Question

(a) The hydrogen sulfite ion \(\left(\mathrm{HSO}_{3}^{-}\right)\) is amphiprotic. Write a balanced chemical equation showing how it acts as an acid toward water and another equation showing how it acts as a base toward water. (b) What is the conjugate acid of HSO \(_{3}^{-}?\) What is its conjugate base?

Short answer

(a) As an acid: \( \mathrm{HSO_{3}^{-}} + \mathrm{H_{2}O} \rightleftharpoons \mathrm{H_{3}O^{+}} + \mathrm{SO_{3}^{2-}} \) As a base: \( \mathrm{HSO_{3}^{-}} + \mathrm{H_{2}O} \rightleftharpoons \mathrm{OH^{-}} + \mathrm{H_{2}SO_{3}} \) (b) Conjugate acid: H₂SO₃ (sulfurous acid); Conjugate base: SO₃²⁻ (sulfite ion)

Step-by-step solution

Part (a): HSO₃⁻ acting as an acid

As an acid, HSO₃⁻ donates a proton (H⁺) to the water molecule, forming a hydronium ion (H₃O⁺) and a corresponding conjugate base. The balanced chemical equation for this reaction is as follows: \( \mathrm{HSO_{3}^{-}} + \mathrm{H_{2}O} \rightleftharpoons \mathrm{H_{3}O^{+}} + \mathrm{SO_{3}^{2-}} \)

Part (a): HSO₃⁻ acting as a base

As a base, HSO₃⁻ accepts a proton (H⁺) from the water molecule, forming a hydroxide ion (OH⁻) and a corresponding conjugate acid. The balanced chemical equation for this reaction is as follows: \( \mathrm{HSO_{3}^{-}} + \mathrm{H_{2}O} \rightleftharpoons \mathrm{OH^{-}} + \mathrm{H_{2}SO_{3}} \)

Part (b): Conjugate acid of HSO₃⁻

When HSO₃⁻ acts as a base, as seen in the previous step, it accepts a proton and forms a conjugate acid. The conjugate acid of HSO₃⁻ is H₂SO₃ (sulfurous acid).

Part (b): Conjugate base of HSO₃⁻

When HSO₃⁻ acts as an acid, as seen in the first step, it donates a proton and forms a conjugate base. The conjugate base of HSO₃⁻ is SO₃²⁻ (sulfite ion). In conclusion, the hydrogen sulfite ion (HSO₃⁻) can act as both an acid and a base when reacting with water. Its conjugate acid is H₂SO₃, and its conjugate base is SO₃²⁻.

Key concepts

Chemical Equilibrium

In the intriguing world of chemical reactions, chemical equilibrium is a state where the rate of the forward reaction equals the rate of the reverse reaction, meaning that the concentrations of reactants and products remain constant over time. This doesn't imply that the reaction has ceased, but rather that it's proceeding in both directions at the same rate.

Consider a simple acid-base reaction taking place in water. At equilibrium, the number of acid molecules donating a proton to become the conjugate base equals the number of conjugate base molecules accepting a proton to revert to the acid form. The phenomenon of the hydrogen sulfite ion acting both as an acid and a base in water, and reaching equilibrium, illustrates this beautifully.

Conjugate Acid-Base Pairs

The concept of conjugate acid-base pairs is at the heart of understanding acid-base reactions. This pair consists of two species that transform into each other by gain or loss of a proton (H+). In the instance of the hydrogen sulfite ion (HSO₃⁻), when it acts as an acid, it donates a proton to form its conjugate base, the sulfite ion (SO₃²⁻). Conversely, when HSO₃⁻ accepts a proton, it forms its conjugate acid, sulfurous acid (H₂SO₃).

These pairs are like two sides of the same coin – they can be flipped back and forth through acid-base reactions, and understanding them is crucial for predicting the direction and extent of such reactions.

Hydrogen Sulfite Ion

The hydrogen sulfite ion, or HSO₃⁻, is an interesting character in the acid-base saga due to its ability to play both roles as required. As an amphiprotic substance, it can lend a proton to become SO₃²⁻, or accept one to form H₂SO₃. This flexibility is due to its molecular structure which harbours both a hydrogen atom available for donation and spots ripe for accepting a new proton.

Understanding the behavior of such ions is fundamental in many chemical processes, including industrial applications and biological systems where pH balance is pivotal. The amphiprotic nature makes HSO₃⁻ incredibly versatile and an essential part of the acid-base dance in chemistry.

Acid-Base Reactions

Delving into acid-base reactions, these are processes where an acid donates a proton to a base. This transaction of protons is not a one-way street but rather a bustling interchange. The acid (proton donor) and base (proton acceptor) in such a reaction can be found in several states – gaseous, liquid, or even solid.

In the case of HSO₃⁻, it can donate a proton to water (acting as a base), or accept one from water (acting as an acid), showing the dynamic nature of these reactions. The ability of a substance to act as either an acid or base in different scenarios is a cornerstone of acid-base chemistry, and it plays a crucial role in many biological systems and industrial chemical processes.