Combustion Reactions — Definition & Examples

What Are Combustion Reactions?

Combustion reactions are a special type of redox reaction, which are chemical reactions where oxygen is the oxidizing agent.

• Our colloquial term for combustion is burning.

• You can often recognize a combustion reaction by the flame. Although technically, a flame is only produced when some of the fuel is vaporized before combusting.

• Common examples of combustion are lighting a fire in the fireplace, striking a match, combusting gasoline in your car, or burning coal for electricity.

• Most often, we are reacting a carbon-based reactant with diatomic oxygen in the air. So, our products are carbon dioxide ($CO_2$) and water.

• There are a few exceptions. For example, our reactant could be sulfur-based or hydrogen gas. In those cases, our product is sulfur dioxide or water.

• They are always exothermic reactions. So, they always release energy as heat.

• The generic balanced chemical equation is $C_{x}H_{y} + zO_{2} \rightarrow xCO_{2} + \frac{y}{2}H_{2}O$.

Ingredients for Combustion

Combustion reactions require three things...

Fuel

• The fuel is whatever substance is burned or consumed during the reaction. The most common fuels are carbon-based organic compounds.

Oxygen

• The oxygen is typically provided by the air around the reaction. But sometimes compounds can release oxygen. For example, when potassium chlorate $KClO_3$ is heated, it decomposes and releases oxygen.

Heat

• Heat is also needed for a combustion reaction. Why? We are breaking covalent bonds! To start the reaction, we need to overcome the activation energy. Often, high temperatures are applied through a flame, spark, or heating apparatus.

• There are rare cases of spontaneous combustion. In most cases, a side reaction produces enough heat for the system to reach the ignition temperature. Some common examples occur in hay bales and compost piles. In them, bacteria are fermenting the organic compounds. Fermentation is an exothermic process. So, the fermentation reaction can sometimes produce enough heat to initiate the combustion reaction.

Example of a Combustion Reaction

A fun experiment to demonstrate combustion involves combusting gummy bears. Gummy bears are made of sucrose. It acts as the fuel source. We start by adding potassium chlorate ($KClO_3$) to a test tube and heating it with a flame. As the compound is heated, it decomposes and releases oxygen.

$2KClO_3\ (s)\ \rightarrow\ 2KCl\ (s)\ +\ 3O_2\ (g)$

The residual heat from decomposing $KClO_3$ is enough of a heat source. The released oxygen is the oxygen source. Once the heat and oxygen are present in the tube, the gummy bear is added. The gummy bear immediately begins to combust! Let's look at the equation for the combustion of sucrose.

$C_{12}H_{22}O_{11}\ (s) +\ 12O_2\ (g)\ \rightarrow\ 12CO_2\ (g)\ +\ 11H_2O\ (l)$

If you do this experiment, you'll observe a purple flame. The color is due to the absorption and emission of potassium. As a side note, this would be a fun reaction to do in a calorimeter to calculate the calories in a gummy bear.

(Video) What Happens If You Mix a Gummy Bear and Potas­si­um Chlo­rate ("Gummy Bear Volcano" Experiment)

by MEL SCIENCE

(Video) Combustion & Incomplete Combustion | Environmental Chemistry | FuseSchool

By: FuseSchool - Global Education

This video covers definition of combustion, combustion materials, complete combustion, incomplete combustion, products of these reactions, and the effects of combustion products.

Combustion reactions are burning reactions. They require oxygen either in the form of pure oxygen or oxygen in the air. These reactions are exothermic and give off heat. Heat is released when a fuel source is burned. Some fuel examples are gas and methane. Other examples include anything that can burn.

Methane or $CH_4$ can undergo complete or incomplete combustion. Incomplete combustion of methane in the presence of air ($O_2$) creates carbon dioxide ($CO_2$) and water ($H_2O$).

Incomplete combustion occurs with a limited supply of oxygen/air. When this happens we get either carbon monoxide ($CO$) and water or carbon ($C$) and water.

The products of combustion have different effects. Water is harmless and has no ill effects of being produced. Carbon dioxide contributes to global warming as a green house gas. Carbon monoxide is a a toxic gas which is almost undetectable. Carbon is a pollutant of the air as a particulate released.

Combustion Reactions: What are combustion reactions?