Inchemistry,areducing agent(also known as areductant,reducer,orelectron donor) is achemical speciesthat "donates" anelectronto anelectron recipient(called theoxidizing agent,oxidant,oxidizer,orelectron acceptor).
Examples of substances that are common reducing agents includehydrogen,thealkali metals,formic acid,[1]oxalic acid,[2]andsulfitecompounds.
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'soxidation statedescribes 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 aredoxreaction, the agent whose oxidation state increases, that "loses/donateselectrons ", that" is oxidized ", and that" reduces "is called thereducerorreducing agent,while the agent whose oxidation state decreases, that "gains/accepts/receives electrons ", that" is reduced ", and that" oxidizes "is called theoxidizeroroxidizing agent.
For example, consider the overall reaction for aerobiccellular respiration:
- C6H12O6(s) + 6O2(g) → 6CO2(g) + 6H2O(l)
Theoxygen(O2) is being reduced, so it is the oxidizing agent. Theglucose(C6H12O6) is being oxidized, so it is the reducing agent.
Characteristics
editConsider the following reaction:
The reducing agent in this reaction isferrocyanide([Fe(CN)6]4−). It donates an electron, becoming oxidized toferricyanide([Fe(CN)6]3−). Simultaneously, that electron is received by the oxidizerchlorine(Cl
2), which is reduced tochloride(Cl−
).
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 thevalence electronsis 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 lowelectronegativity,which is the ability of an atom or molecule to attract bonding electrons, and species with relatively smallionization energiesserve as good reducing agents too.[citation needed]
The measure of a material's ability to reduce is known as itsreduction potential.[3]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, amongsodium(Na),chromium(Cr),cuprous(Cu+) andchloride(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.[citation needed]
| Oxidizing agent | Reducing agent | Reduction Potential (V) | |
|---|---|---|---|
| Li++ e− | ⇌ | Li | −3.04 |
| Na++ e− | Na | −2.71 | |
| Mg2++ 2 e− | Mg | −2.38 | |
| Al3++ 3 e− | Al | −1.66 | |
| 2 H2O (l) + 2 e− | H2(g) + 2 OH− | −0.83 | |
| Cr3++ 3 e− | Cr | −0.74 | |
| Fe2++ 2 e− | Fe | −0.44 | |
| 2 H++ 2 e− | H2 | 0.00 | |
| Sn4++ 2 e− | Sn2+ | +0.15 | |
| Cu2++ e− | Cu+ | +0.16 | |
| Ag++ e− | Ag | +0.80 | |
| Br2+ 2 e− | 2 Br− | +1.07 | |
| Cl2+ 2 e− | 2 Cl− | +1.36 | |
| MnO−4+ 8 H++ 5 e− | Mn2++ 4 H2O | +1.49 | |
| F2+ 2 e− | 2 F− | +2.87 |
Common reducing agents include metals potassium, calcium, barium, sodium and magnesium, and also compounds that contain thehydrideH−ion, those beingNaH,LiH,[5]LiAlH4andCaH2.
Some elements and compounds can be both reducing oroxidizing agents.Hydrogen gasis 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)[a]
Hydrogen (whose reduction potential is 0.0) acts as an oxidizing agent because it accepts an electron donation from the reducing agentlithium(whose reduction potential is -3.04), which causes Li to be oxidized and hydrogen to be reduced.
- H2(g)+ F2(g)→ 2 HF(g)[b]
Hydrogen acts as a reducing agent because it donates its electrons tofluorine,which allows fluorine to be reduced.
Importance
editReducing agents and oxidizing agents are the ones responsible forcorrosion,which is the "degradation of metals as a result of electrochemical activity".[3]Corrosion requires ananodeandcathodeto 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 anelectrolyte.[citation needed]
Examples of redox reaction
edit
Historically, reduction referred to the removal of oxygen from a compound, hence the name 'reduction'.[7]An example of this phenomenon occurred during theGreat Oxidation Event,in which biologically−produced molecular oxygen (dioxygen(O2), an oxidizer and electron recipient) was added tothe early Earth's atmosphere,which was originally a weaklyreducing atmospherecontaining reducing gases likemethane(CH4) andcarbon monoxide(CO) (along with other electron donors)[8]and practically no oxygen because any that was produced wouldreactwith these or other reducers (particularly withirondissolvedinsea water), resulting in theirremoval. By using water as a reducing agent, aquaticphotosynthesizingcyanobacteriaproduced this molecular oxygen as a waste product.[9]ThisO2initially oxidized the ocean's dissolvedferrousiron(Fe(II) − meaning iron in its +2 oxidation state) to forminsolubleferriciron oxidessuch asIron(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 formbanded 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 ledEarthto gain a strongly oxidizing atmosphere containing abundant oxygen (like themodern atmosphere).[10]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 ofiron(III) oxide;
- 4Fe + 3O2→ 4Fe3++ 6O2−→ 2Fe2O3
In the above equation, theIron(Fe) has an oxidation number of 0 before and 3+ after the reaction. Foroxygen(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
