Combustion
Combustion is the chemical term for a process known more commonly as burning. It is certainly one of the earliest chemical changes noted by humans, at least partly because of the dramatic effects it has on materials. Today, the mechanism by which combustion takes place is well understood and is more correctly defined as a form of oxidation that occurs so rapidly that noticeable heat and light are produced.
Probably the earliest reasonably scientific attempt to explain combustion was that of Johannes (or Jan) Baptista van Helmont (1580-1644), a Flemish physician and alchemist. Van Helmont observed the relationship among a burning material, smoke, and flame and said that combustion involved the escape of a spiritus silvestre (wild spirit) from the burning material. This explanation was later incorporated into a theory of combustion--the phlogiston theory--that dominated alchemical thinking for the better part of two centuries.
According to the phlogiston theory, combustible materials contain a substance--phlogiston--that is emitted by the material as it burns. A non-combustible material, such as ashes, will not burn, according to this theory, because all phlogiston contained in the original material (such as wood) had been driven out. The phlogiston theory was developed primarily by the German alchemist Johann Becher and his student Georg Ernst Stahl at the end of the seventeenth century.
Although scoffed at today, the phlogiston theory satisfactorily explained most combustion phenomena known at the time of Becher and Stahl. One serious problem was a quantitative issue. Many objects weigh more after being burned than before. How this could happen when phlogiston escaped from the burning material? One possible explanation was that phlogiston had negative weight, an idea that many early chemists thought absurd, while others were willing to consider. In any case, precise measurements had not yet become an important feature of chemical studies, so loss of weight was not an insurmountable barrier to the phlogiston concept.
As with so many other instances in science, the phlogiston theory fell into disrepute only when someone appeared on the scene who could reject traditional thinking almost entirely and propose a radically new view of the phenomenon. That person was the great French chemist Antoine Laurent Lavoisier (1743-1794). Having knowledge of some recent critical discoveries in chemistry, especially the discovery of oxygen by Karl Wilhelm Scheele (1742-1786) in 1771 and Joseph Priestley (1733-1804) in 1774, Lavoisier framed a new definition of combustion. Combustion, he said, is the process by which some material combines with oxygen. By making the best use of precise quantitative experiments, Lavoisier provided such a sound basis for his new theory that it was widely accepted in a relatively short period of time.
Lavoisier initiated another important line of research related to combustion, one involving the amount of heat generated during oxidation. His earliest experiments involved the study of heat lost by a guinea pig during respiration, which Lavoisier called "a combustion." In this work, he was assisted by a second famous French scientist, Pierre Simon Laplace (1749-1827). As a result of their research, Lavoisier and Laplace laid down one of the fundamental principles of thermochemistry, namely that the amount of heat needed to decompose a compound is the same as the amount of heat liberated during its formation from its elements. This line of research was later developed by the Swiss-Russian chemist Henri Hess (1802-1850) in the 1830s. Hess' development and extension of the work of Lavoisier and Laplace has earned him the title of father of thermochemistry.
From a chemical standpoint, combustion is a process in which chemical bonds are broken and new chemical bonds formed. The net result of these changes is a release of energy, the heat of combustion. For example, suppose that a gram of coal is burned in pure oxygen with the formation of carbon dioxide as the only product. In this reaction, the first step is the destruction of bonds between carbon atoms and between oxygen atoms. In order for this step to occur, energy must be added to the coal/oxygen mixture. For example, a lighted match must be touched to the coal.
Once the carbon-carbon and oxygen-oxygen bonds have been broken, new bonds between carbon atoms and oxygen atoms can be formed. These bonds contain less energy than did the original carbon-carbon and oxygen-oxygen bonds. That energy is released in the form of heat, the heat of combustion. The heat of combustion of one mole of carbon, for example, is about 94 kcal.
Humans have been making practical use of combustion for millennia. Cooking food and heating homes have long been two major applications of the combustion reaction. With the development of the steam engine by Denis Papin, Thomas Savery, Thomas Newcomen, and others at the beginning of the eighteenth century, however, a new use for combustion was found: performing work. Those first engines employed the combustion of some material, usually coal, to produce heat that was used to boil water. The steam produced was then able to move pistons and drive machinery. That concept is essentially the same one used today to operate fossil-fueled electrical power plants.
Before long, inventors found ways to use steam engines in transportation, especially in railroad engines and steam ships. However, it was not until the discovery of a new type of fuel-gasoline and its chemical relatives and a new type of engine--the internal combustion engine--that the modern face of transportation was achieved. Today, most forms of transportation depend on the combustion of a hydrocarbon fuel such as gasoline, kerosene, or diesel oil to produce the energy that drives pistons and moves the vehicles on which modern society depends.
When considering how fuels are burned during the combustion process, "stationary" and "explosive" flames are treated as two distinct types of combustion. In stationary combustion, as generally seen in gas or oil burners, the mixture of fuel and oxidizer flows toward the flame at a proper speed to maintain the position of the flame. The fuel can be either premixed with air or introduced separately into the combustion region. An explosive flame, on the other hand, occurs in a homogeneous mixture of fuel and air in which the flame moves rapidly through the combustible mixture. Burning in the cylinder of a gasoline engine belongs to this category. Overall, both chemical and physical processes are combined in combustion, and the dominant process depends on very diverse burning conditions.
The use of combustion as a power source has had such a dramatic influence on human society that the period after 1750 has sometimes been called the Fossil Fuel Age. Still, the widespread use of combustion for human applications has always had its disadvantages. Pictorial representations of England during the Industrial Revolution, for example, usually include huge clouds of smoke emitted by the combustion of wood and coal in steam engines.
Today, modern societies continue to face environmental problems created by the prodigious combustion of carbon-based fuels. For example, one product of any combustion reaction in the real world is carbon monoxide, a toxic gas that is often detected at dangerous levels in urban areas around the world. Oxides of sulfur, produced by the combustion of impurities in fuels, and oxides of nitrogen, produced at high temperature, also have deleterious effects, often in the form of acid rain and smog. Even carbon dioxide itself, the primary product of combustion, is suspected of causing global climate changes because of the enormous concentrations it has reached in the atmosphere.
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