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Combustion

Combustion /kəmˈbʌs.tʃən/ or burning is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion in a fire produces a flame, and the heat produced can make combustion self-sustaining. Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):

Combustion of an organic fuel in air is always exothermic because the double bond in O₂ is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO₂ and H₂O results in the release of energy. The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH₄ is nearly the same as that of CO₂. The heat of combustion is approximately -418 kJ per mole of O₂ used up in the combustion reaction, and can be estimated from the elemental composition of the fuel.

Round Table

The Round Table is King Arthur's famed table in the Arthurian legend, around which he and his Knights congregate. As its name suggests, it has no head, implying that everyone who sits there has equal status. The table was first described in 1155 by Wace, who relied on previous depictions of Arthur's fabulous retinue. The symbolism of the Round Table developed over time; by the close of the 12th century it had come to represent the chivalric order associated with Arthur's court, the Knights of the Round Table.

History

The Round Table first appears in Wace's Roman de Brut, a Norman language adaptation of Geoffrey of Monmouth's Historia Regum Britanniae finished in 1155. Wace says Arthur created the Round Table to prevent quarrels among his barons, none of whom would accept a lower place than the others.Layamon added to the story when he adapted Wace's work into the Middle English Brut in the early 13th century, saying that the quarrel between Arthur's vassals led to violence at a Yuletide feast. In response a Cornish carpenter built an enormous but easily transportable Round Table to prevent further dispute. Wace claims he was not the source of the Round Table; both he and Layamon credited it instead to the Bretons. Some scholars have doubted this claim, while others believe it may be true. There is some similarity between the chroniclers' description of the Round Table and a custom recorded in Celtic stories, in which warriors sit in a circle around the king or lead warrior, in some cases feuding over the order of precedence as in Layamon. There is a possibility that Wace, contrary to his own claims, derived Arthur's round table not from any Breton source, but rather from medieval biographies of Charlemagne—notably Einhard's Vita Caroli and Notker the Stammerer's De Carolo Magno—in which the king is said to have possessed a round table decorated with a map of Rome.