Redox
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Redox (shorthand for reduction-oxidation reaction) describes all chemical reactionsin which atoms have their oxidation number (oxidation state) changed. This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxideor the reduction of carbon by hydrogen to yield methane (CH4), or it can be a complex process such as the oxidation of sugar in the human body through a series of very complex electron transfer processes.
The term redox comes from the two concepts of reduction and oxidation. It can be explained in simple terms:
- Oxidation describes the loss of electrons or an increase in oxidation state by amolecule, atom or ion.
- Reduction describes the gain of electrons or a decrease in oxidation state by a molecule, atom or ion.
Though sufficient for many purposes, these descriptions are not precisely correct. Oxidation and reduction properly refer to a change in oxidation number — the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation number, and reduction as a decrease in oxidation number. In practice, the transfer of electrons will always cause a change in oxidation number, but there are many reactions that are classed as "redox" even though no electron transfer occurs (such as those involving covalent bonds).
Non-redox reactions, which do not involve changes in formal charge, are known as metathesis reactions.
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[edit]Oxidizing and reducing agents
The chemical way to look at redox processes is that 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 are involved in a particular reaction is called a redox pair.
[edit]Oxidizers
Substances that have the ability to oxidize other substances are said to be oxidative and are known as oxidizing agents, oxidants, or oxidizers. Put another way, the oxidant removes electrons from another substance, and is thus itself reduced. And, because it "accepts" electrons, it is also called an electron acceptor.
Oxidants are usually chemical substances with elements in high oxidation numbers (e.g.,H2O2, MnO4− , CrO3, Cr2O2−7, OsO4) or highly electronegative substances that can gain one or two extra electrons by oxidizing a substance (O, F, Cl, Br).
[edit]Reducers
Substances that have the ability to reduce other substances are said to be reductive and are known as reducing agents, reductants, orreducers. Put in another way, the reductant transfers electrons to another substance, and is, thus, oxidized itself. And, because it "donates" electrons it is also called an electron donor. Reductants in chemistry are very diverse. Metal reduction—electropositive elemental metals can be used (Li, Na, Mg, Fe, Zn, Al). These metals donate or give away electrons readily. Reactions-Reducing Agents Other kinds of reductants arehydride transfer reagents (NaBH4, LiAlH4), these reagents are widely used in organic chemistry[1][2], primarily in the reduction of carbonylcompounds to alcohols. Another useful method is reductions involving hydrogen gas (H2) with a palladium, platinum, or nickel catalyst. Thesecatalytic reductions are primarily used in the reduction of carbon-carbon double or triple bonds.
[edit]Examples of redox reactions
A good example is the reaction between hydrogen and fluorine:
- H2 + F2 → 2 HF
We can write this overall reaction as two half-reactions: the oxidation reaction:
- H2 → 2 H+ + 2 e−
and the reduction reaction:
- F2 + 2 e− → 2 F−
Analyzing each half-reaction in isolation can often make the overall chemical process clearer. Because there is no net change in charge during a redox reaction, the number of electrons in excess in the oxidation reaction must equal the number consumed by the reduction reaction (as shown above).
Elements, even in molecular form, always have an oxidation number of zero. In the first half-reaction, hydrogen is oxidized from an oxidation number of zero to an oxidation number of +1. In the second half-reaction, fluorine is reduced from an oxidation number of zero to an oxidation number of −1.
When adding the reactions together the electrons cancel:
H2 → 2 H+ + 2 e− F2 + 2 e− → 2 F− H2 + F2 → 2 H+ + 2 F−
And the ions combine to form hydrogen fluoride:
- H2 + F2 → 2 H+ + 2 F− → 2 HF
[edit]Displacement reactions
Redox occurs in single displacement reactions or substitution reactions. The redox component of these types of reactions is the change of oxidation state (charge) on certain atoms, not the actual exchange of atoms in the compounds.
For example, in the reaction between iron and copper(II) sulfate solution:
- Fe + CuSO4 → FeSO4 + Cu
The ionic equation for this reaction is:
- Fe + Cu2+ → Fe2+ + Cu
As two half-equations, it is seen that the iron is oxidized:
- Fe → Fe2+ + 2 e−
And the copper is reduced:
- Cu2+ + 2 e− → Cu
[edit]Other examples
- The oxidation of iron(II) to iron(III) by hydrogen peroxide in the presence of an acid:
- Fe2+ → Fe3+ + e−
- H2O2 + 2 e− → 2 OH−
- Overall equation:
- 2 Fe2+ + H2O2 + 2 H+ → 2 Fe3+ + 2 H2O
- The reduction of nitrate to nitrogen in the presence of an acid (denitrification):
- 2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O
- Oxidation of elemental iron to iron(III) oxide by oxygen (commonly known as rusting, which is similar to tarnishing):
- 4 Fe + 3 O2 → 2 Fe2O3
- The combustion of hydrocarbons, such as in an internal combustion engine, which produces water, carbon dioxide, some partially oxidized forms such as carbon monoxide and heat energy. Complete oxidation of materials containing carbon produces carbon dioxide.
- In organic chemistry, the stepwise oxidation of a hydrocarbon by oxygen produces water and, successively, an alcohol, an aldehyde or aketone, a carboxylic acid, and then a peroxide.
