Question

Write stepwise chemical equations for protonation or deprotonation of each of these polyprotic acids and bases in water. (a) \(\mathrm{H}_{2} \mathrm{SO}_{3}\) (b) \(S^{2-}\) (c) \(\mathrm{NH}_{3} \mathrm{CH}_{3} \mathrm{COOH}^{+}\) (glycinium ion, a diprotic acid)

Short answer

Protonation/deprotonation reactions involve sequential transfer of protons from polyprotic acids/bases to/from water, forming new species and hydronium or hydroxide ions.

Step-by-step solution

Identify the Nature of the Species

For each compound, first determine if it is an acid or a base and whether it can donate or accept more than one proton. Polyprotic acids can donate more than one proton, while polyprotic bases can accept more than one proton.

Write Protonation/Deprotonation Reactions for H₂SO₃

Hydrogen sulfite (\(\mathrm{H}_{2}\mathrm{SO}_{3}\)) is a diprotic acid, meaning it can donate two protons. - First deprotonation: \( \mathrm{H}_{2}\mathrm{SO}_{3} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HSO}_{3}^{-} + \mathrm{H}_{3}\mathrm{O}^{+} \)- Second deprotonation: \( \mathrm{HSO}_{3}^{-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{SO}_{3}^{2-} + \mathrm{H}_{3}\mathrm{O}^{+} \)

Write Protonation Reactions for S²⁻

The sulfide ion (\(S^{2-}\)) is a diprotic base, meaning it can accept two protons. - First protonation: \( S^{2-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{SH}^{-} + \mathrm{OH}^{-} \)- Second protonation: \( \mathrm{SH}^{-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{S} + \mathrm{OH}^{-} \)

Write Protonation/Deprotonation Reactions for NH₃CH₃COOH⁺

The glycinium ion (\(\mathrm{NH}_{3} \mathrm{CH}_{3} \mathrm{COOH}^{+}\)) is a diprotic acid and can donate two protons. - First deprotonation: \( \mathrm{NH}_{3} \mathrm{CH}_{3} \mathrm{COOH}^{+} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_{3} \mathrm{CH}_{3} \mathrm{COO} + \mathrm{H}_{3}\mathrm{O}^{+} \)- Second deprotonation: \( \mathrm{NH}_{3} \mathrm{CH}_{3} \mathrm{COO} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_{2} \mathrm{CH}_{3} \mathrm{COO}^{-} + \mathrm{H}_{3}\mathrm{O}^{+} \)

Key concepts

Protonation and Deprotonation

The process of protonation involves the addition of a proton (H⁺), whereas deprotonation refers to the removal of a proton. In chemical terms, these are reversible processes. They occur in aqueous solutions to reach a state of equilibrium. Polyprotic acids and bases are special because they can undergo multiple protonation or deprotonation steps.
For instance, a diprotic acid like \(\mathrm{H}_2\mathrm{SO}_3\) can release two hydrogen ions. Protonation and deprotonation processes are essential as they affect a substance's pH and its chemical reactivity, impacting biological and industrial processes.

• Protonation results in the formation of conjugate acids.
• Deprotonation results in the formation of conjugate bases.

Recognizing which form a substance is in helps predict its behavior in reactions and solutions.

Chemical Equations

Chemical equations are the shorthand notations expressing chemical reactions. They demonstrate the transformation of reactants into products. Each equation must adhere to the law of conservation of mass, meaning the total mass of reactants equals the total mass of products.
In the context of protonation and deprotonation, balancing chemical equations involves ensuring the number of atoms and the charge are equal on both sides.

• Reactants on the left
• Products on the right
• States of matter are often indicated (solid, liquid, gas, aqueous)

Key components of an accurate chemical equation include coefficients to balance atoms, and symbols such as → for yield. Writing these equations helps visualize proton exchange in polyprotic acids and bases.

H2SO3 Reactions

Hydrogen sulfite, \(\mathrm{H}_2\mathrm{SO}_3\), is a diprotic acid and a classic example of a substance that undergoes two deprotonation steps. In water, it can donate two protons consecutively.

• First deprotonation: \( \mathrm{H}_2\mathrm{SO}_3 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HSO}_3^- + \mathrm{H}_3\mathrm{O}^+ \)
• Second deprotonation: \( \mathrm{HSO}_3^- + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{SO}_3^{2-} + \mathrm{H}_3\mathrm{O}^+ \)

Each step releases a proton, resulting in a conjugate base, progressing from slightly acidic \(\mathrm{H}_2\mathrm{SO}_3\) to more basic sulfate ion \(\mathrm{SO}_3^{2-}\). Understanding these steps helps predict the behavior of \(\mathrm{H}_2\mathrm{SO}_3\) in a solution.

S2- Reactions

The sulfide ion \(S^{2-}\) is a robust example of a diprotic base. Unlike acids, diprotic bases gain protons. In aqueous solutions, \(S^{2-}\) can absorb two protons sequentially.

• First protonation: \( S^{2-} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{SH}^- + \mathrm{OH}^- \)
• Second protonation: \( \mathrm{SH}^- + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{S} + \mathrm{OH}^- \)

These protonation steps demonstrate \(S^{2-}\) transitioning to hydrogen sulfide, \(\mathrm{H}_2\mathrm{S}\). Each step combines with water, forming dihydronium ions \(\mathrm{OH}^-\), affecting the surrounding pH and reactivity of compounds.

Glycinium Ion Reactions

The glycinium ion, \(\mathrm{NH}_3\mathrm{CH}_3\mathrm{COOH}^+\), is intriguing because it is a diprotic acid. This means it has the potential to release two hydrogen ions in sequence.

• First deprotonation: \( \mathrm{NH}_3\mathrm{CH}_3\mathrm{COOH}^+ + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_3\mathrm{CH}_3\mathrm{COO} + \mathrm{H}_3\mathrm{O}^+ \)
• Second deprotonation: \( \mathrm{NH}_3\mathrm{CH}_3\mathrm{COO} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_2\mathrm{CH}_3\mathrm{COO}^- + \mathrm{H}_3\mathrm{O}^+ \)

These steps illustrate how the glycinium ion’s structure changes when releasing protons. First transforming to neutral acetate, then to negatively charged acetate ion. Recognizing these transformations aids in understanding the glycinium ion’s role in chemical and biological systems.