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.
Question
Why can the combustion of coal be classified as both a synthesis reaction and a combustion reaction?
When coal (carbon) burns in oxygen, two reactants combine to form a single product, carbon dioxide, which makes it a synthesis reaction. Because the reaction also involves a substance reacting with oxygen and releasing energy, it is also a combustion reaction.
