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

[latex]\large{\text{2 H}}_{2}\text{(}g\text{)}+{\text{O}}_2\text{(}g\text{)}\rightarrow{\text{2 H}}_{2}\text{O(}l\text{)}[/latex]

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.

Decomposition Reactions

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),

[latex]\large{\text{2 NaHCO}}_{3}\text{(}s\text{)}\rightarrow{\text{ Na}}_{2}\text{CO}_{3}\text{(}s\text{)}+{\text{ CO}}_{2}\text{(}g\text{)}+{\text{H}}_{2}\text{O(}l\text{)}[/latex]

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.

Single-Replacement Reactions

A single-replacement reaction (sometimes referred to as a single-displacement reaction) is a chemical reaction in which one element is substituted for another element in a compound, generating a new element and a new compound as products. For example,

[latex]\large\text{2 HCl}\text{(}aq\text{)}+{\text{Zn}}\text{(}s\text{)}\rightarrow{\text{ZnCl}}_{2}\text{(}aq\text{)}+{\text{H}}_{2}\text{(}g\text{)}[/latex]

is an example of a single-replacement reaction. The hydrogen atoms in HCl are replaced by Zn atoms, and in the process a new element—hydrogen—is formed. Another example of a single-replacement reaction is

[latex]\large\text{2 NaCl}\text{(}aq\text{)}+{\text{F}}_{2}\text{(}g\text{)}\rightarrow{\text{2 NaF}}\text{(}aq\text{)}+{\text{Cl}}_{2}\text{(}g\text{)}[/latex]

Double-Replacement Reactions

A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:

[latex]\large2\text{KI(}aq\text{)}+\text{Pb}{\text{(}{\text{NO}}_{3}\text{)}}_{2}\text{(}aq\text{)}\rightarrow{\text{PbI}}_{2}\text{(}s\text{)}+2{\text{KNO}}_{3}\text{(}aq\text{)}[/latex]

This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts.

Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (Figure 1). The properties of pure PbI₂ crystals make them useful for fabrication of X-ray and gamma ray detectors.

Figure 1. A precipitate of PbI₂ forms when solutions containing Pb²⁺ and I⁻ are mixed. (credit: Der Kreole/Wikimedia Commons)

The solubility guidelines discussed above may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation/anion pairing could result in an insoluble compound.

For example, mixing solutions of silver nitrate and sodium fluoride will yield a solution containing Ag⁺, NO^–, Na⁺, and F^– ions. Aside from the two ionic compounds originally present in the solutions, AgNO₃ and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO₃ and AgF. The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equation:

[latex]\large{\text{NaF}}\text{(}aq\text{)}+{\text{ AgNO}}_{3}\text{(}aq\text{)}\rightarrow{\text{AgF}}\text{(}s\text{)}+{\text{ NaNO}}_{3}\text{(}aq\text{)}[/latex]

Combustion Reactions

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:

[latex]\large{\text{CH}}_{4}\text{(}g\text{)}+{\text{ 2 O}}_2\text{(}g\text{)}\rightarrow{\text{CO}}_{2}\text{(}g\text{)}+{\text{2 H}}_{2}\text{O(}g\text{)}[/latex]

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

[latex]\large\text{2 C}_{12}\text{H}_{26}\text{(}l\text{)}+{\text{ 37 O}}_2\text{(}g\text{)}\rightarrow{\text{24 CO}}_{2}\text{(}g\text{)}+{\text{26 H}}_{2}\text{O(}g\text{)}[/latex]

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

[latex]\large\text{C}_{2}\text{H}_{5}\text{OH}\text{(}l\text{)}+{\text{ 3 O}}_2\text{(}g\text{)}\rightarrow{\text{2 CO}}_{2}\text{(}g\text{)}+{\text{3 H}}_{2}\text{O(}g\text{)}[/latex]

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

[latex]\large\text{2 C}_{2}\text{H}_{2}\text{N}_{2}\text{O}_{4}\text{(}l\text{)}+{\text{ O}}_2\text{(}g\text{)}\rightarrow{\text{4 CO}}_{2}\text{(}g\text{)}+{\text{ 2 H}}_{2}\text{O(}g\text{)}+\text{ N}_{2}\text{(}g\text{)}[/latex]

Example 4: Combustion Reactions

Complete and balance each combustion equation.

1. The combustion of propane, C₃H₈
2. The combustion of NH₃

Show Answer

1. The products of the reaction are CO₂ and H₂O, so our unbalanced equation is

   [latex]\large\text{C}_{3}\text{H}_{8}+{\text{ O}}_2\text{(}g\text{)}\rightarrow{\text{CO}}_{2}\text{(}g\text{)}+{\text{H}}_{2}\text{O(}g\text{)}[/latex]

   Balancing (and you may have to go back and forth a few times to balance this), we get

   [latex]\large\text{C}_{3}\text{H}_{8}+{\text{ 5 O}}_2\text{(}g\text{)}\rightarrow{\text{ 3 CO}}_{2}\text{(}g\text{)}+{\text{ 4 H}}_{2}\text{O(}g\text{)}[/latex]

2. The nitrogen atoms in ammonia will react to make N₂, while the hydrogen atoms will react with O₂ to make H₂O.[latex]\large\text{N}\text{H}_{3}\text{(}g\text{)}+{\text{ O}}_2\text{(}g\text{)}\rightarrow{\text{N}}_{2}\text{(}g\text{)}+{\text{H}}_{2}\text{O(}g\text{)}[/latex]

   Balancing (and you may have to go back and forth a few times to balance this), we get

   [latex]\large\text{4 N}\text{H}_{3}\text{(}g\text{)}+{\text{ 3 O}}_2\text{(}g\text{)}\rightarrow{\text{ 2 N}}_{2}\text{(}g\text{)}+{\text{ 6 H}}_{2}\text{O(}g\text{)}[/latex]

Check Your Learning

Complete and balance the combustion equation for cyclopropanol, C₃H₆O.

Show Answer

[latex]\large\text{C}_{3}\text{H}_{6}\text{O}\text{(}l\text{)}+{\text{ 4 O}}_2\text{(}g\text{)}\rightarrow{\text{3 CO}}_{2}\text{(}g\text{)}+{\text{ 3 H}}_{2}\text{O(}g\text{)}[/latex]

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