Identify the conjugate acid-base pairs in each of the following equations: (a) \(\mathrm{NH}_{3}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{NH}_{4}^{+}+\mathrm{OH}^{-}\) (b) \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{HPO}_{4}^{2-}+\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{HCl}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{Cl}^{-}+\mathrm{H}_{3} \mathrm{O}^{+}\)

An acid-base reaction involves the transfer of protons (H⁺ ions) between reactants. According to the Brønsted-Lowry theory, acids are proton donors, while bases are proton acceptors. For example, in the reaction between \(\text{NH}_{3}\) and water, water donates a proton to \(\text{NH}_{3}\), forming \(\text{NH}_{4}^{+}\) and \(\text{OH}^{-}\). This transfer of protons is what defines the acid-base interaction. Acid-base reactions are reversible, meaning that in the reverse reaction, the conjugate base of the acid can act as a base to reform the original reactants.

A conjugate acid is the species formed when a base gains a proton. For instance, in the reaction of \(\text{HC}_{2} \text{H}_{3} \text{O}_{2}\) with water, \(\text{HC}_{2} \text{H}_{3} \text{O}_{2}\) donates a proton to water, producing \(\text{H}_{3} \text{O}^{+}\). Here, \(\text{H}_{3} \text{O}^{+}\) is the conjugate acid of the water molecule. Understanding conjugate acids is crucial because it helps to identify how bases transform upon gaining a proton. Knowing the conjugate acid of a base explains the direction and feasibility of a reaction.

A conjugate base is the species that remains after an acid donates a proton. In the reaction between \(\text{HCl}\) and water, \(\text{HCl}\) donates a proton to form \(\text{Cl}^{-}\). Here, \(\text{Cl}^{-}\) is the conjugate base of \(\text{HCl}\). Each acid has a corresponding conjugate base, and understanding this relationship is important for predicting the outcomes of acid-base reactions. Conjugate bases are capable of accepting a proton in the reverse reaction, thus reforming the original acid.

Proton transfer is the movement of a proton from one molecule (acid) to another (base). This is the central mechanism of acid-base reactions. For example, in the reaction of \(\text{H}_{2} \text{PO}_{4}^{-}\) with \(\text{OH}^{-}\), the \(\text{H}_{2} \text{PO}_{4}^{-}\) donates a proton to the \(\text{OH}^{-}\) ion. The resulting products are \(\text{HPO}_{4}^{2-}\) and water. The ability to transfer protons determines the strength of acids and bases. Strong acids fully transfer their protons, while weak acids only partially do so. Proton transfer is fundamental in understanding the dynamics of chemical reactions in aqueous solutions.

The Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors. This theory extends the concept of acids and bases beyond hydrogen and hydroxide ions. It applies to a broader range of chemical reactions. For example, ammonia (\(\text{NH}_{3}\)) acts as a base by accepting a proton from water, which acts as an acid. The resulting products are \(\text{NH}_{4}^{+}\) and \(\text{OH}^{-}\). This theory helps in understanding the equilibrium state of acid-base reactions. It also assists in predicting the behavior of substances in various solvent systems. The Brønsted-Lowry theory is more versatile than the older definitions and can be used to analyze reactions in non-aqueous solutions.