Identifying the Acid in Acid-Base Reactions

In chemistry, acids and bases are defined in different ways. One common definition is the Brønsted-Lowry definition, which focuses on the transfer of protons (hydrogen ions, \(H^+\)).

• A Brønsted-Lowry acid is a species that donates a proton (\(H^+\)).
• A Brønsted-Lowry base is a species that accepts a proton (\(H^+\)).

Let's analyze the given chemical reaction:

\(HPO_4^{2-}+NH_4^{1+}\rightarrow H_2PO_4^{1-}+NH_3\)

We need to determine which reactant species acts as an acid by donating a proton.

Let's look at the change each reactant undergoes:

• The reactant \(HPO_4^{2-}\) transforms into the product \(H_2PO_4^{1-}\). To go from \(HPO_4^{2-}\) to \(H_2PO_4^{1-}\), the species has gained one hydrogen ion (\(H^+\)). This means \(HPO_4^{2-}\) has accepted a proton. According to the Brønsted-Lowry definition, a species that accepts a proton is a base. Therefore, \(HPO_4^{2-}\) is acting as a base in this reaction.
• The reactant \(NH_4^{1+}\) transforms into the product \(NH_3\). To go from \(NH_4^{1+}\) to \(NH_3\), the species has lost one hydrogen ion (\(H^+\)). This means \(NH_4^{1+}\) has donated a proton. According to the Brønsted-Lowry definition, a species that donates a proton is an acid. Therefore, \(NH_4^{1+}\) is acting as an acid in this reaction.

Based on this analysis, the species that acts as an acid in the reaction is \(NH_4^{1+}\) because it donates a proton to \(HPO_4^{2-}\).

Let's check the given options:

1. \(HPO_4^{2-}\): Acts as a base (accepts a proton).
2. \(NH_4^{1+}\): Acts as an acid (donates a proton).
3. \(NH_3\): A product, formed after \(NH_4^{1+}\) loses a proton. It is the conjugate base.
4. \(H_2PO_4^{1-}\): A product, formed after \(HPO_4^{2-}\) gains a proton. It is the conjugate acid.

The species acting as the acid is \(NH_4^{1+}\).