HSO3 ¯ is amphoteric. For H2SO3 Ka1 = 1.4 x 10-2 and Ka2 = 6.3 x 10-8 . a. Write a chemical reaction showing HSO3 ¯ acting as an acid in water. b. Write a chemical reaction showing HSO3 ¯ acting as a base in water. c. Will an aqueous solution of NaHSO3 be acidic, basic, or neutral?
The Correct Answer and Explanation is :
Answer:
a. When HSO₃⁻ acts as an acid in water:
[
\text{HSO}_3^- + \text{H}_2\text{O} \rightarrow \text{SO}_3^{2-} + \text{H}_3\text{O}^+
]
b. When HSO₃⁻ acts as a base in water:
[
\text{HSO}_3^- + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 + \text{OH}^-
]
c. An aqueous solution of NaHSO₃ will be slightly acidic.
Explanation:
The HSO₃⁻ ion, known as hydrogen sulfite or bisulfite, is amphoteric, meaning it can act as both an acid and a base. This behavior is due to its structure, where it has both a proton (H⁺) that it can donate, and a negatively charged oxygen that allows it to accept an additional proton.
In part (a), we see HSO₃⁻ acting as an acid, donating a proton to water to form SO₃²⁻ and H₃O⁺. The formation of H₃O⁺ ions demonstrates that HSO₃⁻ is capable of increasing the solution’s acidity.
In part (b), HSO₃⁻ acts as a base, accepting a proton from water to form H₂SO₃ (sulfurous acid) and OH⁻. The generation of OH⁻ ions demonstrates that HSO₃⁻ can also increase the basicity of the solution.
To determine if an aqueous solution of NaHSO₃ is acidic, basic, or neutral, we need to consider the relative strengths of HSO₃⁻ as an acid and as a base. The acidity of HSO₃⁻ is quantified by its acid dissociation constants, ( K_a1 = 1.4 \times 10^{-2} ) and ( K_a2 = 6.3 \times 10^{-8} ). The first dissociation constant (Ka1) is larger, indicating that HSO₃⁻ has a stronger tendency to lose a proton and act as an acid rather than accept a proton and act as a base.
Therefore, in an aqueous solution, HSO₃⁻ will predominantly release H⁺ ions, making the solution slightly acidic.