Combustion reactions release energy in the form of heat and light when a fuel and oxidizing agent react. Oxidation involves removing electrons from a substance, often involving oxygen. Spontaneous combustion can occur if the oxidizing agent is strong enough and the fuel ignites easily. Combustion products are typically oxides, and incomplete combustion can produce toxic gases like carbon monoxide. Physical factors like surface area and vapor production affect flammability. Cellular respiration is a slow combustion process that produces heat and maintains body temperature. Combustion is used for warmth, cooking, melting metals, generating electricity, and powering vehicles.
A combustion reaction occurs when a fuel and an oxidizing agent, or oxidizer, react, releasing energy in the form of heat and sometimes light. The best known processes of this type involve the combustion of organic materials containing carbon and hydrogen, which combine with the oxygen in the air to form carbon dioxide and water. Here the fuel is something like wood, gasoline or coal and the oxidizer is oxygen. However, many other types of combustion reactions are possible. Reactions of this type are essential for life and are used to generate energy, provide heat, run motor vehicles and in many other ways.
Oxidation
Oxidation is a chemical process in which electrons are removed from a substance, releasing energy. The term comes from the fact that the most familiar examples involve substances that combine with oxygen, which acquires electrons in the process. Other elements, however, can also act as oxidizers. For example, the element fluorine is a more powerful oxidizer than oxygen. The process can happen extremely rapidly which can lead to an explosion where all the energy is released almost instantaneously and the hot gas expands violently creating a pressure wave and loud noise. Alternatively, it may occur moderately rapidly, as in the case of a fire, or much more slowly, as in the rusting of iron.
Spontaneous combustion
Usually, energy must be supplied to initiate a combustion reaction. This can be as simple as striking a match or generating a spark. After this, the reaction is self-sustaining, meaning that the energy released by the reaction keeps it going.
In some cases, however, sufficient energy may be present at room temperature, or even lower, for the reaction to proceed. It all depends on the oxidizer and the fuel: if the oxidizing agent is strong enough and the fuel ignites easily, they can catch fire when mixed, without the need to provide heat. This is known as spontaneous combustion. Strong oxidants must therefore be handled with great care, as they can cause fires or explosions if they come into contact with flammable materials.
Combustion products
Since combustion usually involves elements in the fuel combining with oxygen, the products are typically oxides. In organic substances, carbon and hydrogen usually combine with oxygen to produce carbon dioxide (CO2) and water (H2O). However, other substances can also burn. For example, sulfur and phosphorus burn easily, producing oxides. Even metals, if pulverized, will burn, forming oxides and often producing brilliantly bright flames: for this reason magnesium, aluminum and other metals are often used in fireworks.
Often a combustion reaction involving organic materials is incomplete. In the case of wood, for example, some of the unburned carbon is released as tiny particles in the form of smoke, and some is usually left behind as charcoal. When there is not enough oxygen available to oxidize all of the carbon in some fuels to carbon dioxide (CO2), another gas called carbon monoxide (CO) can be produced. When this occurs in an enclosed space, as might be the case with a faulty boiler, the consequences can be fatal, as CO is toxic and odourless.
Factors affecting flammability
In addition to chemical factors, such as fuel and oxidant reactivity, there are a number of physical factors that influence flammability. One of them is the surface of the fuel that comes into contact with the oxidizer. Under normal circumstances it is not possible to cause a piece of iron to burn, but in the form of an extremely fine powder, this metal will spontaneously ignite in air.
Liquid fuels don’t actually burn, although they often appear to do. It is the vapor released by fuel that ignites, so the flammability of a liquid depends in part on how much vapor it produces. The temperature at which there is enough vapor in the air to ignite it is known as the flash point; this is important information for the storage and handling of flammable liquids
Cellular respiration
This is the process by which the cells of living organisms oxidize nutrients such as carbohydrates into carbon dioxide and water. Since the end products are the same as if these raw materials were burned, the overall reaction can be considered a combustion, but since it occurs in several separate phases, it is much slower than is usually understood by this term. However, it still produces heat and helps maintain body temperature. In one well-known demonstration, a small amount of sugar is mixed with a strong oxidizer and ignited, causing it to burn hard enough to melt the glass, showing how much energy is locked up in the sugar molecules. In the body, this energy is released much more slowly, but the principle is the same.
it is used
Primitive man first used fire for warmth and then for cooking food, a practice that killed harmful microorganisms and parasites. The Industrial Revolution depended on the burning of fuels – initially wood, and then fossil fuels such as coal and oil – to provide the heat needed to melt metals. Today, combustion is used to generate electricity and produce a wide variety of chemicals and products, and the internal combustion engine also uses the rapid burning of fossil fuels to provide the kinetic energy that powers cars and other vehicles.
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