OXIDATION - REDUCTION
Historically, the term oxidation was first used for reactions in which O2 is actually consumed--for example, the burning in air of free carbon.
C + O2 -----> CO2
2C + O2 -----> 2CO
The burning of hydrogen in air forms water.
2H2 + O2 -----> 2H2O
When iron (Fe) is heated in oxygen, it forms iron oxide, Fe2O3, and other oxides such as FeO. Similar products form slowly when iron rusts in air--although these are usually associated with H2O.
4Fe + 3O2 -----> 2Fe2O3
Chemists soon realized that fundamentally similar combinations may occur with elements other than oxygen. For example, carbon, hydrogen, and iron may "burn" in fluorine.
C + 2F2 -----> CF4
H2 + F2 -----> 2HF
2Fe + 3F2 -----> 2FeF3
Accordingly, the definition of oxidation was broadened. An oxidation is now defined as any reaction in which an atom or molecule gives up electrons. For example, in the oxidation of iron by both O2 and F2, iron undergoes the same transformation: it gives up electrons.
Fe -----> Fe3+ + 3e-
A reduction is the acquisition of electrons by an atom or molecule. Confusion sometimes arises from the fact that a "reduction" is accompanied by an acquisition of something (an electron). Remember that the gain of an electron reduces net electrical charge. In the reaction above between Fe and O2, Fe is oxidized to Fe2+, while oxygen is being reduced in a simultaneous reaction:
3O2 + 12e- -----> 6O2-
Thus the production of Fe2O3 requires both an oxidation and a reduction. The result is a net transfer of 12 electrons from Fe to O. Because there is no change in the total electrical charge, the reactions must occur simultaneously, one at the expense of the other. We refer to reactions in which one or more electrons pass from one molecule to another as oxidation-reduction reactions.
Oxidation may involve only a loss of electrons--as when Na is oxidized to Na+. But often the traveling electron is accompanied by a proton (H+), so that the final reaction may involve not only the addition of oxygen but the removal of hydrogen. This is perhaps the most important form of biological oxidation of organic molecules. Since the ultimate source of oxidizing power is atmospheric O2, it fair to say that the final metabolic task of most aerobic organisms is to reduce O2.
O2 + 4e- + 4H+ -----> 2H2O
The electrons and protons that ultimately perform this critical task in many organisms arise from the oxidation of glucose (and other substrates):
C6H12O6 + 6O2 -----> 6 CO2 + 6 H2O
Reduction of atmospheric oxygen is not accomplished directly. Many hydrogen transfers occur in metabolism before they are collectively transferred in a final sequence to oxygen, the final oxidant (electron acceptor).
Electron removal is usually accompanied by a release of energy. Clearly, energy is released as heat and light in the oxidation of firewood by ordinary combustion. Energy release certainly accompanies oxygen addition or hydrogen removal. Conversely, reduction, or electron addition (which accompanies oxygen removal or hydrogen addition) is associated with utilization of energy. Oxidation occurs as sugar molecules slide down the "energy hill." In short, oxidations are energy-yielding; reductions are energy-consuming.
SUMMARY OF OXIDATION-REDUCTION REACTIONS
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Oxidations Reductions
[with respect to A] [with respect to A]
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Removal of Electrons: Addition of electrons:
A ---> A+ + e- A + e- ---> A-
Addition of oxygen: Removal of oxygen:
A + BO ---> AO + B AO + B ---> A + BO
Removal of hydrogen: Addition of hydrogen:
AH + B ---> A + BH A + BH ---> AH + B
All three mechanisms: All three mechanisms:
Release of energy* Storage of energy
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* in most cases
Nitrate reduction takes place in some bacteria anaerobically. The end products of the reduction depends on the bacterium involved, the availability of various enzymes and the amount of oxygen present.
NO3- + 2e- + 2H+ -----> NO2- + H2O
NO3 -----> NO2 ----->NH3
In this summary reaction, the enzyme nitrate reductase catalyses the conversion of nitrate to nitrite; the conversion of nitrite to ammonia is catalysed by nitrite reductase.
In the first and second reactions, oxygen is removed; in the second reaction hydrogen is also added.
2NO3- -----> 2NO2- -----> 2NO -----> N2O -----> N2
In some bacteria, nitrate is converted to nitrogen gas; the first conversion leads to nitrite and then nitric oxide and this is converted to nitrous oxide and then nitrogen gas. This reaction,
called denitrification, uses 5e- + 6H+ and water is also formed.