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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, structure, proper ...
, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an
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 n ...
to an (called the , , , or ). Examples of substances that are commonly reducing agents include the
Earth metals The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all s ...
, formic acid, oxalic acid, and
sulfite Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid ( sulfurous acid) is elusive, its salts are w ...
compounds. In their pre-reaction states, reducers have extra electrons (that is, they are by themselves reduced) and oxidizers lack electrons (that is, they are by themselves oxidized). This is commonly expressed in terms of their oxidation states. An agent's oxidation state describes its degree of loss of electrons, where the higher the oxidation state then the fewer electrons it has. So initially, prior to the reaction, a reducing agent is typically in one of its lower possible oxidation states; its oxidation state increases during the reaction while that of the oxidizer decreases. Thus in a
redox 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 ...
reaction, the agent whose oxidation state increases, that "loses/ donates electrons", that "is oxidized", and that "reduces" is called the or , while the agent whose oxidation state decreases, that "gains/ accepts/receives electrons", that "is reduced", and that "oxidizes" is called the or . For example, consider the overall reaction for aerobic
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 ...
: : The
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 ...
() is being reduced, so it is the oxidizing agent. The
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 ...
() is being oxidized, so it is the reducing agent. In organic chemistry, reduction usually refers to the addition of hydrogen to a molecule. For example, the oxidizing agent benzene is reduced to cyclohexane by hydrogenation: : This article is about electron-transfer, not hydrogenation.


Characteristics

Consider the following reaction: :2 + → 2 + 2 The reducing agent in this reaction is ferrocyanide (). It donates an electron, becoming oxidized to ferricyanide (). Simultaneously, that electron is received by the oxidizer chlorine (), which is reduced to chloride (). Strong reducing agents easily lose (or donate) electrons. An atom with a relatively large atomic radius tends to be a better reductant. In such species, the distance from the nucleus to the valence electrons is so long that these electrons are not strongly attracted. These elements tend to be strong reducing agents. Good reducing agents tend to consist of atoms with a low electronegativity, which is the ability of an atom or molecule to attract bonding electrons, and species with relatively small
ionization energies Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
serve as good reducing agents too. The measure of a material's ability to reduce is known as its reduction potential. The table below shows a few reduction potentials, which can be changed to oxidation potentials by reversing the sign. Reducing agents can be ranked by increasing strength by ranking their reduction potentials. Reducers donate electrons to (that is, "reduce") oxidizing agents, which are said to "be reduced by" the reducer. The reducing agent is stronger when it has a more negative reduction potential and weaker when it has a more positive reduction potential. The more positive the reduction potential the greater the species' affinity for electrons and tendency to be reduced (that is, to receive electrons). The following table provides the reduction potentials of the indicated reducing agent at 25 °C. For example, among
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 ...
(Na), chromium (Cr), cuprous (Cu+) and chloride (Cl), it is Na that is the strongest reducing agent while Cl is the weakest; said differently, Na+ is the weakest oxidizing agent in this list while Cl is the strongest. Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain the
hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...
H ion, those being
NaH Nah or NAH may refer to: Places * Naha Airport (Indonesia) (IATA airport code: NAH), Tahuna, Sangir Islands, North Sulawesi, Indonesia * Nehbandan or Nah, a city in South Khorasan Province, Iran Other uses * NaH, the chemical formula of sodium hy ...
, LiH, LiAlH4 and CaH2. Some elements and compounds can be both reducing or oxidizing agents. Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals. :2 Li(s) + H2(g) → 2 LiH(s) Hydrogen (whose reduction potential is 0.0) acts as an oxidizing agent because it accepts an electron donation from the reducing agent
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 soli ...
(whose reduction potential is -3.04), which causes Li to be oxidized and hydrogen to be reduced. :H2(g) + F2(g) → 2 HF(g) Hydrogen acts as a reducing agent because it donates its electrons to fluorine, which allows fluorine to be reduced.


Importance

Reducing agents and oxidizing agents are the ones responsible for
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 ...
, which is the "degradation of metals as a result of electrochemical activity". Corrosion requires an
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 ...
and
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 whi ...
to take place. The anode is an element that loses electrons (reducing agent), thus oxidation always occurs in the anode, and the cathode is an element that gains electrons (oxidizing agent), thus reduction always occurs in the cathode. Corrosion occurs whenever there's a difference in oxidation potential. When this is present, the anode metal begins deteriorating, given there is an electrical connection and the presence of an electrolyte.