editThis sectionneeds additional citations forverification.(October 2016) |
- Lithium aluminium hydride(LiAlH4), a very strong reducing agent
- Red-Al(NaAlH2(OCH2CH2OCH3)2), a safer and more stable alternative to lithium aluminum hydride
- Hydrogenwithout or with a suitablecatalyst;e.g. aLindlar catalyst
- Sodium amalgam(Na(Hg))
- Sodium-lead alloy (Na+Pb)
- Zinc amalgam(Zn(Hg)) (reagent forClemmensen reduction)
- Diborane
- Sodium borohydride(NaBH4)
- Ferrouscompounds that contain theFe2+ion, such asiron(II) sulfate
- Stannouscompounds that contain theSn2+ion, such astin(II) chloride
- Sulfur dioxide(sometimes also used as anoxidizing agent),Sulfitecompounds
- Dithionates,e.g.Na2S2O6
- Thiosulfates,e.g. Na2S2O3(mainly in analytical chemistry)[11]
- Iodides,such aspotassium iodide(KI) (mainly inanalytical chemistry)
- Hydrogen peroxide(H
2O
2) – mostly an oxidant but can occasionally act as a reducing agent, typically in analytical chemistry[citation needed] - Hydrazine(Wolff-Kishner reduction)
- Diisobutylaluminium hydride(DIBAL-H)
- Oxalic acid(C
2H
2O
4) - Formic acid(HCOOH)
- Ascorbic acid(C6H8O6)
- Reducing sugars,such aserythrose,seeAldose
- Phosphites,hypophosphites,andphosphorous acid
- Dithiothreitol(DTT) – used in biochemistry labs to avoidSS-bonds
- Carbon monoxide(CO)
- Cyanidesin hydrochemical metallurgical processes
- Carbon(C)
- Tris-2-carboxyethylphosphine hydrochloride(TCEP)
See also
edit- Corrosion– Gradual destruction of materials by chemical reaction with its environment
- Electrochemistry– Branch of chemistry
- Electrolyte– Ionic solids whose dissociation in water frees up ions carrying the electrical current in solution
- Electron acceptor– Chemical entity capable of accepting electrons
- Electron donor– Chemical entity capable of donating electrons to another entity
- Electrosynthesis– Synthesis of chemical compounds in an electrochemical cell
- Organic reduction– Redox reaction that takes place with organic compounds
- Oxidizing agent– Chemical compound used to oxidize another substance in a chemical reaction
- Redox– Chemical reaction in which oxidation states of atoms are changed
- Reducing equivalent– Chemical reaction in which oxidation states of atoms are changed
- Salt-free reduction
Notes
edit- ^Half reactions:2 Li0(s)→ 2 Li+(s)+ 2 e−::::: H20(g)+ 2 e−→ 2 H−(g)
- ^Half reactions:H20(g)→ 2 H+(g)+ 2 e−::::: F20(g)+ 2 e−→ 2 F−(g)
References
edit- ^Garron, Anthony; Epron, Florence (2005). "Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water".Water Research.39(13): 3073–3081.Bibcode:2005WatRe..39.3073G.doi:10.1016/j.watres.2005.05.012.PMID15982701.
- ^"Oxidizing and Reducing Agents".Purdue University.
- ^ab"Electrode Reduction and Oxidation Potential Values".www.EESemi.com.Retrieved12 July2021.
- ^"Standard Electrode Potentials".hyperphysics.phy-astr.gsu.edu.Retrieved29 March2018.
- ^Aufray M, Menuel S, Fort Y, Eschbach J, Rouxel D, Vincent B (2009)."New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis"(PDF).Journal of Nanoscience and Nanotechnology.9(8): 4780–4789.doi:10.1166/jnn.2009.1087.PMID19928149.Archived fromthe original(PDF)on 2020-07-29.Retrieved2019-09-24.
- ^"Metals".Bitesize.BBC.Archivedfrom the original on 2022-11-03.
- ^Olson, Maynard V."oxidation-reduction reaction".Britannica.Retrieved3 May2022.
In hisTraité élémentaire de chimie,heclearly established that combustion consists of a chemical combination between oxygen from the atmosphere and combustible matter [...]. By the end of the century, his ideas were widely accepted and had been successfully applied to the more complex processes of respiration and photosynthesis. Reactions in which oxygen was consumed were classified as oxidations, while those in which oxygen was lost were termed reductions.
- ^Kasting, J.F. (2014). "Modeling the Archean Atmosphere and Climate".Treatise on Geochemistry.Elsevier. pp. 157–175.doi:10.1016/b978-0-08-095975-7.01306-1.ISBN9780080983004.
- ^Buick, Roger (August 27, 2008)."When did oxygenic photosynthesis evolve?".Philosophical Transactions of the Royal Society B.363(1504): 2731–2743.doi:10.1098/rstb.2008.0041.ISSN0962-8436.PMC2606769.PMID18468984.
- ^Sosa Torres, Martha E.; Saucedo-Vázquez, Juan P.; Kroneck, Peter M.H. (2015). "Chapter 1, Section 2: The rise of dioxygen in the atmosphere". In Kroneck, Peter M.H.; Sosa Torres, Martha E. (eds.).Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases.Metal Ions in Life Sciences volume 15. Vol. 15. Springer. pp. 1–12.doi:10.1007/978-3-319-12415-5_1.ISBN978-3-319-12414-8.PMID25707464.
- ^"Cathodic Stripping Voltammetric Procedure for Determination of Some Inorganic Arsenic Species in Water, Soil and Ores Samples".
Further reading
edit- "Chemical Principles: The Quest for Insight", Third Edition. Peter Atkins and Loretta Jones p. F76
External links
edit- Table summarizing strength of reducing agentsat theWayback Machine(archived June 11, 2011)