# 4.15: Classifying Chemical Reactions: Decomposition Reactions

Learning Objectives
• Define decomposition reaction.
• Identify the unique characteristic of a decomposition reaction.

In a decomposition reaction, a single reactant is divided into two or more simpler substances.  The unique characteristic of a decomposition reaction, which can be used to distinguish this pattern from the remaining classifications, is that only a single substance can be represented on the left side of a reaction arrow.

A decomposition reaction can be represented symbolically, as shown below.

$$\ce{A} \rightarrow \ce{D} + \ce{Z}$$

The following pattern also accurately reflects the description that is provided above.

$$\ce{A} \rightarrow \ce{D} + \ce{Q} + \ce{Z}$$

These equations differ in the number of chemicals that are present on the right side of the reaction arrow.  However, both examples are valid representations of decomposition reactions, because the definition that is given above specifies that "two or more" substances can be produced in the reaction.  More importantly, only a single substance, "A," is represented on the left side of the reaction arrow in both of these reaction patterns, as is required for a decomposition reaction.

Note that the definition of a decomposition reaction only requires that the products be "simpler substances" than the corresponding reactant.  Therefore, a decomposition reaction does not require that the reactant be divided into its constituent elements.  Instead, each product molecule must simply contain fewer atoms than the reactant molecule.  As was described in the previous section, the subscripts that are present within a chemical formula are solely dependent on the elemental, ionic, or covalent nature of the corresponding substance.  Therefore, based on the relative classifications of the chemicals that are being combined, the chemical formula for each product can only be determined by applying the appropriate ionic or covalent rules that were established in Chapter 3.

The following reaction is classified as a decomposition because only a single compound, hydrogen peroxide, H2O2, is represented on the left side of the reaction arrow.  Furthermore, this reactant contains a total of four atoms.  The products, water, H2O, and molecular oxygen, O2, each contain a lesser number of atoms, three and two, respectively.  Therefore, the requirement that the products of a decomposition reaction be simpler substances, relative to the corresponding reactant, is satisfied.

$$\ce{2 H_2O_2} \left( aq \right) \rightarrow \ce{2 H_2O} \left( l \right) + \ce{O_2} \left( g \right)$$

Finally, the balancing coefficients that are indicated in this equation are written in order to uphold the Law of Conservation of Matter, which, as stated in Section 4.12, mandates that particles cannot be created or destroyed in the course of a chemical reaction.  The process through which these coefficients are determined will be described in a later section of this chapter.