Inchemistry, areducing agent (also known as areductant,reducer, orelectron donor) is achemical species that "donates" anelectron to anelectron recipient (called theoxidizing agent,oxidant,oxidizer, orelectron acceptor).
Examples of substances that are common reducing agents includehydrogen,carbon monoxide, thealkali metals,formic acid,[1]oxalic acid,[2] andsulfite 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'soxidation 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 aredox reaction, the agent whose oxidation state increases, that "loses/donates electrons", that "is oxidized", and that "reduces" is called thereducer orreducing agent, while the agent whose oxidation state decreases, that "gains/accepts/receives electrons", that "is reduced", and that "oxidizes" is called theoxidizer oroxidizing agent.
For example, consider the overall reaction for aerobiccellular respiration:
Theoxygen (O2) is being reduced, so it is the oxidizing agent. Theglucose (C6H12O6) is being oxidized, so it is the reducing agent.
Consider 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 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 lowelectronegativity, which is the ability of an atom or molecule to attract bonding electrons, and species with relatively smallionization energies serve 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 thehydride H− ion, those beingNaH,LiH,[5]LiAlH4 andCaH2.
Some elements and compounds can be both reducing oroxidizing agents.Hydrogen gas is a reducing agent when it reacts with non-metals and an oxidizing agent when it reacts with metals.
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.
Hydrogen acts as a reducing agent because it donates its electrons tofluorine, which allows fluorine to be reduced.
Reducing agents and oxidizing agents are the ones responsible forcorrosion, which is the "degradation of metals as a result of electrochemical activity".[3] Corrosion requires ananode andcathode 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 anelectrolyte.[citation needed]
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 atmosphere containing reducing gases likemethane (CH4) andcarbon monoxide (CO) (along with other electron donors)[8] and practically no oxygen because any that was produced wouldreact with these or other reducers (particularly withirondissolved insea water), resulting in theirremoval.By using water as a reducing agent, aquaticphotosynthesizingcyanobacteria produced this molecular oxygen as a waste product.[9] ThisO2 initially oxidized the ocean's dissolvedferrousiron (Fe(II) − meaning iron in its +2 oxidation state) to forminsolubleferriciron oxides such 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 ledEarth to 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;
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:
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).
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In hisTraité élémentaire de chimie,he clearly 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.
