Reference Work Entry

Encyclopedia of Astrobiology

pp 1194-1195


  • Concepción AlonsoAffiliated withUniversidad Autonoma de Madrid Email author 


Combustion; Corrosion; Rusting


Electron donor, reducing agent, reductive


Oxidation can be defined with different terms such as: the loss of electrons or hydrogen, or the gain of oxygen by an atom or a molecule.


Oxidation in Terms of Electron Transfer

Oxidation is the loss of electrons.

For example, in the reaction between iron and copper sulfate solution
$$ {\text{Fe}} + {\text{CuS}}{{\text{O}}_4} \to {\text{Cu}} + {\text{FeS}}{{\text{O}}_4} $$
copper sulfate and iron sulfate are both ionic compounds. If rewritten as an ionic equation, it turns out that sulfate ions are spectator ions and the reaction can be written as:
$$ {\text{Fe}} + {\text{C}}{{\text{u}}^{{2} + }} \to {\text{Cu}} + {\text{F}}{{\text{e}}^{{2} + }} $$
where the iron is oxidized from metal iron (Feo) to ferrous iron (Fe2+) by the loss of two electrons:
$$ {\text{Fe}} \to {\text{F}}{{\text{e}}^{{2} + }} + {2}{{\text{e}}^{-} } $$
while the copper (II) ion, gaining two electrons is reduced to copper metal (Cuo). 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.

Oxidation in Terms of Hydrogen Transfer

Oxidation is the loss of hydrogen.
  • Methanol (alcohol) can be oxidized to methanal (aldehyde) by loss of hydrogen
    $$ {\text{C}}{{\text{H}}_3}{\text{OH}} \to {{\text{H}}_2}{\text{CHO}} $$

Oxidation in Terms of Oxygen Transfer

Oxidation is the gain of oxygen.
  • The reaction between elemental iron and oxygen to form iron oxide involves the oxidation of iron (commonly known as rusting)
    $$ {4}\;{\text{Fe}} + {3}\;{{\text{O}}_2} \to {2}\;{\text{F}}{{\text{e}}_2}{{\text{O}}_3} $$

In general terms, oxidation implies an increase in an atom’s oxidation state. However, it is only correct to speak about the oxidation state of a metal atom when the compound is purely ionic. Molecules, atoms or ions that have the ability to reduce others are said to be reductive and are known as reducing agents, reductants, or reducers. That is to say, the reductant transfers electrons to another substance, which is, as a result, oxidized. Since, it “donates” electrons it is also called an electron donor. The electronic transfer reactions known as redox reactions are especially important in biology because most biochemical reactions imply reduction or oxidation reactions.

Substances that act as reductants include, among others, electropositive elemental metals like lithium, sodium, magnesium, iron, zinc and aluminum. These metals readily donate or give away electrons. Hydride transfer reagents, such as NaBH4 and LiAlH4, are widely used in organic chemistry. Another method of reduction involves the use of hydrogen gas (H2) with a palladium, platinum, or nickel catalyst. These catalytic reductions are primarily used in the reduction of carbon-carbon double or triple bonds.

See also

Electron Donor

Redox Potential

Reducing Agent

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© Springer-Verlag Berlin Heidelberg 2011
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