Oxidation-reduction reaction


Combustion, fire, and flame have been observed and speculated about from earliest times. Every civilization had its own explanation for them. The Greeks interpreted combustion in terms of philosophical doctrines, one of which was that a certain "inflammable principle" was contained in all combustible bodies and this principle escaped when the body was burned to react with air. A generalization of the concept was provided by the phlogiston theory, formulated in the 17th century (see above). Treated at first as a purely metaphysical quality, phlogiston was conceived later as a material substance having weight, and, sometimes, negative weight. The inadequacy of the phlogiston theory became apparent only in the late 18th century when it proved unable to explain a host of new facts about combustion that were being observed for the first time as the result of increasing accuracy in laboratory experiments.

The English natural philosopher Sir Francis Bacon observed in 1620 that a candle flame has a structure at about the same time that Robert Fludd, an English mystic, described an experiment on combustion in a closed container in which he determined that an amount of air was used up thereby. A German, Otto von Guericke, using an air pump he had invented in 1650, demonstrated that a candle would not burn in a container from which the air had been pumped. Robert Hooke, an English scientist, in 1665 suggested that air had an active component that, upon heating, combined with combustible substances, giving rise to flame. Another idea ascribed the high temperature of flame to the fast motion of active air particles, and it was learned that sulfur mixed with nitre can burn in the absence of air (nitre is a compound of oxygen which releases oxygen to the sulfur).

The first approximation of the true nature of combustion was posited by Lavoisier, who discovered in 1772 that the products of burned sulfur or phosphorus, in effect their ashes, outweighed the initial substances, and postulated that the increased weight was due to their having combined with air. Interestingly, it was already known that metals transformed by heat to metallic ash weighed less than the metallic ash, but the theory was that in certain cases phlogiston in metals had a negative weight, and upon escaping during combustion, left the ash of the metal heavier than it had been with the phlogiston in it. Later, Lavoisier concluded that the "fixed" air that had combined with the sulfur was identical to a gas obtained by Priestley on heating the metallic ash of mercury--that is, the "ashes" obtained when mercury was burned could be made to release the gas with which the metal had combined. This gas was also identical to that described by the Swedish chemist, Carl Wilhelm Scheele, as an active fraction of air that sustained combustion. Lavoisier called the gas "oxygen."

Lavoisier's theory that combustion was a reaction between the burning substance and the gas oxygen, present only to a limited extent in the atmosphere, was based on scientific principles, the most important of which was the law of the conservation of matter (after Einstein's relativity theory, of matter and energy): the total amount of matter in the universe is constant. Even ancient philosophers had guessed this law and it was substantiated in the 17th century. Lavoisier also clarified the concept of element into a modern generalization, that it was a substance that could not be broken down, and this, too, supported his theory. Soon after, studies of gases by John Dalton, and the first table of atomic weights that Dalton compiled, as well as many new gases discovered by other scientists, were important in supporting not only Lavoisier's theory of combustion but his whole new system of chemistry based on accurate measurement. The discoveries of nitrogen and hydrogen in the latter half of the 18th century, added to the earlier discoveries of carbon dioxide and carbon monoxide, and the discovery that the composition of air is remarkably constant though it is a mixture, all supported Lavoisier's theory. The proper explanation of combustion, perhaps the oldest recognized chemical reaction, is usually said to have been a keystone in the development of modern science.

From 1815 to 1819, Sir Humphry Davy experimented on combustion, including measurements of flame temperatures, investigations of the effect on flames of rarefied gases, and dilution with various gases; he also discovered catalytic combustion--the oxidation of combustibles on a catalytic surface accompanied by the release of heat but without flame.

Despite these discoveries, the materialistic theory of combustion lacked a clear concept of energy and, therefore, of the critical role that energy considerations play in an accurate explanation of combustion. It was Sir Benjamin Thompson's experiments with heat in 1798 that revealed evidence for the concept of heat as a movement of particles. Development of a kinetic theory of gases, based on the premise that heat results from the motion of molecules and atoms, of thermodynamics, and of thermochemistry, all in the 19th century, finally elucidated the energy aspects of combustion.

Investigation of burning velocities, experiments on the order of events in the combustion of gas mixtures, and study of the breaking down of gas molecules by heat (thermal dissociation), in the last half of the 19th century, played a vital part in the refinement of theories concerning combustion mechanism. Studies of light emitted by flames led to its analysis in the spectroscope, a device that separates a mixture of light waves into the component waves, and to spectral analysis generally, including theories of atomic and molecular spectra, which, in turn, contributed to an understanding of the nature of flames. The Bunsen burner was also of importance in the study of flame structure. Progress in industry was a powerful stimulus in the search for clarification of flame phenomena. Explosion hazards in coal mines had drawn attention to flame propagation as far back as 1815, when Davy invented his safety lamp. In 1881 detonation was discovered and led at the beginning of the 20th century to a detonation theory, based on the assumption that a gas behaved as a fluid under certain conditions. Chemical kinetics after the 1930s became an indispensable part offlame propagation theory.

Encyclopedia Brittanica

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