Combustion Definition in Chemistry

Combustion is a chemical reaction that occurs between a fuel and an oxidizing agent that produces energy, usually in the form of heat and light. Combustion is considered an exergonic or exothermic chemical reaction. It is also known as burning. Combustion is considered to be one of the first chemical reactions intentionally controlled by humans.

The reason combustion releases heat is because the double bond between oxygen atoms in O₂ is weaker than the single bonds or other double bonds. So, although energy is absorbed in the reaction, it is released when the stronger bonds are formed to make carbon dioxide (CO₂) and water (H₂O). While the fuel plays a role in the energy of the reaction, it's minor in comparison because the chemical bonds in the fuel are comparable to the energy of the bonds in the products.

Mechanics

Combustion occurs when fuel and an oxidant react to form oxidized products. Typically, energy must be supplied to initiate the reaction. Once combustion starts, the released heat can make combustion self-sustaining.

For example, consider a wood fire. Wood in the presence of oxygen in the air does not undergo spontaneous combustion. Energy must be supplied, as from a lit match or exposure to heat. When the activation energy for the reaction is available, the cellulose (a carbohydrate) in wood reacts with oxygen in the air to produce heat, light, smoke, ash, carbon dioxide, water, and other gases. The heat from the fire allows the reaction to proceed until the fire becomes too cool or the fuel or oxygen is exhausted.

Example Reactions

A simple example of a combustion reaction is the reaction between hydrogen gas and oxygen gas to produce water vapor:

2H₂(g) + O₂(g) → 2H₂O(g)

A more familiar type of combustion reaction is the combustion of methane (a hydrocarbon) to produce carbon dioxide and water:

CH₄ + 2O₂ → CO₂ + 2H₂O

which leads to one general form of a combustion reaction:

hydrocarbon + oxygen → carbon dioxide and water

Oxidants

The oxidation reaction may be thought of in terms of electron transfer rather than the element oxygen. Chemists recognize several fuels capable of acting as oxidants for combustion. These include pure oxygen and also chlorine, fluorine, nitrous oxide, nitric acid, and chlorine trifluoride. For example, hydrogen gas burns, releasing heat and light, when reacted with chlorine to produce hydrogen chloride.

Catalysis

Combustion isn't usually a catalyzed reaction, but platinum or vanadium can act as catalysts.

Complete Versus Incomplete Combustion

Combustion is said to be "complete" when the reaction produces a minimal number of products. For example, if methane reacts with oxygen and produces only carbon dioxide and water, the process is complete combustion.

Incomplete combustion occurs when there is insufficient oxygen for the fuel to convert completely to carbon dioxide and water. Incomplete oxidation of a fuel may also occur. It also results when pyrolysis occurs before combustion, as is the case with most fuels. In pyrolysis, organic matter undergoes thermal decomposition at high temperatures without reacting with oxygen. Incomplete combustion may yield many additional products, including char, carbon monoxide, and acetaldehyde.