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

2 H₂(g) + O₂(g) → 2 H₂O(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, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),

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

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 + 2 O₂ → CO₂ + 2 H₂O

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

2 C₁₂H₂₆ + 37 O₂ → 24 CO₂ + 26 H₂O

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 + 3 O₂ → 2 CO₂ + 3 H₂O

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

2 C₂H₂N₂O₄ + O₂ → 4 CO₂ + 2 H₂O + 2 N₂

Example 10

Complete and balance each combustion equation.

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

Solution

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 + 5 O₂ → 3 CO₂ + 4 H₂O

2. The nitrogen atoms in ammonia will react to make N₂, while the hydrogen atoms will react with O₂ to make H₂O:

   NH₃ + O₂ → N₂ + H₂O

   To balance this equation without fractions (which is the convention), we get

   4 NH₃ + 3 O₂ → 2 N₂ + 6 H₂O

Test Yourself

Complete and balance the combustion equation for cyclopropanol, C₃H₆O.

Answer

C₃H₆O + 4 O₂ → 3 CO₂ + 3 H₂O

Propane is a fuel used to provide heat for some homes. Propane is stored in large tanks like that shown here.
Source: “flowers and propane” by vistavision is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic