A combustion reaction 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. 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. Heat provides the activation energy to start the chemical reaction. The combination of oxygen, fuel, and heat forms the fire triangle, which is one way to represent the requirements for combustion.
The general form of a combustion reaction is:
hydrocarbon + oxygen → carbon dioxide + water + heat
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:
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.
Technically, oxidation doesn’t always require oxygen, so combustion can occur without oxygen, too.
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.
Like other chemical reactions, combustion is subject to a limiting reactant and doesn’t always proceed to completion.
