combustion

Chemistry

2022

We explain what combustion is, how it occurs and what are the stages of the reaction. Also, classification and examples.

Combustion is a chemical reaction that releases light and heat energy.

What is combustion?

Combustion is a type of exothermic chemical reaction. Can involve matter in gaseous state or in a heterogeneous state (liquid-gas or solid-gas). Generate light Y heat in most cases, and it occurs considerably quickly.

Traditionally, combustion is understood as a process of oxidation speed of certain fuel elements, that is, made up mainly of hydrogen, carbon and sometimes sulfur. Furthermore, it necessarily takes place in the presence of oxygen.

Actually, the combustions are redox reactions (reduction-oxidation) that can occur both in a controlled way, such as in internal combustion engines, or uncontrolled, such as in explosions. These reactions involve exchange of electrons between the atoms of matter during the reaction.

Most of the time, the combustions generate thermal energy Y light and they also produce other gaseous and solid substances, such as carbon dioxide (CO2) and water steam, or the solid residues of the fuel (the substance consumed in the reaction) and of the oxidizer (the substance that promotes the reaction). The substances generated depend on the chemical nature of the reagents involved in combustion.

In this way, although in the traditional image of combustion there is always fire involved, it is possible that fire is not generated, since it is nothing more than a form of plasma (ionized gas) product of the release of heat from the chemical reaction of combustion, which to form depends on the conditions and reactants of each specific reaction.

How does combustion take place?

Combustion always results in CO2, water vapor, energy, and another compound.

Combustion is a type of redox reaction, that is, a reduction-oxidation reaction. This means that in them one reactant is oxidized (loses electrons), while the other is reduced (gains electrons).

In the case of combustion, the oxidizing agent (oxygen) obtains electrons from the reducing agent (fuel), or what is the same, the oxidizer (oxygen), obtains electrons from the fuel. This is generally given according to the following formula:

Combustion compounds can vary in each combustion reaction, depending on their nature, just as the levels of fuel can vary. Energy generated. But carbon dioxide and water are produced in some way in all combustions.

Types of combustion

There are three types of combustion:

  • Complete or perfect combustions. They are those reactions in which the combustible material is totally oxidized (consumed) and other oxygenated compounds are produced, such as carbon dioxide (CO2) or sulfur dioxide (SO2), as the case may be, and water (H2O).
  • Stoichiometric or neutral combustions. This is the name given to the ideal complete combustions, which use just the right amounts of oxygen for their reaction and which generally occur only in the environment controlled from a laboratory.
  • Incomplete combustions. They are those reactions in which compounds that are not completely oxidized (also called unburned) appear in the combustion gases. Such compounds can be carbon monoxide (CO), hydrogen, carbon particles, and so on.

Combustion reaction

Combustion processes actually comprise a set of fast and simultaneous chemical reactions. Each of these reactions can be called a stage or phase. The three fundamental stages of combustion are:

  • Pre-reaction or first stage. The hydrocarbons present in the combustible material decompose and begin their reaction with the oxygen in the air, forming radicals (molecularly unstable compounds). This starts a chain reaction of appearance and disappearance of chemical compounds where, in general, more compounds are formed than decomposed.
  • Oxidation or second stage. At this stage, most of the heat energy of the reaction is generated. As oxygen reacts with the radicals from the previous stage, a process of displacement violent of electrons. In the case of explosions, a high number of radicals leads to a massive and violent reaction.
  • End of reaction or third stage. It occurs when the oxidation of radicals is completed and the molecules stable that will be the products of combustion.

Examples of combustion

Combustion occurs within engines that releases energy for movement.

Some simple examples of combustion in everyday life are:

  • The lighting of a match / match. It is the most emblematic case of combustion. When the phosphor head (covered with phosphorous and sulfur) is scraped against a rough surface, it is heated by the friction and triggers a rapid combustion, which in turn produces a short flame.
  • The lighting of a gas stove. Domestic kitchens work by burning a hydrocarbon gas, generally mixture of propane (C3H8) and butane (C4H10), which the appliance extracts from a pipe or a container. Placed in contact with the air and provided with an initial charge of heat energy (such as the pilot's flame, or that of a phosphor), the gas begins its reaction; but to keep the flame burning, fuel must be supplied continuously.
  • Strong foundations and organic material. Most of the bases strong (hydroxides) such as caustic soda, caustic potash and other pH extreme basic, they generate violent oxidation reactions when they come into contact with the organic material. This means that we can get burned by contact with these substances and even start fires with them, since these reactions are usually very exothermic.
  • Internal combustion engines. These devices are present in cars, boats and other vehicles that operate with fossil fuels such as diesel, gasoline or kerosene. They are an example of the use of controlled combustion. In them, the hydrocarbons in the fuel are consumed and small explosions are generated that, within the piston system, are transformed into movement, also producing polluting gases, which are released into the atmosphere.
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