Combustion is an exothermic chemical reaction between a fuel and an oxidizer that forms an oxidized product. In general chemistry, it is one of the main types of chemical reactions. Technically, oxidation doesn’t always require oxygen, so combustion can occur without oxygen, too. However, if you’re a student in a general chemistry class, combustion is a reaction between a hydrocarbon fuel (e.g., coal, propane, wood, methane) and molecular oxygen (O2), producing carbon dioxide (CO2), water (H2O), and heat.
General Form of Combustion Reaction Equation
The general form of a combustion reaction is:
hydrocarbon + oxygen → carbon dioxide + water + heat
CxHy + O2 → CO2 + H2O
Examples of Combustion Reactions
Combustion is also called burning. So, any example of burning you can think of is a combustion reaction, including burning matches, candles, campfires, and gas burners. Here are examples of balanced equations for combustion reactions:
- Combustion of methane
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
- Burning of naphthalene
C10H8 + 12 O2 → 10 CO2 + 4 H2O
- Combustion of ethane
2 C2H6 + 7 O2 → 4 CO2 + 6 H2O
- Combustion of butane (commonly found in lighters)
2C4H10(g) +13O2(g) → 8CO2(g) +10H2O(g)
- Combustion of methanol (also known as wood alcohol)
2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(g)
- Combustion of propane (used in gas grills, fireplaces, and some cookstoves)
2C3H8(g) + 7O2(g) → 6CO2(g) + 8H2O(g)
How to Recognize a Combustion Reaction
You’ll know you have a combustion reaction when you see a hydrocarbon (molecule containing carbon and hydrogen) and oxygen gas (O2) on the reactant side (left side) of the reaction arrow and carbon dioxide (CO2) and water (H2O) on the product side (right side) of the reaction arrow. Also, combustion using oxygen always produces heat. The reaction still requires activation energy to start, but more heat is released by combustion than is absorbed starting it.
Many combustion reaction produce flames. If you see fire, it indicates a combustion reaction. However, combustion often occurs without fire. For example, smoldering is combustion without flames.
Sometimes it’s harder to recognize a combustion reaction because the reactant contains its own oxidizer (oxygen) or because combustion is incomplete, forming other products besides carbon dioxide and water. For example, some rockets rely on the reaction between Aerozine 50 (C2H12N4) and nitrogen tetroxide (N2O4). If you’re clever, you’ll see Aerozine 50 contains the necessary chemical bonds to act as a fuel (carbon-hydrogen and carbon-nitrogen) and nitrogen tetroxide supplies oxygen for combustion.
Then, there are forms of combustion that don’t even involve oxygen.
Combustion Without Oxygen
An oxidizer accepts electrons, usually by supplying oxygen to a chemical reaction. Other oxidizers include the halogens (fluorine, chlorine, etc.). Metallic fuels burn using fluoropolymers (e.g., Teflon, Viton), without the need for any oxygen at all.
Complete Versus Incomplete Combustion
Like other chemical reactions, combustion is subject to a limiting reactant and doesn’t always proceed to completion.
- Complete combustion or “clean combustion” occurs when the oxidation of a hydrocarbon only produces carbon dioxide and water. Burning candle wax is a good example of complete combustion. Heat from the burning wick vaporizes wax (a hydrocarbon). Wax reacts with oxygen, releasing carbon dioxide and water. The wax burns away and the carbon dioxide and water dissipate into the air.
- Incomplete combustion or “dirty combustion” is incomplete hydrocarbon oxidation, producing carbon monoxide (CO), carbon (soot), and other products, in addition to carbon dioxide and water. Wood and most fossil fuels undergo incomplete combustion, releasing these additional waste products.
- Lackner, Maximilian; Winter, Franz; Agarwal, Avinash K., eds. (2010). Handbook of Combustion. Wiley-VCH. ISBN 978-3-527-32449-1.
- Law, C.K. (2006). Combustion Physics. Cambridge University Press. ISBN 9780521154215.
- Schmidt-Rohr, K (2015). “Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2“. J. Chem. Educ. 92 (12): 2094–2099. doi:10.1021/acs.jchemed.5b00333