Combustion

We explain what combustion is, how it is produced and what are the stages of the reaction. In addition, classification and examples.

1. What is combustion?

Combustion is a type of exothermic chemical reaction . It may involve matter in the gaseous state or in the heterogeneous state (liquid-gaseous or gaseous-gaseous). It generates light and heat and is produced more quickly or less quickly.

Traditionally, combustion is understood as a process of rapid oxidation of certain combustible elements, that is, mainly constituted by hydrogen, carbon and sometimes sulfur. In addition, it necessarily takes place in the presence of oxygen .

In fact, combustions are forms of redox reaction (reduction-oxidation) that can occur both in a controlled manner, as in internal combustion engines, or uncontrolled, as in explosions They involve the exchange of electrons between the atoms of matter during the reaction. Therefore, they generate thermal and light energy.

In addition, other gaseous and solid substances, such as carbon dioxide (CO2) and water vapor, or solid fuel residues (the substance consumed in the reaction) result n) and the oxidizer (the substance that promotes the reaction), always according to its chemical nature.

Thus, although in the traditional image of combustion there is always fire involved, it is possible that this is not generated, since it is nothing more than a form of plasma (ionized gas) resulting from the release of heat from the chemical reaction.

See also: Enthalpy

1. How does combustion occur?

Combustions are a type of redox reaction, that is, oxidation reduction. This means that in them one material oxidizes (loses electrons), while the other one reduces (gains electrons).

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

Compound + O ₂ = Other compound + CO ₂ + H ₂ O + Energy

The compounds may vary, depending on their nature, as well as energy levels may vary. But carbon dioxide and water vapor are constant in all types of combustion.

1. Types of combustion

There are three types of combustion, which are the following:

• Complete or perfect combustion . Those reactions in which the combustible material is completely oxidized (consumed), and other oxygenated compounds are subproduced, such as carbon dioxide, sulfur dioxide or water vapor.
• Stoichiometric or neutral combustion . This is called the ideal complete combustion, which uses just amounts of oxygen for its reaction and that usually occurs only in the controlled environment of a laboratory.
• Incomplete Combustion Those in which compounds appear to oxidize (also called unburned) of flue gases, such as carbon monoxide (CO), hydrogen, carbon particles, and so on.

1. Combustion reaction

Combustion processes actually comprise a set of rapid and simultaneous chemical reactions, which may well be considered as one, which goes through the following phases or stages:

• Pre-reaction or first stage . The hydrocarbons present in the combustible material decompose and begin their reaction with the oxygen in the air, thus forming radicals, which are molecularly unstable compounds. This initiates a chain reaction of appearance and disappearance of chemical compounds, always tending to create more than it destroys.
• Oxidation or second stage . In this stage most of the heat energy of the reaction is generated, as oxygen reacts with the radicals of the previous stage, thus giving violent electron displacement process. A high number of radicals leads to a massive and violent reaction, known as an explosion.
• End of the reaction or third stage . It occurs when the oxidation of the radicals is complete and the molecules of the released gases are formed. The combustible material will be completely depleted.

1. Examples of combustion

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 head of the match (phosphorus and sulfur cover) is scraped against a rough surface, it is heated by friction and triggers a rapid combustion, which in turn produces a flame brief.
• The lighting of a gas stove . Domestic kitchens work by burning a hydrocarbon gas, usually a mixture of propane (C ₃ H ₈ ) and butane (C ₄ H ₁₀ ), which the device draws from a pipe of a bowl Placed in contact with the air, and provided an initial charge of caloric energy (such as the pilot's flame, or that of a phosphorus), the gas begins its reaction; but to keep the flame going, fuel must be supplied continuously.
• Strong bases and organic matter . Most of the strong bases (hydroxides) such as caustic soda, caustic potash and other substances of extreme pH, generate violent oxidation reactions upon contact with the organic matter. This means that we can burn by contact and even start fires with them, since these types of reactions tend to be very exothermic.
• The internal combustion engines . Present in cars, boats and other vehicles that operate with fossil fuel such as diesel, gasoline or kerosene, these devices are an example of controlled combustion. In them the hydrocarbons of the fuel are consumed and small explosions are generated that, within the piston system, are transformed into movement, also byproducing polluting gases, which are released into the atmosphere.

Continue with: Endothermic reactions