So far in this lesson, you have learned the characteristics of two classes of chemical reactions: synthesis and decomposition. In a synthesis reaction two or more substances react to form a single product, and in a decomposition reaction a single compound breaks down into two or more simpler substances.
Recall that some reactions fit into more than one class. For example, coal power plants generate electric power from the energy released when coal, which is mostly carbon, is burned in the presence of oxygen to produce carbon dioxide. The reaction is represented by this equation:
\(\text{C}_{(s)} + \text{O}_{2\text{(g)}} → \text{CO}_{2\text{(g)}}\)
This reaction can be classified as a synthesis reaction, since two substances combine to produce a single product. This reaction can also be classified as a combustion reaction. In a combustion reaction a substance combines with oxygen and releases energy.
However, not all combustion reactions are also synthesis reactions. For example, natural gas primarily consists of methane \((\text{CH}_{4})\). Methane gas is burned in the presence of oxygen according to this equation:
\(\text{CH}_{4\text{(g)}} + 2\text{O}_{2\text{(g)}} → \text{CO}_{2\text{(g)}} + 2\text{H}_{2}\text{O}_{(g)}\)
Like the combustion of coal, the combustion of methane in natural gas also releases energy that can be used to generate electric power. Methane is a hydrocarbon, which means that it is composed of the elements carbon and hydrogen. When combustion is discussed in chemistry, it is often limited to reactions that occur between a hydrocarbon and oxygen to produce carbon dioxide and water.
Let's Watch
Watch the video to see what the equations for these types of combustion reactions look like.
You may want to use the study guide to follow along. If so, click below to download the study guide.
For the last few types of chemical reactions, we have been able to use elements from all over the periodic table, but this next reaction type only involves three elements: carbon, hydrogen, and oxygen. We're familiar with combustion reactions in our every day life. Any fire is combustion reaction. Most cars have internal combustion engines, so their energy comes from just a series of combustion reactions.
Well, from the viewpoint of chemistry, a combustion reaction is a chemical reaction that occurs between a hydrocarbon and oxygen to produce carbon dioxide and water.
Now, this word hydrocarbon is new. A hydrocarbon is just a molecule made up of carbon and hydrogen, just like the name suggests. So we can express the chemical formula for hydrocarbons broadly using variables x and y. Butane is an example of a hydrocarbon, because it has the chemical formula C4H10.
An example of a combustion reaction would be C6H6 reacting with oxygen - and remember that because oxygen is a HONCLBRIF element, it has to be here as the diatomic O2 - to produce CO2, carbon dioxide, and H2O, water.
Now, you might notice that this chemical equation isn't balanced. The balanced chemical equation for this reaction is 2C6H6 plus 15O2 yields 12CO2 plus 6H2O.
You see these types of reactions every day, but now you know what's happening on a molecular level with combustion reactions
Let's Practice
Which one?
Which of the following would be a product of a combustion reaction?
Combustion reactions always produce carbon dioxide and water.
Combustion reactions always produce carbon dioxide and water.
Combustion reactions always produce carbon dioxide and water.
Which is a reactant in a combustion reaction?
Combustion reactions always occur between a hydrocarbon and oxygen.
Combustion reactions always occur between a hydrocarbon and oxygen.
Combustion reactions always occur between a hydrocarbon and oxygen.
Which of these would be required for the combustion of gasoline \((\text{C}_{8}\text{H}_{10})\)?
Combustion reactions always occur between a hydrocarbon and oxygen.
Combustion reactions always occur between a hydrocarbon and oxygen.
Combustion reactions always occur between a hydrocarbon and oxygen.
Which of these would be a product of the combustion of propane \((\text{C}_{3}\text{H}_{8})\)?
Combustion reactions always produce carbon dioxide and water.
Combustion reactions always produce carbon dioxide and water.
Combustion reactions always produce carbon dioxide and water.
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