5.2 Redox

Oxygen is a reactive element, and the introduction of free oxygen into the atmosphere by the first photosynthetic lifeforms actually caused a mass extinction. Nearly all elements have at least one known oxide, the only exceptions being some of the noble gases.

Reacting free oxygen with another element or compound is an example of oxidation. Such a reaction may have the effect of turning a free element into an oxide, turning an alcohol into an aldehyde and then into a carboxylic acid, turning a thioether into a sulfoxide, turning two thiols into a disulfide, turning an alkane into an alkene, etc. Oxidation might add oxygen to a molecule, or it might subtract hydrogen from a molecule.

Burning something in a fire is also an example of oxidation. Organic matter reacts with oxygen in the air at high temperature, forming carbon dioxide and water vapor and releasing energy as heat that keeps the reaction going. (Some carbon monoxide is also produced, but carbon monoxide itself will also burn.)

There are other examples of oxidation as well. Household bleach contains sodium hypochlorite NaOCl, which performs all its many reactions not by releasing oxygen but by releasing chlorine, which is a strong oxidizer. Chlorine can react directly with metals, oxidizing them to chlorides, and it reacts reversibly with water, forming oxidizing acids that wreak havoc on single-celled organisms.

But wait, you might be wondering, oxidation without oxygen? Why yes! Oxidation happens any time electrons are lost. In fact, oxidation is only one half of the reaction; whenever something is oxidized, something else is reduced, meaning it gains electrons, and the whole reaction is called a redox. Burning organic matter oxidizes it, but also reduces some of the oxygen in the air.

The mnemonic OIL RIG can be used to remember where electrons go in a redox reaction: Oxidation Is Loss of electrons, and Reduction Is Gain of electrons.

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