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21.6: Brønsted-Lowry Acid-Base Reactions

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  • The Roman god Janus was considered the god of gates and doors, beginnings and endings. He is portrayed with two faces, looking in two directions at once. Janus would have been in good position to look at the acid-base reactions we see in this concept, since they are equilibrium reactions involving two different forms of both acids and bases.

    Brønsted-Lowry Acid-Base Reactions

    An acid-base reaction according to the Brønsted-Lowry definition is a transfer of a proton from one molecule or ion to another. When ammonia is dissolved in water, it undergoes the following reversible reaction.

    \[\begin{array}{ccccccc} \ce{NH_3} \left( aq \right) & + & \ce{H_2O} \left( l \right) & \rightleftharpoons & \ce{NH_4^+} \left( aq \right) & + & \ce{OH^-} \left( aq \right) \\ \text{base} & & \text{acid} & & \text{acid} & & \text{base} \end{array}\]

    In this reaction, the water molecule is donating a proton to the ammonia molecule. The resulting products are the ammonium ion and the hydroxide ion. The water is acting as a Brønsted-Lowry acid, while the ammonia is acting as a Brønsted-Lowry base. The hydroxide ion that is produced causes the solution to be basic.

    We can also consider the reverse reaction in the above equation. In that reaction, the ammonia ion donates a proton to the hydroxide ion. The ammonium ion is a Brønsted-Lowry acid, while the hydroxide ion is a Brønsted-Lowry base. Most Brønsted-Lowry acid-base reactions can be analyzed in this way. There is one acid and one base as reactants, and one acid and one base as products.

    In the above reaction, water acted as an acid, which may seem a bit unexpected. Water can also act as a base in a Brønsted-Lowry acid-base reaction, as long as it reacts with a substance that is a better proton donor. Shown below is the reaction of water with they hydrogen sulfate ion.

    \[\begin{array}{ccccccc} \ce{HSO_4^-} \left( aq \right) & + & \ce{H_2O} \left( l \right) & \rightleftharpoons & \ce{H_3O^+} \left( aq \right) & + & \ce{SO_4^{2-}} \left( aq \right) \\ \text{acid} & & \text{base} & & \text{acid} & & \text{base} \end{array}\]

    So, water is capable of being either an acid or a base, a characteristic called amphoterism. An amphoteric substance is one that is capable of acting as either an acid or a base by donating or accepting hydrogen ions.

    Conjugate Acids and Bases

    When a substance that is acting as a Brønsted-Lowry acid donates its proton, it becomes a base in the reverse reaction. In the reaction above, the hydrogen sulfate ion \(\left( \ce{HSO_4^-} \right)\) donates a proton to water and becomes a sulfate ion \(\left( \ce{SO_4^{2-}} \right)\). The \(\ce{HSO_4^-}\) and the \(\ce{SO_4^{2-}}\) are linked to one another by the presence or absence of the \(\ce{H^+}\) ion. A conjugate acid-base pair is a pair of substances related by the loss or gain of a single hydrogen ion. A conjugate acid is the particle produced when a base accepts a proton. The hydrogen sulfate ion is the conjugate acid of the sulfate ion. A conjugate base is the particle produced when an acid donates a proton. The sulfate ion is the conjugate base of the hydrogen sulfate ion.

    In the reaction illustrated below, water serves both as acid and base simultaneously. One water molecule serves as an acid and donates a proton. The other water molecule functions as a base by accepting the proton.

    Figure 21.6.1: Water molecules as a conjugate acid-base pair. (CC BY-NC; CK-12)

    A typical Brønsted-Lowry acid-base reaction contains two conjugate acid-base pairs as shown below.

    \[\ce{HNO_2} \left( aq \right) + \ce{PO_4^{3-}} \left( aq \right) \rightleftharpoons \ce{NO_2^-} \left( aq \right) + \ce{HPO_4^{2-}} \left( aq \right)\]

    One conjugate acid-base pair is \(\ce{NHO_2}\)/\(\ce{NO_2^-}\), while the other pair is \(\ce{HPO_4^{2-}}\)/\(\ce{PO_4^{3-}}\).


    • Conjugate acids and bases are defined.
    • Examples of conjugate acid-base pairs are given.


    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.