Redox (portmanteau for reduction-oxidation) reactions describe all chemical reactions in which atoms have their oxidation state changed. This can be either a simple redox process, such as the oxidation of carbon to yield carbon dioxide (CO2) or the reduction of carbon by hydrogen to yield methane (CH4), or a complex process such as the oxidation of glucose (C6H12O6) in the human body through a series of complex electron transfer processes.
Redox reactions, or oxidation-reduction reactions, have a number of similarities to acid-base reactions. Fundamentally, redox reactions are a family of reactions that are concerned with the transfer of electrons between species.
The term comes from the two concepts of reduction and oxidation. It can be explained in simple terms:
Like acid-base reactions, redox reactions are a matched set, that is, there cannot be an oxidation reaction without a reduction reaction happening simultaneously. The oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction. When writing half-reactions, the gained or lost electrons are typically included explicitly in order that the half-reaction be balanced with respect to electric charge.
Though sufficient for many purposes, these descriptions are not precisely correct. Oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation state, and reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, 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.
The word oxidation originally implied reaction with oxygen to form an oxide, since (di)oxygen was historically the first recognized oxidizing agent. Later the meaning was generalized to include all processes involving loss of electrons.
The word reduction originally referred to the loss in weight on heating a metallic ore such as a metal oxide to extract the metal. Lavoisier showed that this loss of weight was due to the loss of oxygen as a gas. Later it was realized that the metal atom gains electrons in this process, and the meaning of reduction was generalized to include all processes involving gain of electrons.
The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes respectively when they occur at electrodes[1]. These words are analogous to protonation and deprotonation, but they have not been widely adopted by chemists.
In redox processes, 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. A redox couple is a reducing species and its corresponding oxidized form, e.g., Fe2+/Fe3+.
Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers. Put another way, the oxidant (oxidizing agent) removes electrons from another substance; i.e., it oxidizes other substances, 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 states (e.g., H2O2, MnO−
4, CrO3, Cr2O2−
7, OsO4), or else highly electronegative elements that can gain extra electrons by oxidizing another substance (O2, F2, Cl2, Br2).
Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as reducing agents, reductants, or reducers. The reductant (reducing agent) transfers electrons to another substance; i.e., it reduces others, and is thus itself oxidized. And, because it "donates" electrons, it is also called an electron donor. Electron donors can also form charge transfer complexes with electron acceptors.
Reductants in chemistry are very diverse. Electropositive elemental metals, such as lithium, sodium, magnesium, iron, zinc, and aluminium, are good reducing agents. These metals donate or give away electrons readily. Hydride transfer reagents, such as NaBH4 and LiAlH4, are widely used in organic chemistry,[2][3] primarily in the reduction of carbonyl compounds to alcohols. Another method of reduction involves the use of hydrogen gas (H2) with a palladium, platinum, or nickel catalyst. These catalytic reductions are used primarily in the reduction of carbon-carbon double or triple bonds.
Each half-reaction has a standard electrode potential (E0cell), which is equal to the potential difference (or voltage) (E0cell) at equilibrium under standard conditions of an electrochemical cell in which the cathode reaction is the half-reaction considered, and the anode is a standard hydrogen electrode where hydrogen is oxidized: ½ H2 → H+ + e-.
The electrode potential of each half-reaction is also known as its reduction potential E0red, or potential when the half-reaction takes place at a cathode. The reduction potential is a measure of the tendency of the oxidizing agent to be reduced. Its value is zero for H+ + e- → ½ H2 by definition, positive for oxidizing agents stronger than H+ (e.g., +2.866 V for F2) and negative for oxidizing agents which are weaker than H+ (e.g. –0.763 V for Zn2+)[4].
For a redox reaction which takes place in a cell, the potential difference E0cell = E0cathode – E0anode
Historically, however, the potential of the reaction at the anode was sometimes expressed as an oxidation potential, E0ox = – E0. The oxidation potential is a measure of the tendency of the reducing agent to be oxidized, but does not represent the physical potential at an electrode. With this notation, the cell voltage equation is written with a plus sign E0cell = E0cathode + E0ox (anode)
A good example is the reaction between hydrogen and fluorine in which hydrogen is being oxidized and fluorine is being reduced:
We can write this overall reaction as two half-reactions:
the oxidation reaction:
and the reduction reaction:
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 state of zero. In the first half-reaction, hydrogen is oxidized from an oxidation state of zero to an oxidation state of +1. In the second half-reaction, fluorine is reduced from an oxidation state of zero to an oxidation state of −1.
