Redox
Redox reactions include all
chemical processes in which atoms have their oxidation number changed.
This can be a simple redox process, such as the oxidation 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. It can be explained in simple terms:However, these descriptions are not truly correct.
Encyclopedia
Redox reactions include all
chemical processes in which atoms have their oxidation number changed.
This can be a simple redox process, such as the oxidation 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. It can be explained in simple terms:
- Oxidation describes the loss of an electron by a molecule, atom or ion
- Reduction describes the gain of an electron by a molecule, atom or ion
However, these descriptions are not truly 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 which are classed as "redox", though no electrons are transferred .
Oxidizing and reducing agents
Substances that have the ability to
oxidize 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 or highly electronegative substances that can gain one or two extra electrons by oxidizing a substance .
Substances that have the ability to
reduce other substances are said to be
reductive and are known as
reducing 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 . These metals donate or
give away electrons readily. Other kinds of reductants are
hydride transfer reagents , 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 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, 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 industry
Oxidation is used in a wide variety of industries such as in the production of cleaning products.
Redox reactions are the foundation of
electrochemical cells.
Examples of redox reactions
A good example is the reaction between
hydrogen and
fluorine:
We can write this overall reaction as two half-reactions: the oxidation reaction
and the reduction reaction:
Analysing 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 .
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:
And the ions combine to form
hydrogen fluoride:
Other examples
- Fe2+ ? Fe3+ + e-
- H2O2 + 2 e- ? 2 OH-
overall equation for the above:
- 2Fe2+ + H2O2 + 2H+ ? 2Fe3+ + 2H2O
- 2NO3- + 10e- + 12 H+ ? N2 + 6H2O
- iron oxidizes to iron oxide and oxygen is reduced forming iron oxide :
- 4Fe + 3O2 ? 2 Fe2O3.
...
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 , which then contributes to the creation of a proton gradient, which drives the synthesis of
adenosine triphosphate 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 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. Redox signaling involves the control of cellular processes by redox processes.
Mnemonics
The key terms involved in redox can be confusing. For example, an element that is oxidized loses electrons; however, that element is referred to as the reducing agent. Likewise, an element that is reduced gains electrons and is referred to as the oxidizing agent. Several acronyms and
mnemonics are often used to remember what is happening:
- "OIL RIG"—Oxidation Is Loss, Reduction Is Gain.
- "LEO the lion says GER"—Losing Electrons is Oxidation, Gaining Electrons is Reduction.
- Visualize OXen going up a mountain—just as the oxidation number of an OXidation goes up—and RED blood flowing down the mountain—just as the oxidation number of a REDuction goes down.
- "EOH" remind you to include an electron and oxygen and hydrogen atom in your complex equations.
Redox Cycling
A wide variety of aromatic compounds are enzymatically reduced to form free radicals that contain one more electron than their parent compounds. In general, the electron donor is any of a wide variety of flavoenzymes and their coenzymes. Once formed, these anion free radicals reduce molecular oxygen to superoxide and regenerate the unchanged parent compound. The net reaction is the oxidation of the flavoenzyme's coenzymes and the reduction of molecular oxygen to form superoxide. This catalytic behavior has been described as futile cycle or redox cycling. Examples of redox cycling-inducing molecules are the herbicide paraquat and other viologens and quinones such as menadione.
References
See also
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External links
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- balances equations of any half-cell and full reactions
