# 11.6: Combustion Reactions

Roasting marshmallows over an open fire is popular whilst camping and during cookouts. The trick is to get the marshmallow a nice golden brown, without catching it on fire. Too often, the person roasting the marshmallow is not successful, and the marshmallow is seen suddenly burning on the stick—a combustion reaction is taking place right in front of them!

## Combustion Reactions

A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve $$\ce{O_2}$$ as one reactant. The combustion of hydrogen gas produces water vapor:

$2 \ce{H_2} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow 2 \ce{H_2O} \left( g \right)$

Notice that this reaction also qualifies as a combination reaction. Figure $$\PageIndex{1}$$: Explosion of the Hindenberg. (CC BY-NC; CK-12)

The Hindenberg was a hydrogen-filled airship that suffered an accident upon its attempted landing in New Jersey in 1937. The hydrogen immediately combusted in a huge fireball, destroying the airship and killing 36 people. The chemical reaction was a simple one: hydrogen combining with oxygen to produce water.

Many combustion reactions occur with a hydrocarbon, a compound made up solely of carbon and hydrogen. The products of the combustion of hydrocarbons are carbon dioxide and water. Many hydrocarbons are used as fuel because their combustion releases very large amounts of heat energy. Propane $$\left( \ce{C_3H_8} \right)$$ is a gaseous hydrocarbon that is commonly used as the fuel source in gas grills.

$\ce{C_3H_8} \left( g \right) + 5 \ce{O_2} \left( g \right) \rightarrow 3 \ce{CO_2} \left( g \right) + 4 \ce{H_2O} \left( g \right)$

Example $$\PageIndex{1}$$

Ethanol can be used as a fuel source in an alcohol lamp. The formula for ethanol is $$\ce{C_2H_5OH}$$. Write the balanced equation for the combustion of ethanol.

Solution

Step 1: Plan the problem.

Ethanol and oxygen are the reactants. As with a hydrocarbon, the products of the combustion of an alcohol are carbon dioxide and water.

Step 2: Solve.

Write the skeleton equation:

$\ce{C_2H_5OH} \left( l \right) + \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + \ce{H_2O} \left( g \right)$

Balance the equation.

$\ce{C_2H_5OH} \left( l \right) + 3 \ce{O_2} \left( g \right) \rightarrow 2 \ce{CO_2} \left( g \right) + 3 \ce{H_2O} \left( g \right)$