8.20: Brønsted-Lowry Acids and Bases: Conjugate Bases

Learning Objectives

• Define conjugate pair.
• Define conjugate base.
• Write the chemical formula that corresponds to the conjugate base of a Brønsted-Lowry acid.

Recall that a Brønsted-Lowry acid is a proton, H⁺¹, donor in solution, and a Brønsted-Lowry base is a proton, H⁺¹, acceptor in solution.  Because a specific ion, a proton, H⁺¹, is always transferred from an acid to a base, the identities of the products that are formed during a Brønsted-Lowry acid/base reaction can be readily predicted.

Since a Brønsted-Lowry acid donates, or loses, a proton, H⁺¹, during an acid/base reaction, the particle that is formed as a result of this transfer contains one less proton than the acid from which it was generated.  In order to emphasize the relationship between an acid reactant and the chemical that is produced upon the dissociation of a proton, H⁺¹, from that acid, Brønsted and Lowry designated these substances as conjugates, or, more formally, as a conjugate pair.  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.  Finally, relative to the acid that was initially present, the conjugate product is lacking and, consequently, has the capacity to gain, a proton.  Therefore, the substance that is generated upon the loss of a proton, H⁺¹, from a Brønsted-Lowry acid has the potential to be a proton, H⁺¹, acceptor and, consequently, is the conjugate base of that acid.

For example, write the chemical formula that corresponds to the conjugate base of nitric acid, which can be classified as a Brønsted-Lowry acid.

As stated above, a conjugate base is produced when a Brønsted-Lowry acid donates a proton, H⁺¹.  Nitric acid, HNO₃, is comprised of one hydrogen, H, one nitrogen, N, and three oxygens, O, and bears a net neutral charge.  Because, 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, the loss of a proton, H⁺¹, from this acid generates a particle that is comprised of one nitrogen, N, and three oxygens, O, does not contain any hydrogen, H, and bears a net –1 charge.  Therefore, the conjugate base of nitric acid, HNO₃, is the nitrate ion, which is symbolized as NO₃^–1.

Exercise \(\PageIndex{1}\)

Write the chemical formula that corresponds to the conjugate base of hydrofluoric acid, which can be classified as a Brønsted-Lowry acid.

Answer

As stated above, a conjugate base is produced when a Brønsted-Lowry acid donates a proton, H⁺¹.  Hydrofluoric acid, HF, is comprised of one hydrogen, H, and one fluorine, F, and bears a net neutral charge.  Because, 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, the loss of a proton, H⁺¹, from this acid generates a particle that is comprised of one fluorine, F, does not contain any hydrogen, H, and bears a net –1 charge.  Therefore, the conjugate base of hydrofluoric acid, HF, is the fluoride ion, which is symbolized as F^–1.

Exercise \(\PageIndex{2}\)

Write the chemical formula that corresponds to the conjugate base of phosphoric acid, which can be classified as a Brønsted-Lowry acid.

Answer

As stated above, a conjugate base is produced when a Brønsted-Lowry acid donates a proton, H⁺¹.  Phosphoric acid, H₃PO₄, is comprised of three hydrogens, H, one phosphorus, P, and four oxygens, O, and bears a net neutral charge.  Because, 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, the loss of a proton, H⁺¹, from this acid generates a particle that contains two hydrogens, H, one phosphorus, P, and four oxygens, O, and bears a net –1 charge.  Therefore, the conjugate base of phosphoric acid, H₃PO₄, is symbolized as H₂PO₄^–1.

Exercise \(\PageIndex{3}\)

Write the chemical formula that corresponds to the conjugate base of acetic acid, which can be classified as a Brønsted-Lowry acid.

Answer

As stated above, a conjugate base is produced when a Brønsted-Lowry acid donates a proton, H⁺¹.  Acetic acid, HC₂H₃O₂, is comprised of four hydrogens, H, two carbons, C, and two oxygens, O, and bears a net neutral charge.  Because, 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, the loss of a proton, H⁺¹, from this acid generates a particle that contains three hydrogens, H, two carbons, C, and two oxygens, O, and bears a net –1 charge.  The elemental symbol of hydrogen, H, is present two distinct places in the chemical formula of acetic acid, HC₂H₃O₂.  Because this substance contains a polyatomic anion, which bonds and reacts as an indivisible charged unit, the hydrogen, H, that is donated by this acid cannot be lost from the acetate ion, C₂H₃O₂^–1.  Therefore, during a Brønsted-Lowry acid/base reaction, the first hydrogen, H, that is written in the chemical formula of acetic acid, HC₂H₃O₂, is ionized, and the conjugate base that is produced, the acetate ion, is symbolized as C₂H₃O₂^–1.

Exercise \(\PageIndex{4}\)

Write the chemical formula that corresponds to the conjugate base of water, which can be classified as a Brønsted-Lowry acid.

Answer

As stated above, a conjugate base is produced when a Brønsted-Lowry acid donates a proton, H⁺¹.  Water, H₂O, is comprised of two hydrogens, H, and one oxygen, O, and bears a net neutral charge.  Because, 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, the loss of a proton, H⁺¹, from this acid generates a particle that is comprised of one hydrogen, H, and one oxygen, O, and bears a net –1 charge.  Therefore, the conjugate base of water, H₂O, is the hydroxide ion, which is symbolized as OH^–1.