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In Br酶nsted-Lowry theory, an acid is defined as a **proton donor**. This means it gives up a proton (hydrogen ion, H鈦�) to another substance during a chemical reaction. It's crucial to identify the proton donor in any acid-base reaction to understand how the process works.
For example, in the equation \(\text{H}_{3}\text{PO}_{4}(aq)+\text{H}_{2}\text{O}(l) \rightleftharpoons \text{H}_{2}\text{PO}_{4}^{-}(aq)+\text{H}_{3}\text{O}^{+}(aq)\), phosphoric acid (\(\text{H}_{3}\text{PO}_{4}\)) donates a proton to water (\(\text{H}_{2}\text{O}\)), becoming dihydrogen phosphate (\(\text{H}_{2}\text{PO}_{4}^{-}\)).
Another example is the equation \(\text{H}_{3}\text{PO}_{4}(aq)+\text{NH}_{3}(aq) \rightleftharpoons \text{H}_{2}\text{PO}_{4}^{-}(aq)+\text{NH}_{4}^{+}(aq)\). Here, phosphoric acid (\(\text{H}_{3}\text{PO}_{4}\)) again donates a proton, this time to ammonia (\(\text{NH}_{3}\)), forming dihydrogen phosphate (\(\text{H}_{2}\text{PO}_{4}^{-}\)) and ammonium ion (\(\text{NH}_{4}^{+}\)). Identifying the proton donor is the first step to recognize the acid in the reaction.

A **proton acceptor** is a substance that accepts a proton during a chemical reaction. According to Br酶nsted-Lowry theory, a base is defined as a proton acceptor. Knowing which species accepts the proton helps identify the base in the reaction.
For instance, consider the equation \(\text{CO}_{3}^{2-}(aq)+\text{H}_{2}\text{O}(l) \rightleftharpoons \text{HCO}_{3}^{-}(aq)+\text{OH}^{-}(aq)\), carbonate (\(\text{CO}_{3}^{2-}\)) accepts a proton from water (\(\text{H}_{2}\text{O}\)) to form bicarbonate (\(\text{HCO}_{3}^{-}\)). This makes carbonate the proton acceptor, or the base, in the reaction.
Another example is the equation \(\text{H}_{3}\text{PO}_{4}(aq)+\text{NH}_{3}(aq) \rightleftharpoons \text{H}_{2}\text{PO}_{4}^{-}(aq)+\text{NH}_{4}^{+}(aq)\). In this case, ammonia (\(\text{NH}_{3}\)) accepts a proton from phosphoric acid (\(\text{H}_{3}\text{PO}_{4}\)), forming ammonium ion (\(\text{NH}_{4}^{+}\)). Recognizing the proton acceptor helps us identify the base involved in the reaction.

An acid-base reaction involves the transfer of a proton from the acid (proton donor) to the base (proton acceptor). Understanding how these reactions work is important for grasping many concepts in chemistry.
Let's look at the reactions provided in the problem:
- In the reaction \(\text{H}_{3}\text{PO}_{4}(aq)+\text{H}_{2}\text{O}(l) \rightleftharpoons \text{H}_{2}\text{PO}_{4}^{-}(aq)+\text{H}_{3}\text{O}^{+}(aq)\), phosphoric acid (\(\text{H}_{3}\text{PO}_{4}\)) donates a proton to water (\(\text{H}_{2}\text{O}\)). In turn, water accepts the proton, becoming hydronium (\(\text{H}_{3}\text{O}^{+}\)). This tells us that phosphoric acid and water are involved in a classic acid-base reaction.
Another example is \(\text{CO}_{3}^{2-}(aq)+\text{H}_{2}\text{O}(l) \rightleftharpoons \text{HCO}_{3}^{-}(aq)+\text{OH}^{-}(aq)\). Here, water (\(\text{H}_{2}\text{O}\)) donates a proton to carbonate (\(\text{CO}_{3}^{2-}\)), forming bicarbonate (\(\text{HCO}_{3}^{-}\)) and hydroxide (\(\text{OH}^{-}\)). These species are also engaged in an acid-base reaction. By recognizing these fundamental processes, you can better understand the dynamics and outcomes of various chemical reactions.