3 minute read

Oxidation-Reduction Reaction

History, Oxidation Numbers, Corrosion, Biological Processes, Current And Future UsesExamples of oxidation-reduction reactions

Oxidation-reduction reactions, also known as redox reactions, are chemical processes in which electrons are transferred from one atom, ion, or molecule to another. Explosions, fires, batteries, and even our own bodies are powered by oxidation-reduction reactions. When iron rusts or colored paper bleaches in the sun, oxidation-reduction has taken place.

Oxidation-reduction reactions can be thought of as a combination of two processes: oxidation, in which electrons are lost, and reduction, in which electrons are gained. The two processes cannot occur independently of each other. A mnemonic device used by chemists to help keep things straight is "LEO says 'GER,'" which stands for Loss of Electrons, Oxidation. Gain of Electrons, Reduction.

The driving force of oxidation-reduction reactions is the transfer of electrons. Although it is sometimes difficult to remember what happens to electrons during oxidation and what happens during reduction, a look at familiar processes can help keep this straight. Some of the first oxidation-reduction reactions understood by chemists were those involving oxygen. Oxygen, the most plentiful element on Earth, combines readily with numerous other elements. When combined with other elements in a compound or molecule, oxygen frequently is an electron "hog." It takes electrons away from many other elements and this oxidizes them. The oxygen takes the negatively charged electrons and becomes a negatively charged ion. The oxygen has been reduced, somewhat like taking in negative thoughts will reduce a person's positive attitude. An example of this is the reaction between oxygen in the air and iron. The iron metal becomes positively charged and the oxygen becomes negatively charged. The two charged ions now attract each other and hang around together in the form of iron oxide, or rust.


Let us look at an oxidation-reduction more chemically as we examine what happened to the Hindenburg in 1937. The Hindenburg was a dirigible filled with hydrogen, which gave it the lift it needed to keep afloat. The Hindenburg was a luxurious mode of transportation complete with a dining room and 25 private rooms. However, its first voyage, from Germany to the United States, ended tragically with the destruction of the airship and the loss of 36 lives because of the explosive combination of hydrogen and oxygen illustrated by this equation. The oxidation numbers of each element are indicated below the chemical formulas:

Hydrogen underwent a loss of electrons; it was oxidized. Oxygen underwent a gain of electrons; it was reduced. In terms of half reactions, the oxidation half reaction shows what happens to the hydrogen:

while the reduction half reaction illustrates what happens to the oxygen:

Hydrogen and oxygen combined once again to produce a fireball in the sky in 1986. This time, the space shuttle Challenger was destroyed by explosion and all seven crew members aboard were killed. Cold temperatures before the launch fatigued the O-rings that sealed the Challenger's booster tanks containing 500,000 gal (1.9 million l) of liquide hydrogen and oxygen. The controlled combination of hydrogen and oxygen was intended to provide power needed to launch the Challenger just as the combustion of gasoline provides power to a car. A spark ignited the two liquids and set off a massive uncontrolled oxidation-reduction reaction

Oxidation-reduction reactions are ofter accompanied by release of heat and sometimes, flame. Combustion reactions are oxidation-reduction reactions that occur when oxygen oxidizes another material. For example, burning carbon in a lump of coal produces carbon dioxide. The reaction can be illustrated as:

In this reaction, carbon is oxidized, going from an oxidation number of 0 to +4. The oxygen is reduced from an oxidation number of 0 to -2. A similar reaction occurs when hydrocarbon fuel is burned.

Additional topics

Science EncyclopediaScience & Philosophy: Overdamped to Peat