What is Combustion in Chemistry?

by Kate Onissiphorou

Combustion in chemistry is a type of exothermic chemical reaction. It happens when oxygen combines with an element or a compound, forming an oxide. If the compound is organic, such as methane, the final products of a complete combustion will be carbon dioxide and water.

Combustion, or burning, releases energy as heat and light, such as with a fire. It’s a very important chemical reaction that we rely on to cook our food, warm our homes, power industries, travel the world, and illuminate the dark. In fact, the taming of fire is what paved the way for the development of human civilisation. 

What Are the Different Types of Combustion?

Combustion is one of the five main types of chemical reactions and can be divided into several subcategories.

Oxygen is always present in combustion reactions, typically in diatomic form. A high temperature or spark is needed to trigger this reaction. Oxygen may either combine with an element or a compound, which can either be inorganic or organic.

Combustion always produces heat and light as byproducts, in addition to several chemical products. It can be classified into several subcategories based on the speed and energy of the reaction:

  • Complete combustion: This is where there’s sufficient supply of oxygen and the reactant compound is completely consumed. If the reactant is a hydrocarbon compound or mixture, the final products are carbon dioxide and water in the form of steam. An example of this reaction is in car engines. However, it seldom occurs naturally, so cars are fitted with catalytic converters to aid in the complete combustion of the fuel.Internal car engine
  • Incomplete combustion: This occurs when the supply of oxygen is too low to completely consume the reactant. It also occurs when the reactant has impurities that aren’t completely flammable. If the reactant is a hydrocarbon or any organic compound, byproducts include soot and carbon monoxide.
  • Rapid combustion: As the name suggests, rapid combustion has a fast rate of reaction. This type of reaction releases relatively large amounts of heat and light energy, which produce either fire or an explosion. The engines of cars and other internal-combustion machines are designed based on the rapid combustion of fossil fuels.
  • Smouldering combustion: This is a very slow reaction. Unlike rapid combustion, it doesn’t produce a flame and has a relatively low temperature. Typically, the surface of the fuel, such as wood, is slowly consumed by oxygen. It’s a type of combustion that produces the steady glow of embers you see in a dying fire. 
  • Spontaneous combustion: Many highly volatile substances, like hydrocarbons, can combust without a spark or flame as a trigger. Volatile compounds have high vapour pressure but low ignition temperature. If they’re inside a container, heat and pressure can build up, which may lead to spontaneous combustion. Some coals also spontaneously combust when exposed to high levels of oxygen.
  • Turbulent combustion: This is usually a practical application of combustion in engines and gas turbines. The turbulence is intended to enhance the efficient reactions between the fuel and oxygen.
  • Microgravity combustion: This type of combustion behaves very differently compared to normal combustion on Earth. Experiments are done in space stations to see the behaviour of flames. Under very low gravity in orbit due to the continuous ‘falling’ of the space station, or any spacecraft in orbit, a flame takes the form of a sphere.

What Type of Reaction is Combustion?

Combustion is a thermochemical reaction, which means that heat is always generated. It can also be classified as an oxidation reaction because it produces an oxide. It requires three ingredients in order to occur: oxygen, fuel, and a trigger.

  • Oxygen: Although some chemical reactions, like rusting, also involve oxygen, combustion is different because it’s relatively fast and highly energetic.
  • The fuel: This could be anything that can be consumed by oxygen in a relatively fast manner, such as magnesium, wood, or other fossil fuels.
  • A trigger: This could be a spark, a flame, or simply the heat buildup of a highly volatile liquid. It could also be exposure to high levels of oxygen.

A graphic showing how combustion worksWhat is the Difference Between Complete and Incomplete Combustion?

Combustion reactions are either ‘complete’ or ‘incomplete’. This means that the fuel or reactant can either fully or partially combine with oxygen. 

If it partially combines, residues of the original substance may remain, or some other less oxidised forms of products may be produced. Two main factors affect combustion: temperature and the amount of oxygen.

What is Complete Combustion?

Complete combustion is a reaction wherein the fuel reactant is completely consumed by oxygen. It usually occurs at high temperatures. 

For example, the industrial preparation of sulphur trioxide, also known as nisso sulfan, is done at very high temperatures. Sulphur trioxide (SO3) is the most important sulphur oxide because it serves as a precursor to sulphuric acid. It is prepared by oxidising sulphur dioxide at 400-600°C with the aid of a catalyst.

In the combustion of hydrocarbons, the final products of a complete reaction are carbon dioxide and water. If there are some non-combustible impurities like metals, these will become ashes or oxides. See the illustration below for an example:

A graphic showing the combustion of hydrocarbonsAs you can see, one molecule of methane reacts with two diatomic molecules of oxygen to form one molecule of carbon dioxide and two molecules of water.

What is Incomplete Combustion?

Incomplete combustion typically happens when there is insufficient oxygen to completely consume the fuel. It may also happen if the fuel is not thoroughly mixed with oxygen. In the internal combustion chambers of some modern car engines, fuel is injected with the aid of a computerised system to maximise combustion.

However, even with a high degree of precision, the fuel may still only partially combine with oxygen. To ensure that pollutants are minimised, catalytic converters are used as after-burning devices that aid the complete oxidation of carbon monoxide exhaust. In some designs, the exhaust gases are rerouted to the engine for complete combustion. 

What is the Equation For Combustion?

The general chemical equation for combustion depends on the type of fuel. If the fuel is an inorganic substance like magnesium, it may simply form an oxide. However, if the fuel is organic, the products are water and carbon dioxide as shown in the general chemical equation below:

A + O2 →  H2O + CO2 + heat and light energy


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