8.21: Brønsted-Lowry Acids and Bases: Conjugate Acids

Learning Objectives

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

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.  As stated in the previous section of this chapter, 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 base accepts, or gains, a proton, H⁺¹, during an acid/base reaction, the particle that is formed as a result of this transfer contains one more proton than the base from which it was generated.  In order to emphasize the relationship between a basic reactant and the chemical that is produced upon the absorption of a proton, H⁺¹, by that base, 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 base that was initially present, the conjugate product has an additional proton, and, consequently, has the ability to lose that proton.  Therefore, the substance that is generated upon the gain of a proton, H⁺¹, by a Brønsted-Lowry base has the potential to be a proton, H⁺¹, donor and, consequently, is the conjugate acid of that base.

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

As stated above, a conjugate acid is produced when a Brønsted-Lowry base accepts 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 gain of a proton, H⁺¹, by this base generates a particle that is comprised of three hydrogens, H, and one oxygen, O, and bears a net +1 charge.  Therefore, the conjugate acid of water, H₂O, is the hydronium ion, which is symbolized as H₃O⁺¹.

Exercise \(\PageIndex{1}\)

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

Answer

As stated above, a conjugate acid is produced when a Brønsted-Lowry base accepts a proton, H⁺¹.  Nitrogen trihydride, NH₃, is comprised of one nitrogen, N, and three hydrogens, H, 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 gain of a proton, H⁺¹, by this base generates a particle that is comprised of one nitrogen, N, and four hydrogens, H, and bears a net +1 charge.  Therefore, the conjugate acid of nitrogen trihydride, NH₃, which is more commonly-known as "ammonia," is the ammonium ion, which is symbolized as NH₄⁺¹.

Exercise \(\PageIndex{2}\)

Write the chemical formula that corresponds to the conjugate acid of ethanol, C₂H₅OH, which can be classified as a Brønsted-Lowry base.

Answer

As stated above, a conjugate acid is produced when a Brønsted-Lowry base accepts a proton, H⁺¹.  Ethanol, C₂H₅OH, is comprised of two carbons, C, six 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 gain of a proton, H⁺¹, by this base generates a particle that is comprised of two carbons, C, seven hydrogens, H, and one oxygen, O, and bears a net +1 charge.  Because the proton, H⁺¹, bonds with the oxygen, O, that is present in ethanol, the conjugate acid of ethanol, C₂H₅OH, is symbolized as C₂H₅OH₂⁺¹.