History demonstrates that combustion is inseparable from human civilization. Combustion reactions are highly variable in terms of the molecules involved and the amounts of energy produced, but most reactions can be classified as combustion if they include oxygen and release energy in the form of heat and light. Modern societies have developed numerous practical applications for combustion reactions.
Reactants, Products and Energy
Like all chemical reactions, combustion reactions transform existing molecules, known as reactants, into different molecules, known as products. The defining reactant in combustion reactions is oxygen, and the products vary according to the other molecules that react with oxygen. Many chemical reactions also lead to the movement of energy between the reactive system and the surrounding environment; endothermic reactions absorb energy and exothermic reactions release it. Combustion reactions are exothermic because they release heat and light energy, and most types of combustion release this energy in the form of a flame.
Heat from Hydrocarbons
In the most familiar type of combustion reactions, hydrocarbons react with oxygen to produce carbon dioxide, water vapor and energy. A hydrocarbon is an organic compound containing hydrogen and carbon. The majority of common fuels -- including natural gas, propane, kerosene, gasoline and wood -- are composed of hydrocarbons. Many furnace and engine systems rely upon the combustion of hydrocarbons because these reactions release large amounts of heat, which can be directly used to warm up a building or converted into mechanical rotation that powers a vehicle.
Energy From Alcohol
Alcoholic substances have long served as fuels for combustion reactions. Though largely ignored during the years when petroleum was inexpensive and abundant, alcohol-based fuels are now being intensely researched as potential alternatives to fossil fuels. Perhaps the most familiar example is ethanol, which is now mixed with gasoline for use in standard internal combustion engines. The ethanol molecule is composed of hydrogen and carbon like standard hydrocarbon fuels, but it also contains oxygen. The combustion of ethanol produces carbon dioxide and water vapor, as with hydrocarbon combustion, but the amount of energy released is significantly lower than the amount released by gasoline combustion.
Power Without Carbon
Hydrogen is a pervasive element in natural environments because it combines with oxygen to form water, which of course is non-flammable. Under different circumstances, however, hydrogen and oxygen form a highly explosive mixture. Researchers are exploring the combustion reaction of pure hydrogen gas and oxygen as a source of environmentally-friendly energy because the supply of hydrogen is virtually unlimited, and because this particular reaction has only one major product: water. Many scientists assert that rising levels of atmospheric carbon could lead to devastating climatic consequences. Hydrogen, in contrast to hydrocarbons and alcohols, releases no carbon dioxide when burned.
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