen electrode The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis ...
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 H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and fluorine, 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 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 Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
is deposited when
zinc Zinc is a chemical element with the 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 (IIB) of the periodi ...
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 h ...
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 to
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
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. Combus ...
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 hydrophobic, and their odors are usually weak or ...
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 c ...
, 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 atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
, 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: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
. Complete oxidation of materials containing
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
produces carbon dioxide. * The stepwise oxidation of a hydrocarbon by oxygen, in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the 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.Clayden, ...
, produces water and, successively: an alcohol, 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 grou ...
or a ketone, a carboxylic acid, and then a
peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen ...
.


Corrosion and rusting

* The term
corrosion Corrosion is a 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. Corrosion engi ...
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 iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
ing, the formation of iron oxides, 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, Abundanc ...
metal. Common rust often refers to iron(III) oxide, 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 viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
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 can ...
reaction is one in which a single substance is both oxidized and reduced. For example,
thiosulfate Thiosulfate ( IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, ...
ion with sulfur in oxidation state +2 can react in the presence of acid to form elemental sulfur (oxidation state 0) and sulfur dioxide (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" to act as the anode. The sacrificial metal instead of the protected metal, then, corrodes. A common application of cathodic protection is in
galvanized Galvanization or galvanizing ( 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-dip galvanizing, in which the parts are submerged ...
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, removin ...
and oxidizing
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
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 nitri ...
. 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 Electrosynthesis in chemistry is the synthesis of chemical compounds in an electrochemical cell. Compared to ordinary redox reaction, electrosynthesis sometimes offers improved selectivity and yields. Electrosynthesis is actively studied as a scie ...
. Metal
ore Ore is natural 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. Retrieved 7 Apr ...
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, iron, copper, and other base metals. Smelting uses heat and a ...
at high temperature in the presence of a reducing agent. The process of electroplating uses redox reactions to coat objects with a thin layer of a material, as in chrome-plated automotive parts,
silver plating Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improv ...
cutlery,
galvanization Galvanization or galvanizing ( 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-dip galvanizing, in which the parts are submerge ...
and gold-plated
jewelry Jewellery ( UK) or jewelry ( U.S.) consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a w ...
.


Redox reactions in biology

Top: ascorbic acid (
reduced form In statistics, and particularly in econometrics, the reduced form of a system of equations is the result of solving the system for the endogenous variables. This gives the latter as functions of the exogenous variables, if any. In econometrics, the ...
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 'serum' ingredient to treat melasma (dark pigment spots) ...
)
Bottom:
dehydroascorbic acid Dehydroascorbic acid (DHA) is an oxidized form of ascorbic acid (vitamin C). It is actively imported into the endoplasmic reticulum of cells via glucose transporters. It is trapped therein by reduction back to ascorbate by glutathione and othe ...
( 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 'serum' ingredient to treat melasma (dark pigment spots) ...
)
Many important biological 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 molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
(C6H12O6) to CO2 and the reduction of
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
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 convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
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 atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
into sugars 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 (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 th ...
, which drives the synthesis of
adenosine triphosphate Adenosine triphosphate (ATP) is an organic compound that provides energy to drive many processes in living cells, such as muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis. Found in all known forms o ...
(ATP) and is maintained by the reduction of oxygen. In animal cells, mitochondria perform similar functions. See the '' Membrane potential'' 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 Gerontology Life extension Metabo ...
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 ...
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. 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, beta-hydroxybutyrate, and acetoacetate), whose interconversion is dependent on these ratios. An abnormal redox state can develop in a variety of deleterious situations, such as hypoxia,
shock Shock may refer to: Common uses Collective noun *Shock, a historic commercial term for a group of 60, see English numerals#Special names * Stook, or shock of grain, stacked sheaves Healthcare * Shock (circulatory), circulatory medical emergen ...
, 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 regulatio ...
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 groupin ...
are enzymatically reduced to form free radicals that contain one more electron than their parent compounds. In general, the electron donor is any of a wide variety of flavoenzymes and their
coenzyme A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...
s. Once formed, these anion free radicals reduce molecular oxygen to
superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...
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 A futile cycle, also known as a substrate cycle, occurs when two metabolic pathways run simultaneously in opposite directions and have no overall effect other than to dissipate energy in the form of heat. The reason this cycle was called "futile" c ...
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 oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
(Fe3O4). Titanium is mined as its dioxide, usually in the form of
rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at visib ...
(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 A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding. Mnemonics make use of elaborative encoding, retrieval cues, and imager ...
are commonly used by students to help memorise the terminology: * "
OIL RIG {{about, , the mnemonic OIL RIG, Redox An oil rig is any kind of apparatus constructed for oil drilling. Kinds of oil rig include: * Drilling rig, an apparatus for on-land oil drilling * Drillship, a floating apparatus for offshore oil drilling ...
" — oxidation is loss of electrons, reduction is gain of electrons * "LEO the lion says GER rr — 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 cell An electrolytic cell is an electrochemical cell that utilizes an external source of electrical energy to force a chemical reaction that would not otherwise occur. The external energy source is a voltage applied between the cell′s two electrod ...
s 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 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 Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain. In aerobic organisms undergoing r ...
*
Bessemer process The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation ...
*
Bioremediation Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi, and plants), living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluent ...
* Calvin cycle *
Chemical equation A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the product entities on the right-hand side with a plus sign between ...
*
Chemical looping combustion Chemical looping combustion (CLC) is a technological process typically employing a dual fluidized bed system. CLC operated with an interconnected moving bed with a fluidized bed system, has also been employed as a technology process. In CLC, a met ...
*
Citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...
*
Electrochemical series The data values of standard electrode potentials (''E''°) are given in the table below, in volts relative to the standard hydrogen electrode, and are for the following conditions: * A temperature of . * An effective concentration of 1 mol ...
*
Electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
* Electrolysis *
Electron equivalent Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
* Electron transport chain *
Electrosynthesis Electrosynthesis in chemistry is the synthesis of chemical compounds in an electrochemical cell. Compared to ordinary redox reaction, electrosynthesis sometimes offers improved selectivity and yields. Electrosynthesis is actively studied as a scie ...
*
Galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...
*
Hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organ ...
* Membrane potential *
Microbial fuel cell Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system that generates electric current by diverting electrons produced from the microbial oxidation of reduced compounds (also known as fuel or electron donor) on the anode to oxid ...
* Murburn concept *
Nucleophilic abstraction Nucleophilic abstraction is a type of an organometallic reaction which can be defined as a nucleophilic attack on a ligand which causes part or all of the original ligand to be removed from the metal along with the nucleophile.Spessard, Gary; Mie ...
* Organic redox reaction * Oxidative addition and reductive elimination * Oxidative phosphorylation *
Partial oxidation Partial oxidation (POX) is a type of chemical reaction. It occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen-rich syngas which can then be put to further use, for example in a fuel cell. A d ...
* Pro-oxidant *
Redox gradient A redox gradient is a series of reduction-oxidation (redox) reactions sorted according to redox potential. The redox ladder displays the order in which redox reactions occur based on the free energy gained from redox pairs. These redox gradients ...
*
Redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
*
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 me ...
*
Reducing atmosphere A reducing atmosphere is an Atmosphere, atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidizing gases or vapours, and which may contain actively Reducing agent, reducing gases such as hydrogen, carbon monoxi ...
*
Reduction potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
* Thermic reaction * Transmetalation * Sulfur cycle


References


Further reading

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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