Combustion Reactions in Chemistry

3D illustration of methane combustion
A methane combustion reaction.

JSquish / Wikimedia Commons / CC BY 3.0

A combustion reaction is a major class of chemical reactions, commonly referred to as "burning." Combustion usually occurs when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. In the more general sense, combustion involves a reaction between any combustible material and an oxidizer to form an oxidized product. Combustion is an exothermic reaction, so it releases heat, but sometimes the reaction proceeds so slowly that a temperature change is not noticeable. Good signs that you are dealing with a combustion reaction include the presence of oxygen as a reactant and carbon dioxide, water, and heat as products. Inorganic combustion reactions might not form all of the products but are recognizable by the reaction of oxygen.

Combustion doesn't always result in fire, but when it does, a flame is a characteristic indicator of the reaction. While the activation energy must be overcome to initiate combustion (i.e.: using a lit match to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining.

General Form of a Combustion Reaction

hydrocarbon + oxygen → carbon dioxide + water

Examples of Combustion Reactions

Here are several examples of balanced equations for combustion reactions. It's important to remember that combustion reactions are easy to recognize because the products always contain carbon dioxide and water. In these examples, oxygen gas is present as a reactant, but trickier examples of the reaction exist where the oxygen comes from another reactant.

  • 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 and fireplaces)
    2C3H8(g) + 7O2(g) → 6CO2(g) + 8H2O(g)

Complete Versus Incomplete Combustion

Combustion, like all chemical reactions, does not always proceed with 100% efficiency. It is prone to limiting reactants the same as other processes. So, there are two types of combustion you are likely to encounter:

  • Complete Combustion - Also called "clean combustion", clean combustion is oxidation of a hydrocarbon that produces only carbon dioxide and water. An example of clean combustion is burning of candle wax, where the heat from the wick vaporizes wax (a hydrocarbon), which reacts with oxygen in the air to release carbon dioxide and water. Ideally, all the wax burns so nothing remains once the candle is consumed. The water vapor and carbon dioxide dissipate into air.
  • Incomplete Combustion - Also called "dirty combustion", incomplete combustion is hydrocarbon oxidation that produces carbon monoxide and/or carbon (soot) in addition to carbon dioxide. An example of incomplete combustion would be burning coal, where a lot of soot and carbon monoxide is released. Many of the fossil fuels burn incompletely, releasing waste products.