Examples of redox reaction

Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'. An example of this phenomenon occurred during the Great Oxidation Event, in which biologically−produced molecular oxygen ( dioxygen (), an oxidizer and electron recipient) was added to the early Earth's atmosphere, which was originally a weakly reducing atmosphere containing reducing gases like
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
() and carbon monoxide () (along with other electron donors) and practically no oxygen because any that was produced would
react REACT or React may refer to: Science and technology * REACT (telescope), a telescope at Fenton Hill Observatory, New Mexico, US Computing * React (JavaScript library) , a JavaScript library for building user interfaces, from Facebook ** React Nat ...
with these or other reducers (particularly with
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
dissolved in sea water), resulting in their . By using water as a reducing agent, aquatic photosynthesizing cyanobacteria produced this molecular oxygen as a waste product. This initially oxidized the ocean's dissolved ferrous
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
(Fe(II) − meaning iron in its +2 oxidation state) to form
insoluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...
ferric iron oxides such as Iron(III) oxide (Fe(II) lost an electron to the oxidizer and became Fe(III) − meaning iron in its +3 oxidation state) that precipitated down to the ocean floor to form banded iron formations, thereby removing the oxygen (and the iron). The rate of production of oxygen eventually exceeded the availability of reducing materials that removed oxygen, which ultimately led
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
to gain a strongly oxidizing atmosphere containing abundant oxygen (like the modern atmosphere). The modern sense of donating electrons is a generalization of this idea, acknowledging that other components can play a similar chemical role to oxygen. The formation of iron(III) oxide; :4Fe + 3O2 → 4Fe3+ + 6O2− → 2Fe2O3 In the above equation, the
Iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
(Fe) has an oxidation number of 0 before and 3+ after the reaction. For
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 ...
(O) the oxidation number began as 0 and decreased to 2−. These changes can be viewed as two " half-reactions" that occur concurrently: #Oxidation half reaction: Fe0 → Fe3+ + 3e #Reduction half reaction: O2 + 4e → 2 O2− Iron (Fe) has been oxidized because the oxidation number increased. Iron is the reducing agent because it gave electrons to the oxygen (O2). Oxygen (O2) has been reduced because the oxidation number has decreased and is the oxidizing agent because it took electrons from iron (Fe).


Common reducing agents

* Lithium aluminium hydride ( Li Al H4), a very strong reducing agent * Red-Al (NaAlH2(OCH2CH2OCH3)2), a safer and more stable alternative to lithium aluminum hydride *
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-to ...
without or with a suitable catalyst; e.g. a
Lindlar catalyst A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on calcium carbonate or barium sulfate which is then poisoned with various forms of lead or sulfur. It is used for the hydrogenation of alkynes to alkenes (i.e. ...
* Sodium amalgam ( Na( Hg)) *Sodium-lead alloy ( Na + Pb) * Zinc amalgam ( Zn( Hg)) (reagent for Clemmensen reduction) * Diborane * Sodium borohydride ( Na BH4) * Ferrous compounds that contain the Fe2+ ion, such as iron(II) sulfate * Stannous compounds that contain the Sn2+ ion, such as tin(II) chloride *
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 volcanic a ...
(sometimes also used as an oxidizing agent),
Sulfite Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid ( sulfurous acid) is elusive, its salts are w ...
compounds *
Dithionate The dithionate (or metabisulfate) anion, , is a sulfur oxoanion derived from dithionic acid, H2S2O6. Its chemical formula is sometimes written in a semistructural format, as 3SSO3sup>2−. It is the first member of the polythionates. The su ...
s, e.g. Na2 S2 O6 *
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, e ...
s, e.g. Na2S2O3 (mainly in analytical chemistry) * Iodides, such as potassium iodide ( K I) (mainly in analytical chemistry) * Hydrogen peroxide ()mostly an oxidant but can occasionally act as a reducing agent (typically in analytical chemistry.) * Hydrazine ( Wolff-Kishner reduction) * Diisobutylaluminium hydride (DIBAL-H) * Oxalic acid () * Formic acid (HCOOH) * Ascorbic acid (C6H8O6) *
Reducing sugars A reducing sugar is any sugar that is capable of acting as a reducing agent. In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reacti ...
, such as erythrose, see Aldose * Phosphites, hypophosphites, and phosphorous acid * Dithiothreitol (DTT) – used in biochemistry labs to avoid SS-bonds * Carbon monoxide (CO) *
Cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
s in hydrochemical metallurgical processes *
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 ma ...
(C) * Tris-2-carboxyethylphosphine hydrochloride (TCEP)


See also

* * * * * * * * * *


Notes


References


Further reading

* "Chemical Principles: The Quest for Insight", Third Edition. Peter Atkins and Loretta Jones p. F76


External links

* {{DEFAULTSORT:Reducing Agent Chemical reactions Redox