Redox

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Redox reactions include all chemical processes in which atoms have their oxidation number (oxidation state) changed.

This can be a simple redox process, such as the combustion of carbon to yield carbon dioxide, it could be the reduction of carbon by hydrogen to yield methane, or it could be 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.

Reduction describes the uptake of an electron by a molecule, atom, or ion.

Oxidation describes the loss of an electron by a molecule, atom, or ion.

These two terms go together, because in a chemical reaction, one cannot occur without the other; electrons lost by one compound must be gained by another. Reduction can also be considered to be the reducing of an atom's positive charge, and oxidation its opposite (gaining positive charge).

Oxidizing and reducing agents

Substances that have the ability to oxidize (Commonwealth English oxidise) other substances are said to be oxidative and are known as oxidizing agents, oxidants or oxidizers. Put in another way, the oxidant removes electrons from the other substance, and is thus reduced itself. Oxidants are usually chemical substances with elements in high oxidation numbers (e.g. H₂O₂, MnO₄^-, CrO₃, Cr₂O₇^2-, OsO₄) or highly electronegative substances that can gain one or two extra electrons by oxidizing a substance (O₂, O₃, F₂, Cl₂, Br₂).

Substances that have the ability to reduce other substances are said to be reductive and are known as reductive agents, reductants, or reducers. Put in another way, the reductant transfers electrons to the substance. 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. Other kinds of reductants are hydride transfer reagents (NaBH₄, LiAlH₄), these reagents are widely used in organic chemistry, primarily in the reduction of carbonyl compounds to alcohols. Another useful method is reductions involving 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.

The chemical way to look at redox processes is that the reductant transfers electrons to the oxidant. Thus, at the end of the reaction, the reductant will have been oxidized and the oxidant will have been reduced.

Former meaning (oxygen/hydrogen)

Formerly, oxidation simply meant the addition of oxygen or the removing of hydrogen (hence the name oxidation), and reduction was removal of oxygen or the addition of hydrogen. Currently, however, the terms are normally used in a more general sense, describing electron movement.

Examples of redox reactions

A good example is the reaction between hydrogen and fluorine:

H₂ + F₂ → 2HF

We can write this overall reaction as two half-reactions: an oxidation reaction:

H₂ → 2H⁺ + 2e^-

and a reduction reaction:

F₂ + 2e^- → 2F^-

Elements 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 → 2H⁺ + 2e^-

+ 2e^- + F₂ → 2F^-

---------------------

H₂ + F₂ → 2H⁺ + 2F^-

And the ions combine to form hydrogen fluoride:

2H⁺ + 2F^- → 2HF

Other examples

• iron(II) oxidizes to iron(III):

Fe²⁺ → Fe³⁺ + e^-

• hydrogen peroxide reduces to hydroxide:

H₂O₂ + 2 e^- → 2 OH^-

overall equation for the above:

2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O

• denitrification, nitrate reduces to nitrogen:

2NO₃^- + 10e^- + 12 H⁺ → N₂ + 6H₂O

• iron oxidizes to iron(III) oxide and oxygen is reduced forming iron(III) oxide (commonly known as rusting or tarnishing):

4Fe + 3O₂ → 2 Fe₂O₃.

• Combustion of hydrocarbons 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, stepwise oxidation of a hydrocarbon produces water and, successively, an alcohol, an aldehyde or a ketone, carboxylic acid, and then a peroxide.

Redox reactions in biology

Much biological energy is stored and released by means of redox reactions. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. The reverse reaction, respiration, oxidizes sugars to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce nicotinamide adenine dinucleotide (NAD⁺), which then contributes to the creation of a proton gradient, which drives the synthesis of adenosine triphosphate (ATP) and is maintained by the reduction of oxygen. In animal cells, mitochondria perform similar functions. See Membrane potential article.

The term redox state is often used to describe the balance of NAD⁺/NADH and NADP⁺/NADPH in a biological system such as a cell or organ. The redox state is reflected in the balance of several sets of metabolites (e.g., lactate and pyruvate, beta-hydroxybutyrate and acetoacetate) whose interconversion is dependent on these ratios. An abnormal redox state can develop in a variety of deleterious situations, such as hypoxia, shock, and sepsis.

Mnemonics

A simple way to remember this is the mnemonic "LEO the lion goes GER". LEO means "Lose Electrons, Oxidize" and GER means "Gain Electrons, Reduce".

Another mnemonic is "OIL RIG", "Oxidation Is Loss", "Reduction Is Gain".

See also

• Bioremediation
• Calvin cycle
• Citric acid cycle
• Electrochemical cell
• Electrochemistry
• Galvanic cell
• Membrane potential
• Thermic reaction

References

External links

• Redox Reactions Calculatorca:Oxidació

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