When adding the reactions together the electrons are canceled:
| H2 | → | 2 H+ + 2 e− |
| F2 + 2 e− |
→ | 2 F− |
|
|
||
| H2 + F2 | → | 2 H+ + 2 F− |
And the ions combine to form hydrogen fluoride:
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:
The ionic equation for this reaction is:
As two half-equations, it is seen that the iron is oxidized:
And the copper is reduced:
The primary process of reducing ore to produce metals is discussed in the article on Smelting.
Oxidation is used in a wide variety of industries such as in the production of cleaning products and oxidizing ammonia to produce nitric acid, which is used in most fertilizers.
Redox reactions are the foundation of electrochemical cells.
The process of electroplating uses redox reactions to coat objects with a thin layer of a material, as in chrome-plated automotive parts, silver plating cutlery, and gold-plated jewelry.
The production of compact discs depends on a redox reaction, which coats the disc with a thin layer of metal film.
Many important biological processes involve redox reactions.
Cellular respiration, for instance, is the oxidation of glucose (C6H12O6) to CO2 and the reduction of oxygen to water. The summary equation for cell respiration is:
The process of cell respiration also depends heavily on the reduction of NAD+ to NADH and the reverse reaction (the oxidation of NADH to NAD+). Photosynthesis and Cellular respiration are complementary but photosynthesis is not the reverse of the redox reaction in cell respiration:
Biological energy is frequently 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.
Free radical reactions are redox reactions that occur as a part of homeostasis and killing microorganisms, where an electron detaches from a molecule and then reattaches almost instantaneously. Free radicals are a part of redox molecules and can become harmful to the human body if they do not reattach to the redox molecule or an antioxidant. Unsatisfied free radicals can spur the mutation of cells they encounter and are thus causes of cancer.
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. Redox signaling involves the control of cellular processes by redox processes.
Redox proteins and their genes must be co-located for redox regulation according to the CoRR hypothesis for the function of DNA in mitochondria and chloroplasts.
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.[5]
In geology, redox is important to both the formation of minerals, mobilization of minerals, and in some depositional environments. In general, the redox state of most rocks can be seen in the color of the rock. Red is associated with oxidizing conditions of formation, and green is typically associated with reducing conditions. White (bleached rock) can also be associated with reducing conditions. Famous examples of redox conditions affecting geological processes include uranium deposits and Moqui marbles.
Describing the overall electrochemical reaction for a redox process requires a balancing of the component half-reactions for oxidation and reduction. In general, for reactions in aqueous solution, this involves adding H+, OH−, H2O, and electrons to compensate for the oxidation changes.
In acidic media, H+ ions and water are added to half reactions to balance the overall reaction.
For example, when manganese(II) reacts with sodium bismuthate:
| Unbalanced reaction: | Mn2+(aq) + NaBiO3(s) → Bi3+(aq) + MnO4− (aq) |
| Oxidation: | 4 H2O(l) + Mn2+(aq) → MnO− 4(aq) + 8 H+(aq) + 5 e− |
| Reduction: | 2 e− + 6 H+ + BiO− 3(s) → Bi3+(aq) + 3 H2O(l) |
The reaction is balanced by scaling the two half-cell reactions to involve the same number of electrons (multiplying the oxidation reaction by the number of electrons in the reduction step and vice versa):
Adding these two reactions eliminates the electrons terms and yields the balanced reaction:
In basic media, OH− ions and water are added to half reactions to balance the overall reaction.
For example, in the reaction between potassium permanganate and sodium sulfite:
| Unbalanced reaction: | KMnO4 + Na2SO3 + H2O → MnO2 + Na2SO4 + KOH |
| Reduction: | 3 e− + 2 H2O + MnO4− → MnO2 + 4 OH− |
| Oxidation: | 2 OH− + SO32− → SO42− + H2O + 2 e− |
Balancing the number of electrons in the two half-cell reactions gives:
Adding these two half-cell reactions together gives the balanced equation:
The key terms involved in redox are often confusing to students.[6][7] 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.[8] Acronyms or mnemonics are commonly used[9] to help remember what is happening: