8.22: Brønsted-Lowry Acids and Bases: Conjugate Acid/Base Pairs

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Learning Objectives

Identify the conjugate acid/base pairs in a Brønsted-Lowry acid/base equation.
Identify the acid, the base, the conjugate acid, and the conjugate base in a Brønsted-Lowry acid/base equation.
Define amphoteric.

Recall that, during a Brønsted-Lowry acid/base reaction, a proton, H⁺¹, is transferred from a Brønsted-Lowry acid to a Brønsted-Lowry base. Sections 8.18 and 8.19 presented and applied the process for generating Brønsted-Lowry acid/base equations from aqueous and non-aqueous solution equations, respectively. The resultant acid/base equations symbolically-represent the synergistic relationship that exists between a Brønsted-Lowry acid, which is defined as a proton, H⁺¹, donor in solution, and a Brønsted-Lowry base, which, by definition, accepts protons, H⁺¹, that are created in solution.

Subsequently, Sections 8.20 and 8.21 used these definitions to predict the products that are generated during a Brønsted-Lowry acid/base reaction. Because the products that are generated during an acid/base reaction always differ from their corresponding Brønsted-Lowry reactants by exactly and only one proton, H⁺¹, these substances are designated as conjugates, or, more formally, as a conjugate pair. Relative to an acid that was initially present, its conjugate product is lacking a proton, and, therefore, has the potential to be a proton, H⁺¹, acceptor. As a result, the substance that is generated upon the loss of a proton, H⁺¹, from a Brønsted-Lowry acid is the conjugate base of that acid. In contrast, a conjugate product has an additional proton, relative to a base that was initially present, and, consequently, has the potential to be a proton, H⁺¹, donor. Therefore, the substance that is generated upon the gain of a proton, H⁺¹, by a Brønsted-Lowry base is the conjugate acid of that base.

Finally, the chemical formulas of a Brønsted-Lowry acid, a Brønsted-Lowry base, and the conjugate products that are generated when these reactants lose and gain protons, H⁺¹, are all represented in a Brønsted-Lowry acid/base equation. Therefore, as will be described and exemplified in the following paragraphs, the conjugate pairs that are symbolized in a Brønsted-Lowry acid/base equation can be identified, and, subsequently, each of given chemical formulas can be labeled according to its reactivity, using one of the terms that is indicated above.

For example, identify the conjugate pairs in the following Brønsted-Lowry acid/base equation, and label each of the given chemical formulas as corresponding to a Brønsted-Lowry acid, a Brønsted-Lowry base, a conjugate acid, or a conjugate base.

\(\ce{HNO_3}\) + \(\ce{H_2O}\) \(\longrightarrow\) \(\ce{H_3O^{+1}}\) + \(\ce{NO_3^{–1}}\)

By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Additionally, Brønsted-Lowry acids and bases are used to prepare solutions and, therefore, must be represented on the left side of the arrow in a given acid/base equation. Conjugate bases and conjugate acids, respectively, are formed from these reactants and, consequently, the formulas of these generated substances must be written on the product side of the reaction arrow in an equation.

Because the formulas of the first reactant, nitric acid, HNO₃, and the second product, the nitrate ion, NO₃^–1, both contain one nitrogen, N, and three oxygens, O, and differ by exactly one proton, H⁺¹, the corresponding chemicals are conjugates. Additionally, water, H₂O, and the hydronium ion, H₃O⁺¹, are a conjugate pair, as their chemical formulas both contain one oxygen, O, and differ by exactly one proton, H⁺¹. Furthermore, because nitric acid, HNO₃, loses a proton, H⁺¹, to generate its conjugate in the reaction that is shown above, this reactant can be classified as a Brønsted-Lowry acid, and, consequently, the nitrate ion, NO₃^–1, is the conjugate base of this acid. Finally, since water, H₂O, gains a proton, H⁺¹, to produce its conjugate in the given reaction, this reactant can be labeled as a Brønsted-Lowry base, and the hydronium ion, H₃O⁺¹, is the conjugate acid of this base.

Exercise \(\PageIndex{1}\)

Identify the conjugate pairs in the following Brønsted-Lowry acid/base equation, and label each of the given chemical formulas as corresponding to a Brønsted-Lowry acid, a Brønsted-Lowry base, a conjugate acid, or a conjugate base.

\(\ce{C_2H_5OH}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{C_2H_5OH_2^{+1}}\) + \(\ce{OH^{–1}}\)

Answer: By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Additionally, Brønsted-Lowry acids and bases are used to prepare solutions and, therefore, must be represented on the left side of the arrow in an acid/base equation. Conjugate bases and conjugate acids, respectively, are formed from these reactants and, consequently, the formulas of these generated substances must be written on the product side of the reaction arrow in an equation.

Because the formulas of the second reactant, water, H₂O, and the final product, the hydroxide ion, OH^–1, both contain one oxygen, O, and differ by exactly one proton, H⁺¹, the corresponding chemicals are conjugates. Additionally, the first reactant, C₂H₅OH, and the first product C₂H₅OH₂⁺¹, are a conjugate pair, as their chemical formulas both contain two carbons, C, and one oxygen, O, and differ by exactly one proton, H⁺¹. Furthermore, because water, H₂O, loses a proton, H⁺¹, to generate its conjugate in the reaction that is shown above, this reactant can be classified as a Brønsted-Lowry acid, and, consequently, the hydroxide ion, OH^–1, is the conjugate base of this acid. Finally, since the first reactant, C₂H₅OH, gains a proton, H⁺¹, to produce its conjugate in the given reaction, this reactant can be labeled as a Brønsted-Lowry base, and C₂H₅OH₂⁺¹, is the conjugate acid of this base.

Water, H₂O, is a reactant in both of the examples that are provided above. In the in-text example that was presented, water gained a proton, H⁺¹, to produce its conjugate and, therefore, was labeled as a Brønsted-Lowry base. However, in Exercise 8.22.1, water lost a proton, H⁺¹, to generate its conjugate and, consequently, was classified as a Brønsted-Lowry acid. Because of its ability to act as a Brønsted-Lowry acid in some solutions and as a Brønsted-Lowry base in others, water is classified as an amphoteric molecule. Because of the dual reactivity of this type of chemical, an amphoteric substance cannot be definitively categorized as a Brønsted-Lowry acid or as a Brønsted-Lowry base by independently analyzing its chemical formula. Instead, after classifying the other given reactant as an acid or a base, the reactivity of an amphoteric chemical is determined "by default."

Exercise \(\PageIndex{2}\)

\(\ce{HF}\) + \(\ce{H_2O}\) \(\longrightleftharpoons\) \(\ce{F^{–1}}\) + \(\ce{H_3O^{+1}}\)

Answer: By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Additionally, Brønsted-Lowry acids and bases are used to prepare solutions and, therefore, must be represented on the left side of the arrow in an acid/base equation. Conjugate bases and conjugate acids, respectively, are formed from these reactants and, consequently, the formulas of these generated substances must be written on the product side of the reaction arrow in an equation.

Because the formulas of the first reactant, hydrofluoric acid, HF, and the first product, the fluoride ion, F^–1, both contain one fluorine, F, and differ by exactly one proton, H⁺¹, the corresponding chemicals are conjugates. Additionally, water, H₂O, and the hydronium ion, H₃O⁺¹, are a conjugate pair, as their chemical formulas both contain one oxygen, O, and differ by exactly one proton, H⁺¹. Furthermore, because hydrofluoric acid, HF, loses a proton, H⁺¹, to generate its conjugate in the reaction that is shown above, this reactant can be classified as a Brønsted-Lowry acid, and, consequently, the fluoride ion, F^–1, is the conjugate base of this acid. Finally, since water, H₂O, gains a proton, H⁺¹, to produce its conjugate in the given reaction, this reactant can be labeled as a Brønsted-Lowry base, and the hydronium ion, H₃O⁺¹, is the conjugate acid of this base.

Exercise \(\PageIndex{3}\)

\(\ce{H_2O}\) + \(\ce{H_3PO_4}\) \(\longrightleftharpoons\) \(\ce{H_2PO_4^{–1}}\) + \(\ce{H_3O^{+1}}\)

Answer: By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Additionally, Brønsted-Lowry acids and bases are used to prepare solutions and, therefore, must be represented on the left side of the arrow in an acid/base equation. Conjugate bases and conjugate acids, respectively, are formed from these reactants and, consequently, the formulas of these generated substances must be written on the product side of the reaction arrow in an equation.

Because the formulas of the second reactant, phosphoric acid, H₃PO₄, and the first product, H₂PO₄^–1, both contain one phosphorus, P, and four oxygens, O, and differ by exactly one proton, H⁺¹, the corresponding chemicals are conjugates. Additionally, water, H₂O, and the hydronium ion, H₃O⁺¹, are a conjugate pair, as their chemical formulas both contain one oxygen, O, and differ by exactly one proton, H⁺¹. Furthermore, because phosphoric acid, H₃PO₄, loses a proton, H⁺¹, to generate its conjugate in the reaction that is shown above, this reactant can be classified as a Brønsted-Lowry acid, and, consequently, H₂PO₄^–1, is the conjugate base of this acid. Finally, since water, H₂O, gains a proton, H⁺¹, to produce its conjugate in the given reaction, this reactant can be labeled as a Brønsted-Lowry base, and the hydronium ion, H₃O⁺¹, is the conjugate acid of this base.

Exercise \(\PageIndex{4}\)

\(\ce{HC_2H_3O_2}\) + \(\ce{NH_3}\) \(\longrightarrow\) \(\ce{NH_4^{+1}}\) + \(\ce{C_2H_3O_2^{–1}}\)

Answer: By definition, the chemical formulas of conjugate particles must differ by exactly and only one proton, H⁺¹, and should otherwise be identical to one another. Additionally, Brønsted-Lowry acids and bases are used to prepare solutions and, therefore, must be represented on the left side of the arrow in an acid/base equation. Conjugate bases and conjugate acids, respectively, are formed from these reactants and, consequently, the formulas of these generated substances must be written on the product side of the reaction arrow in an equation.

Because the formulas of the first reactant, acetic acid, HC₂H₃O₂, and the second product, the acetate ion, C₂H₃O₂^–1, both contain two carbons, C, and two oxygens, O, and differ by exactly one proton, H⁺¹, the corresponding chemicals are conjugates. Additionally, nitrogen trihydride, NH₃, and the ammonium ion, NH₄⁺¹, are a conjugate pair, as their chemical formulas both contain one nitrogen, N, and differ by exactly one proton, H⁺¹. Furthermore, because acetic acid, HC₂H₃O₂, loses a proton, H⁺¹, to generate its conjugate in the reaction that is shown above, this reactant can be classified as a Brønsted-Lowry acid, and, consequently, the acetate ion, C₂H₃O₂^–1, is the conjugate base of this acid. Finally, since nitrogen trihydride, NH₃, gains a proton, H⁺¹, to produce its conjugate in the given reaction, this reactant can be labeled as a Brønsted-Lowry base, and the ammonium ion, NH₄⁺¹, is the conjugate acid of this base.