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Oxidation and Reduction
From Grolier's Multimedia Encyclopedia

Oxidation and reduction are complementary chemical processes that involve a loss of electrons (oxidation) by one reactant and a corresponding gain (reduction) by another. Both processes must occur simultaneously and in equivalent amounts. The most familiar oxidative processes utilize oxygen from the atmosphere; these include the rusting of iron, combustion, and respiration. In each case oxygen is reduced. Reductive processes include the recovery of metals from their ores, the photosynthetic production of carbohydrates, and the hydrogenation of fats.

OXIDIZING AGENTS

The substance that acquires electrons during an oxidation-reduction reaction is an oxidizing agent. In the course of the reaction, the oxidizing agent is reduced; a strong oxidizing agent reacts and becomes a weak reducing agent. Of the chemical elements, the most electronegative elements have the greatest tendency to participate in reactions as oxidizing agents, because they form negative ions (gain electrons) very readily. Fluorine, the most highly electronegative atom, is the most active oxidizing agent among the elements. It reacts to form the fluoride ion, the weakest reducing agent. Oxygen (O2) is highly active, particularly so in the form of ozone (O3). All of the halogen elements can act as strong oxidizing agents.

Certain oxygen-containing compounds readily give up oxygen to another reactant, becoming reduced in the process. Some important examples are hydrogen peroxide, nitric acid, concentrated sulfuric acid, potassium nitrate, and the permanganate, dichromate, chlorate, and hypochlorite ions. Potassium permanganate (KMnO4) is a useful laboratory oxidizing agent. It is strongest in acid solution, where it is reduced to Mn2+, and less active in alkaline solution, where the product is MnO4 2−.

REDUCING AGENTS

Elements that readily form positive ions are active reducing agents and, as expected from their low electronegativities, the most active are the alkali metals followed by the alkaline earth metals. Lithium is the strongest reducing agent of the elements. Useful reducing agents in organic chemistry include hydrogen, lithium aluminum hydride, and sodium borohydride.

In general, the strength of a reactant in an oxidation-reduction process depends on the reaction conditions. Hydrogen reduces the carbon-carbon triple bond to single bonds when a platinum catalyst is used, but the reaction stops at the double-bond stage when Raney nickel is the catalyst. To oxidize or reduce certain functional groups on a complex molecule but leave others unaffected, a chemist must carefully select the appropriate agent, catalyst, concentrations, temperature, and pressure.

AUTO-OXIDATION

Certain compounds can act as both oxidizing and reducing agents. The oxygen in hydrogen peroxide (H2 O2) is in an intermediate state of oxidation between free oxygen and oxides. In the decomposition of hydrogen peroxide both water and molecular oxygen are formed: 2H2 O2 → H2 O + O2. This process is called auto-oxidation and is catalyzed by impurities in the sample.

BIOLOGICAL OXIDATION AND REDUCTION

The vast majority of living organisms rely on oxygen to generate oxidative power. The actual mechanism is not a direct chemical reaction but a series of electron transfers through a number of intermediate compounds that readily accept and release electrons, alternating between an oxidized and a reduced form. This route is called the electron transport chain and is similar in all organisms. As the strongest oxidizing agent of the chain, oxygen is the final electron acceptor. Its vital role in living organisms is essentially as a substance on which to "dump" electrons. Many microorganisms are anaerobic; that is, they do not require oxygen for survival. These organisms are able to utilize other substances, such as sulfur, as oxidizing agents.

All organisms generate reducing power through the reversible biochemical reactions of substances such as nicotin-amide-adenine dinucleotide (NAD), flavins, and cytochromes, which can exist in an oxidized or a reduced form. By participating in the electron transport chain, the reduced form is continually regenerated from the oxidized form.

Stephen Fleishman