To determine which species is acting as an acid in the reverse reaction, we need to understand the definitions of acids and bases. According to the Brønsted-Lowry theory:
- Acid: A species that donates a proton (H^+).
- Base: A species that accepts a proton (H^+).
In the given reaction:
[tex]\[ HCO_3^{-} + HPO_4^{2-} \leftrightarrow CO_3^{2-} + H_2PO_4^{-} \][/tex]
We need to examine the reverse reaction:
[tex]\[ CO_3^{2-} + H_2PO_4^{-} \leftrightarrow HCO_3^{-} + HPO_4^{2-} \][/tex]
In this reverse reaction:
1. Identify proton transfer:
- [tex]\( H_2PO_4^{-} \)[/tex] could potentially donate a proton (H^+) to become [tex]\( HPO_4^{2-} \)[/tex].
- [tex]\( CO_3^{2-} \)[/tex] is not likely to donate a proton because it’s already in its basic form and would prefer to accept a proton to become [tex]\( HCO_3^{-} \)[/tex].
2. Determine the acid:
- [tex]\( H_2PO_4^{-} \)[/tex] (dihydrogen phosphate ion) donates a proton and transforms into [tex]\( HPO_4^{2-} \)[/tex] (hydrogen phosphate ion).
Therefore, the species that acts as an acid by donating a proton in the reverse reaction is [tex]\( \mathbf{H_2PO_4^{-}} \)[/tex].
The correct answer is:
d. [tex]\( H_2PO_4^{-} \)[/tex]
