# 4.16: Classifying Chemical Reactions: Single Replacement Reactions

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

In a single replacement reaction, which can also be referred to as a single displacement reaction, an elemental reactant and a portion of a compound exchange their relative positions.  The unique characteristic of a single replacement reaction is that one of the components that is being repositioned initially exists in an elemental form.

A single replacement reaction can be represented symbolically, as shown below.

$$\ce{A} + \ce{QZ} \rightarrow \ce{Q} + \ce{AZ}$$

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

$$\ce{A} + \ce{QZ} \rightarrow \ce{Z} + \ce{QA}$$

Both of these equations share a common pair of reactants.  In order to explicitly distinguish the reactants from one another, a single placeholder letter, "A," is utilized to indicate the elemental reactant, and two letters, "QZ," are incorporated into the symbolic representation of the compound.  In order for the patterns that are shown above to be classified as single replacement reactions, the elemental reactant, "A," must displace one of the components of the compound, either "Q" or "Z," in order to occupy its position.  In the first reaction that is shown above, "A" displaces "Q."  In the remaining reaction, "A" replaces "Z."  Note that because the letters "A," "Q," and "Z" are simply placeholders that are not associated with defined reactivities, the choice of whether to replace "Q" or "Z" has no chemical consequences.  As a result, both of these equations are valid representations of single replacement reactions.

In contrast, if a single replacement reaction were to occur between a pair of authentic chemicals, only one of the patterns that is shown above would result in the formation of a viable compound.  For example, consider the following partially-complete single replacement reaction, in which, aluminum, Al, is present in its elemental form, and the remaining reactant, silver sulfide, Ag2S, is an ionic compound.

$$\ce{Ag_2S} \left( s \right) + \ce{Al} \left( s \right) \rightarrow$$

If the pattern in the second reaction that is shown above were followed, aluminum, Al, would displace sulfur, S.  The resultant products of this hypothetical reaction would be elemental sulfur, S, and a compound that contains both aluminum, Al, and the unaltered reaction component, silver, Ag.  However, because both aluminum, Al, and silver, Ag, are metals, neither the ionic nor the covalent rules that were established in Chapter 3 are applicable to this pair of elements.  Since these elements cannot interact with one another to form a viable compound, the displacement of sulfur by aluminum is chemically-invalidated and, therefore, does not occur.

If the pattern in the remaining reaction were followed, aluminum, Al, would displace silver, Ag.  The resultant products of this reaction would be elemental silver, Ag, and a compound that contains both aluminum, Al, and the unaltered reaction component, sulfur, S.  Because aluminum, Al, is a metal, and sulfur, S, is a non-metal, these elements would successfully interact to form an ionic compound, aluminum sulfide, Al2S3, in accordance with the rules that were established in Chapter 3.  Therefore, the products that result upon the reaction of silver sulfide, Ag2S, and aluminum, Al, are elemental silver, Ag, and aluminum sulfide, Al2S3, as shown below.

$$\ce{3 Ag_2S} \left( s \right) + \ce{2 Al} \left( s \right) \rightarrow\ \ce{6 Ag} \left( s \right) + \ce{Al_2S_3} \left( s \right)$$

As stated previously, the subscripts that are present within a chemical formula are solely dependent on the elemental, ionic, or covalent nature of the corresponding substance.  Therefore, the chemical formula of the compound that is formed in the reaction that is shown above cannot be obtained simply by rewriting the chemical information, as given, after exchanging the relative positions of the appropriate components.  Because aluminum, Al, is a metal, and sulfur, S, is a non-metal, the formula of the compound that is produced in the reaction that is shown above can only be established by applying the Chapter 3 rules for determining ionic chemical formulas.

In order for a replacement to occur successfully, the elemental reactant and the component of the compound that is being displaced must have identical metallic classifications.  In the example that is shown above, both aluminum, the given elemental reactant, and silver, the component of the compound that was successfully displaced, are classified as metals.  Therefore, if the given elemental reactant were a non-metal, a viable single replacement reaction would only occur if the non-metal component of the compound were displaced.

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.