In each equation, label the acids, bases, and conjugate pairs: (a) \(\mathrm{NH}_{4}^{+}+\mathrm{CN}^{-} \rightleftharpoons \mathrm{NH}_{3}+\mathrm{HCN}\) (b) \(\mathrm{H}_{2} \mathrm{O}+\mathrm{HS}^{-} \rightleftharpoons \mathrm{OH}^{-}+\mathrm{H}_{2} \mathrm{~S}\) (c) \(\mathrm{HSO}_{3}^{-}+\mathrm{CH}_{3} \mathrm{NH}_{2} \rightleftharpoons \mathrm{SO}_{3}^{2-}+\mathrm{CH}_{3} \mathrm{NH}_{3}{ }^{+}\)

To understand acid-base reactions, you need to grasp the fundamentals of Brønsted-Lowry acids and bases. This theory defines acids as proton (H\textsuperscript{+}) donors and bases as proton acceptors. This means:

• An acid donates a proton to another substance.
• A base accepts a proton from another substance.

In the reaction \(\mathrm{NH}_{4}^{+} + \mathrm{CN}^{-} \rightleftharpoons \mathrm{NH}_{3} + \mathrm{HCN}\), \(\mathrm{NH}_{4}^{+}\) is the acid because it donates a proton to become \(\mathrm{NH}_{3}\), and \(\mathrm{CN}^{-}\) is the base because it accepts a proton to become \(\mathrm{HCN}\). Understanding which species donate or accept protons is essential for identifying acids and bases in a reaction.

In any acid-base reaction, you'll find conjugate acid-base pairs. When an acid donates a proton, it forms its conjugate base. Similarly, when a base accepts a proton, it forms its conjugate acid. For example:

• In the reaction \(\mathrm{H}_{2} \mathrm{O} + \mathrm{HS}^{-} \rightleftharpoons \mathrm{OH}^{-} + \mathrm{H}_{2} \mathrm{~S}\), \(\mathrm{H}_{2} \mathrm{O}\) (acid) donates a proton to become \(\mathrm{OH}^{-}\) (its conjugate base),
• while \(\mathrm{HS}^{-}\) (base) accepts a proton to become \(\mathrm{H}_{2} \mathrm{~S}\) (its conjugate acid).

Conjugate acid-base pairs help us understand how acids and bases transform during reactions. Another example is \(\mathrm{HSO}_{3}^{-} + \mathrm{CH}_{3} \mathrm{NH}_{2} \rightleftharpoons \mathrm{SO}_{3}^{2-} + \mathrm{CH}_{3} \mathrm{NH}_{3} \textsuperscript{+}\). Here, \(\mathrm{HSO}_{3}^{-}\) (acid) donates a proton to become \(\mathrm{SO}_{3}^{2-}\) (conjugate base), and \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (base) accepts a proton to become \(\mathrm{CH}_{3} \mathrm{NH}_{3} \textsuperscript{+}\) (conjugate acid).

Proton transfer is the movement of a proton from one molecule to another in an acid-base reaction. This process is fundamental to understanding how acids and bases interact:

• When an acid donates a proton, it undergoes proton transfer to its conjugate base.
• When a base accepts a proton, it undergoes proton transfer to its conjugate acid.

For example, in \(\mathrm{H}_{2} \mathrm{O} + \mathrm{HS}^{-} \rightleftharpoons \mathrm{OH}^{-} + \mathrm{H}_{2} \mathrm{~S}\), proton transfer occurs when \(\mathrm{H}_{2} \mathrm{O}\) donates a proton to \(\mathrm{HS}^{-}\), ultimately producing \(\mathrm{OH}^{-}\) and \(\mathrm{H}_{2} \mathrm{~S}\). In essence, proton transfer not only determines which species act as acids or bases but also leads to the formation of conjugate acid-base pairs.