Oxidation-reduction

Originally chemists viewed oxidation as a class of chemical reactions in which a chemical species (e.g., a chemical element, compound) reacts with oxygen to form an oxygen-containing product referred to as an 'oxide'. In modern terminology, chemists would describe that as the chemical species reacting with an oxygen molecule (O2) such that it combines with an atom of oxygen (O) to form an oxygen-containing product, as when hydrogen (H2) reacts with O2 to form the oxygen-containing product, H2O, namely water:


 * 2H2 + O2 → 2H2O [or, 2HOH]

In that reaction, each molecular pair of hydrogen atoms has been 'oxidized' by gaining an oxygen atom.

The 'Father of Chemistry', Antoine-Laurent Lavoisier (1743-1794), in his Elements of Chemistry (originally published in French in 1789), writes of oxidation in relation to the products formed when metals (e.g., mercury, iron) are exposed to air and a certain amount of heat:

The term oxidation, or calcination, is chiefly used to signify the process by which metals exposed to a certain degree of heat are converted to oxides, by absorbing oxygen from the air.

Lavoisier recognized calcination of metals as oxidation, the resuling oxides referred to as calxes (sing., calx).

Lavoisier's work revealed the common link between the air-requiring processes of burning (combustion), rusting and other such so-called calcinations of metals, and breathing (respiration) by animals &mdash; namely the requirement for the oxygen component of air, the chemical reaction of oxidation.

Prior to Lavoisier, the chemist, Georg Ernst Stahl (1660-1734), taught that oxidation, as it came to be called later, involved the escape of a common constituent of combustible substances, including metals capable of calcination, referred to as phlogiston, whose concentration differed among combustible/calcinible substances. Coal burning violently he considered nearly all phlogiston. The phenomenon of converting a calx back to the metal it started as, by reacting the calx with burning coal, Stahl explained as returning the phlogiston back to the metal. That phenomenon later came to be known as 'reduction', i.e., reducing the calx back to the metal. As the concept of oxidation matured with the development of the atomic theory, the concept of reduction also became refined, and more importantly, became inextricably linked to oxidation as a simultaneous event that accompanied every oxidation event (see below).

How Lavoisier came to replace the the phlogiston theory with the oxygenation theory is one of the great stories in the annals of the history of chemistry.