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4.17: Classifying Chemical Reactions: Double Replacement Reactions

  • Page ID
    217475
  • Learning Objectives
    • Define double replacement reaction.
    • Identify the unique characteristic of a double replacement reaction.

    In a double replacement reaction, which can also be referred to as a double displacement reaction, either the cationic or the anionic portions of two compounds exchange their relative positions.  The unique characteristic of a double replacement reaction is that the components that are repositioned must both exist within compounds.

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

    \(\ce{AD} + \ce{QZ} \rightarrow \ce{QD} + \ce{AZ}\)

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

    \(\ce{AD} + \ce{QZ} \rightarrow \ce{AZ} + \ce{QD}\)

    The inclusion of two letters within each of the symbolic representations that are shown above indicates that the reactants and products for both reactions are all compounds.  Furthermore, these equations share a common pair of reactants and products.  The only discernable difference between these patterns is the order in which the products, "QD" and "AZ," are written.  However, the order in which chemicals are written has no impact on the classification of a reaction, as long as the positions of those substances are consistent, relative to the reaction arrow.

    In order to classify the patterns that are shown above as double replacement reactions, either the cationic or the anionic portions of the reactants must be exchanged.  Based on the Chapter 3 rules for determining chemical formulas, the symbol of the cationic component of an ionic compound is always written before the symbol of the corresponding anion.  Therefore, in the patterns that are shown above, the placeholder letters "A" and "Q," which are written in the first positions in their respective chemical formulas, represent cations, and "D" and "Z" symbolize the anionic components of these compounds.

    In the first reaction that is shown above, "A" and "Q" replace one another.  Therefore, the double replacement reaction that is represented in this equation occurs through cation exchange.  The remaining reaction occurs through anion displacement, because "D" and "Z" are interchanged.  Because the only difference between these patterns is the order in which the products are written, executing a double replacement reaction through a cation exchange has the same net result as repositioning the corresponding anions.

    However, note that only the cations or anions should be exchanged during a double replacement reaction, as the process of repositioning the cations,

    \(\ce{AD} + \ce{QZ} \rightarrow \ce{QD} + \ce{AZ}\)

    followed by the anions

    \(\ce{QD} + \ce{AZ} \rightarrow \ce{QZ} + \ce{AD}\)

    results in the generation of chemicals that are identical in composition to the initial reactants.  Therefore, no net reaction occurs, because executing a second exchange negates the effects of the initial displacement.

    The following reaction is classified as a double replacement because the anionic components of two compounds, the iodide ion, I–1, and the nitrate ion, NO3–1, respectively, exchange positions.  Recall that the nitrate ion, NO3–1, is a polyatomic anion.  Because these multi-atom ions generally bond and react as indivisible units, the entire ion must be repositioned, in order for the reaction to be classified as a double replacement.

    \(\ce{2 KI} \left( aq \right) + \ce{Pb(NO_3)_2} \left( aq \right) \rightarrow \ce{2 KNO_3} \left( aq \right) + \ce{PbI_2} \left( s \right)\)

    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 compounds that are 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.  Based on the classifications of potassium, K, as a metal and the nitrate anion, NO3–1, as a polyatomic anion, the formula of the first 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.  These rules must also be applied to derive the chemical formula of the remaining product, which is an ionic compound that results from pairing lead, Pb, a metal, with iodine, I, a non-metal.

    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.

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