Combustion Reactions in Chemistry

An Introduction to Combustion (Burning) Reactions

lighting a candle with a match

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A combustion reaction is a major class of chemical reactions, commonly referred to as "burning." In the most general sense, combustion involves a reaction between any combustible material and an oxidizer to form an oxidized product. It usually occurs when a hydrocarbon reacts with oxygen to produce carbon dioxide and water. Good signs that you're 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 those products but remain recognizable by the reaction of oxygen.

Combustion Doesn't Necessarily Mean Fire

Combustion is an exothermic reaction, meaning it releases heat, but sometimes the reaction proceeds so slowly that the change in temperature is not noticeable. 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

It's important to remember that combustion reactions are easy to recognize because the products always contain carbon dioxide and water. Here are several examples of balanced equations for combustion reactions. Note that while oxygen gas is always present as a reactant, in the trickier examples, 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, fireplaces, and some cookstoves)
    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's prone to limiting reactants the same as other processes. As a result, there are two types of combustion you're likely to encounter:

  • Complete Combustion: Also called "clean combustion," complete combustion is the oxidation of a hydrocarbon that produces only carbon dioxide and water. An example of clean combustion would be burning a wax candle: The heat from the flaming wick vaporizes the wax (a hydrocarbon), which in turn, 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, while the water vapor and carbon dioxide dissipate into the 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 (a fossil fuel), during which quantities of soot and carbon monoxide are released. In fact, many fossil fuels—including coal—burn incompletely, releasing waste products into the environment.