# 4.14: Classifying Chemical Reactions: Combination Reactions

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

Recall that the chemical formula for a reactant, which is a chemical that is present before a reaction has occurred, is written on the left side of a reaction arrow.  The chemical formulas for the final substances that are generated at the end of a chemical reaction, which are called products, are written on the right side of a reaction arrow.  If multiple reactants or products are involved in a particular reaction, a plus sign, "+", is used to separate their formulas.

In a combination reaction, the components of two or more reactants bond with one another to form a single product.  The unique characteristic of a combination reaction, which can be used to distinguish this pattern from the remaining classifications, is that only a single substance can be represented on the right side of a reaction arrow.

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

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

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

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

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

Finally, note that the identity of the product that is generated, "Z," is unique, relative to each of the indicated reactants.  While the product of a combination reaction will contain the atomic and ionic components of the associated reactants, the chemical formula of the product cannot be obtained simply by removing the plus sign from the reactant side of the equation and rewriting the remaining chemical information as a single formula.  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 the corresponding product can only be determined by applying the appropriate ionic or covalent rules that were established in Chapter 3.

This challenging, yet highly-important, aspect of chemical reactivity is relevant to the reaction that is shown below.

$$\ce{H_2} \left( g \right) + \ce{I_2} \left( g \right) \rightarrow \ce{2 HI} \left( g \right)$$

This reaction is classified as a combination because only a single molecule, hydrogen iodide, HI, is represented on the right side of the reaction arrow.  Furthermore, this product contains both hydrogen, H, and iodine, I, which are both present in the chemical formulas of the given reactants.  However, the subscripts that are associated with each of these elemental symbols on the reactant side of the reaction arrow are not present on the product side.  Because hydrogen and iodine are both non-metals, these elements will bond to form a covalent molecule.  Therefore, in order to determine the chemical formula for the molecule that results upon the combination of these elements, a corresponding Lewis structure must first be drawn.  The electron dot structures for each element must be written, as shown below,

so that the unpaired electrons on each symbol can be correctly paired with one another, in order to satisfy the valences of each atom, as follows.

Finally, while not necessary for the current application, all shared pairs of electrons can be replaced with lines, in order to generate a structure that is more visually-appealing.

Based on the second and third structures that are shown above, which are both chemically-correct representations of a covalent compound, the molecule that results upon the combination of hydrogen, H, and iodine, I, contains only one of each of these elements, as is reflected in the chemical formula of the product in the corresponding combination chemical equation.

Finally, the balancing coefficient that is indicated on the right side of this equation, a "2," is 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 this coefficient is determined will be described in a later section of this chapter.