Question

Write balanced equations for proton-transfer reactions between the listed pairs. Indicate the conjugate pairs, and determine the favored direction for each equilibrium. (a) \(\mathrm{HCl}\) and \(\mathrm{PO}_{4}^{3-}\) (b) \(\mathrm{HCN}\) and \(\mathrm{SO}_{4}^{2-}\) (c) \(\mathrm{HClO}_{4}\) and \(\mathrm{NO}_{2}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{O}^{-}\) and \(\mathrm{HF}\)

Short answer

(a) Right; (b) Left; (c) Right; (d) Right.

Step-by-step solution

Understanding Proton-Transfer Reactions

Proton-transfer reactions involve the transfer of a proton (H+) from an acid to a base. The acid donates a proton, becoming its conjugate base, while the base accepts a proton, becoming its conjugate acid.

Identifying Reactants and Products

For each pair, identify which compound is the acid (donor of H+) and which is the base (acceptor of H+).

Writing Balanced Equations and Conjugate Pairs

Write balanced equations for each pair's reaction and identify the conjugate acid-base pairs.

Evaluating Equilibrium Favored Side

Determine which side of the reaction is favored based on the strength of the acids and bases involved.

Part (a): Reaction of HCl and PO4^3-

**Equation:**\[ \mathrm{HCl} + \mathrm{PO}_4^{3-} \rightleftharpoons \mathrm{Cl}^- + \mathrm{HPO}_4^{2-} \]**Conjugate Pairs:**- Acid: \( \mathrm{HCl} \), Conjugate Base: \( \mathrm{Cl}^- \)- Base: \( \mathrm{PO}_4^{3-} \), Conjugate Acid: \( \mathrm{HPO}_4^{2-} \)**Favored Direction:** To the right because \( \mathrm{HCl} \) is a strong acid and dissociates completely.

Part (b): Reaction of HCN and SO4^2-

**Equation:**\[ \mathrm{HCN} + \mathrm{SO}_4^{2-} \rightleftharpoons \mathrm{CN}^- + \mathrm{HSO}_4^- \]**Conjugate Pairs:**- Acid: \( \mathrm{HCN} \), Conjugate Base: \( \mathrm{CN}^- \)- Base: \( \mathrm{SO}_4^{2-} \), Conjugate Acid: \( \mathrm{HSO}_4^- \)**Favored Direction:** To the left because \( \mathrm{HCN} \) is a weaker acid than \( \mathrm{HSO}_4^- \).

Part (c): Reaction of HClO4 and NO2^-

**Equation:**\[ \mathrm{HClO}_4 + \mathrm{NO}_2^- \rightleftharpoons \mathrm{ClO}_4^- + \mathrm{HNO}_2 \]**Conjugate Pairs:**- Acid: \( \mathrm{HClO}_4 \), Conjugate Base: \( \mathrm{ClO}_4^- \)- Base: \( \mathrm{NO}_2^- \), Conjugate Acid: \( \mathrm{HNO}_2 \)**Favored Direction:** To the right because \( \mathrm{HClO}_4 \) is a strong acid.

Part (d): Reaction of CH3O^- and HF

**Equation:**\[ \mathrm{CH}_3 ext{O}^- + \mathrm{HF} \rightleftharpoons \mathrm{CH}_3 ext{OH} + \mathrm{F}^- \]**Conjugate Pairs:**- Acid: \( \mathrm{HF} \), Conjugate Base: \( \mathrm{F}^- \)- Base: \( \mathrm{CH}_3 ext{O}^- \), Conjugate Acid: \( \mathrm{CH}_3 ext{OH} \)**Favored Direction:** To the right because \( \mathrm{HF} \) is a relatively strong acid compared to the stability of its conjugate base \( \mathrm{F}^- \).

Key concepts

Conjugate Acid-Base Pairs

In a proton-transfer reaction, two entities play a vital role: the acid and the base. The acid is the substance that donates a proton (H\(^+\)), while the base is the substance that accepts the proton. When these reactions occur, the donating acid becomes its conjugate base. Conversely, the base turns into its conjugate acid.
For example, in the reaction between hydrochloric acid (\(\mathrm{HCl}\)) and phosphate ion (\(\mathrm{PO}_4^{3-}\)), \(\mathrm{HCl}\) donates a proton, transforming into \(\mathrm{Cl}^-\), its conjugate base. Meanwhile, \(\mathrm{PO}_4^{3-}\), which accepts the proton, becomes \(\mathrm{HPO}_4^{2-}\), its conjugate acid.
Recognizing these pairs is crucial, as it helps us understand how substances behave as acids or bases in different situations. When identifying conjugate pairs, remember:

• The acid always forms the conjugate base.
• The base always forms the conjugate acid.

Equilibrium Direction

In chemical reactions, not all reactions proceed entirely in one direction. For proton-transfer reactions, the equilibrium can favor either the forward or reverse direction. Deciding which direction is favored involves comparing the strengths of the acids and bases involved.
A strong acid like \(\mathrm{HCl}\) dissociates completely, meaning the reaction generally favors the direction in which it forms its weaker conjugate base, \(\mathrm{Cl}^-\).
Consider the reaction: \[ \mathrm{HCl} + \mathrm{PO}_4^{3-} \rightleftharpoons \mathrm{Cl}^- + \mathrm{HPO}_4^{2-} \] Here, because \(\mathrm{HCl}\) is strong, the equilibrium is favored to the right. However, in a reaction like: \[ \mathrm{HCN} + \mathrm{SO}_4^{2-} \rightleftharpoons \mathrm{CN}^- + \mathrm{HSO}_4^- \] Since \(\mathrm{HCN}\) is a weaker acid compared to \(\mathrm{HSO}_4^-\), the equilibrium favors the left side. Remember, the rule of thumb is that equilibria favor the side producing the weaker acid or base.

Reaction Equations

Writing balanced equations for proton-transfer reactions is essential. It involves deducing which species in the pair acts as the proton donor (acid) and acceptor (base) and rearranging them into products that form their conjugate counterparts.
Take the example of \(\mathrm{CH}_3\mathrm{O}^-\) and hydrogen fluoride (\(\mathrm{HF}\)). In this pair, \(\mathrm{HF}\) acts as the acid, donating a proton to \(\mathrm{CH}_3\mathrm{O}^-\), which acts as the base: \[ \mathrm{CH}_3\mathrm{O}^- + \mathrm{HF} \rightleftharpoons \mathrm{CH}_3\mathrm{OH} + \mathrm{F}^- \] The balanced equation highlights the proton donation and acceptance, resulting in the formation of \(\mathrm{CH}_3\mathrm{OH}\) and \(\mathrm{F}^-\), the conjugate acid and base, respectively.
When arranging reaction equations, it’s crucial to ensure that both the number of atoms and the charges are balanced on both sides of the equation. This reflects the conservation of mass and charge in chemical reactions. Make sure you're consistent with these rules to represent the chemical processes accurately.