Redox Redux

Oxidation is a type of reaction that happens when oxygen or another electronegative element changes an atom or molecule. Fire is an example of oxidation; combustible materials are oxidized at high temperatures by oxygen in the air. The tarnishing of metals and rusting of iron are another good example. When a metal combines with oxygen, it forms an oxide. The metal is oxidized by the reaction.

The opposite of oxidation is reduction. When a combustible material burns, it reduces some of the oxygen in the air. When a metal tarnishes or rusts, it also reduces oxygen.

Oxygen is not the only oxidizer. The halogens (fluorine, chlorine, bromine, and iodine) are also oxidizers. Halogens react with metals, and we say they oxidize them to halides, even though no oxygen is involved. Equally, metals reduce halogens. The reaction of sodium metal with chlorine gas oxidizes the sodium, reduces the chlorine, and produces table salt. A reaction that oxidizes one substance and reduces another is called a redox reaction.

Most organic compounds are flammable. Ethyl alcohol burns in air to form carbon dioxide and water. The two carbon atoms combine with four oxygen atoms and produce two molecules of CO₂. The six hydrogen atoms combine with the molecule's existing oxygen atom plus two more oxygens, and make three molecules of H₂O. But other, more subtle redox reactions are possible. Cyclohexane can be oxidized to benzene, using an oxidizer strong enough to remove six of its hydrogen atoms. This is still an oxidation reaction, even though we are not adding any oxygen or halogens to the molecule. Taking away hydrogen atoms is another type of oxidation.

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