Three reactions involving `H_(2)PO_(4)^(-)` are given below
`I. H_(3)PO_(4)+H_(2)OrarrH_(3)O^(+)+H_(2)PO_(4)^(-)`
`II. H_(2)PO_(4)^(-)+H_(2)OrarrHPO_(4)^(2-)+H_(3)O^(+)`
`III. H_(2)PO_(4)^(-)+OH^(-)rarrH_(3)PO_(4)+O^(2+)`
In which of the above does `H_(2)PO_(4)^(-)` act as an acid?

Options

A. `II` only
B. `I` and `II`
C. `III` only
D. `I` only

Explanation

To determine in which of the given reactions `H₂PO₄⁻` acts as an acid, we need to analyze each reaction based on the definitions of acids and bases. An acid is a substance that donates a proton (H⁺), while a base is a substance that accepts a proton.

### Step-by-Step Solution:

**Reaction I:**
\[ H₃PO₄ + H₂O \rightleftharpoons H₃O^+ + H₂PO₄^- \]

- In this reaction, `H₃PO₄` (phosphoric acid) donates a proton (H⁺) to water (H₂O), forming `H₃O^+` (hydronium ion) and `H₂PO₄⁻` (dihydrogen phosphate ion).
- Here, `H₂PO₄⁻` is the conjugate base of `H₃PO₄`, and thus it does not act as an acid in this reaction.

**Reaction II:**
\[ H₂PO₄^- + H₂O \rightleftharpoons HPO₄^{2-} + H₃O^+ \]

- In this reaction, `H₂PO₄⁻` donates a proton (H⁺) to water, resulting in the formation of `HPO₄^{2-}` (hydrogen phosphate ion) and `H₃O^+`.
- Since `H₂PO₄⁻` donates a proton, it acts as an acid in this reaction.

**Reaction III:**
\[ H₂PO₄^- + OH^- \rightleftharpoons H₃PO₄ + O^{2-} \]

- In this reaction, `H₂PO₄⁻` reacts with hydroxide ion (OH⁻). Here, `H₂PO₄⁻` accepts a proton (H⁺) from the hydroxide ion, resulting in the formation of `H₃PO₄` and `O^{2-}` (oxide ion).
- Since `H₂PO₄⁻` is accepting a proton, it acts as a base in this reaction.

### Conclusion:
`H₂PO₄⁻` acts as an acid only in **Reaction II**.