Three classifications of chemical reactions will be reviewed in this section. Predicting the products in some of them may be difficult, but the reactions are still easy to recognize.

A composition reaction (sometimes also called a combination reaction or a synthesis reaction) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction

2H₂(g) + O₂(g) → 2H₂O(ℓ)

water is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance—water—as a product. So this is a composition reaction.

A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, if we look at the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate):

2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(ℓ)

We can see that sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.

Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.

A combustion reaction occurs when a reactant combines with oxygen, many times from the atmosphere, to produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N₂. Many reactants, called fuels, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO₂ and H₂O. For example, the balanced chemical equation for the combustion of methane, CH₄, is as follows:

CH₄ + 2O₂ → CO₂ + 2H₂O

Kerosene can be approximated with the formula C₁₂H₂₆, and its combustion equation is

2C₁₂H₂₆ + 37O₂ → 24CO₂ + 26H₂O

Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C₂H₅OH, whose combustion equation is

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

If nitrogen is present in the original fuel, it is converted to N₂, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C₂H₂N₂O₄, we have

2C₂H₂N₂O₄ + O₂ → 4CO₂ + 2H₂O + 2N₂

Example 4.10

Problems

Complete and balance each combustion equation.

1. the combustion of propane, C₃H₈
2. the combustion of ammonia, NH₃

Solutions

1. The products of the reaction are CO₂ and H₂O, so our unbalanced equation is C₃H₈ + O₂ → CO₂ + H₂O. Balancing (and you may have to go back and forth a few times to balance this), we get C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
2. The nitrogen atoms in ammonia will react to make N₂, while the hydrogen atoms will react with O₂ to make H₂O, thus NH₃ + O₂ → N₂ + H₂O. To balance this equation without fractions (which is the convention), we get 4NH₃ + 3O₂ → 2N₂ + 6H₂O.

Test Yourself

Complete and balance the combustion equation for cyclopropanol, C₃H₆O.

Answer

C₃H₆O + 4O₂ → 3CO₂ + 3H₂O

Media Attributions

Example 4.10

• “flowers and propane” by vistavision © CC BY-NC-ND (Attribution NonCommercial NoDerivs)