Problem 31 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The following equilibrium is established when hydrogen chloride is dissolved in acetic acid. $$ \mathrm{HCl}+\mathrm{CH}_{3} \mathrm{COOH} \rightleftharpoons \mathrm{Cl}^{-}+\mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} $$ The set that characterises the conjugate acid-base pairs is (a) \(\left(\mathrm{HCl}, \mathrm{CH}_{3} \mathrm{COOH}\right)\) and \(\left(\mathrm{CH}_{2} \mathrm{COOH}_{2}^{+}, \mathrm{Cl}^{-}\right)\) (b) \(\left(\mathrm{HCl}, \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}\right)\)and \(\left(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{Cl}^{-}\right)\) (c) \(\left(\mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}, \mathrm{HCl}\right)\) and \(\left(\mathrm{Cl}^{-}, \mathrm{CH}_{3} \mathrm{COOH}\right)\) (d) \(\left(\mathrm{HCl}, \mathrm{Cl}^{-}\right)\)and \(\left(\mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}, \mathrm{CH}_{3} \mathrm{COOH}\right)\)

Step-by-Step Solution

Answer

Option (d) is correct: \((\mathrm{HCl}, \mathrm{Cl}^{-})\) and \((\mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}, \mathrm{CH}_{3} \mathrm{COOH})\)."

Step 1: Understanding the Equilibrium Reaction

The given reaction is \( \mathrm{HCl} + \mathrm{CH}_{3} \mathrm{COOH} \rightleftharpoons \mathrm{Cl}^{-} + \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \). In this reaction, \( \mathrm{HCl} \) donates a proton to \( \mathrm{CH}_{3} \mathrm{COOH} \), forming \( \mathrm{Cl}^{-} \) and \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \). We need to identify the conjugate acid-base pairs.

Step 2: Identify Conjugate Acid-Base Pairs

A conjugate acid-base pair consists of two species that differ by a proton (\( \mathrm{H}^{+} \)). \( \mathrm{HCl} \) is the acid because it donates a proton, becoming \( \mathrm{Cl}^{-} \), the conjugate base. \( \mathrm{CH}_{3} \mathrm{COOH} \) accepts the proton, becoming \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \), the conjugate acid.

Step 3: Match Pairs with Options

The correct pairs are: \( \left(\mathrm{HCl}, \mathrm{Cl}^{-}\right) \) and \( \left(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}\right) \). Option (d) corresponds to these pairs: \( \left(\mathrm{HCl}, \mathrm{Cl}^{-}\right) \) and \( \left(\mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}, \mathrm{CH}_{3} \mathrm{COOH}\right) \), which are presented in reverse order for \( \left(\mathrm{CH}_{3} \mathrm{COOH}, \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}\right) \).

Key Concepts

Equilibrium Reactions

Equilibrium reactions play a fundamental role in chemistry, particularly in reactions involving acids and bases. Such a reaction reaches a state where the rates of the forward and reverse reactions are equal. This implies that the concentrations of reactants and products remain constant over time, although they are not necessarily equal. When hydrogen chloride (HCl) is dissolved in acetic acid, an equilibrium is established where HCl donates a proton to acetic acid:

• The forward reaction: HCl donates a proton to form Cl^- and CH₃COOH₂⁺.
• The reverse reaction: Cl^- and CH₃COOH₂⁺ recombine to form HCl and CH₃COOH.

This dynamic equilibrium means that both reactions occur simultaneously, allowing us to observe a balance between reactant and product forms. Understanding equilibrium helps explain how substances interact and transform in reversible chemical processes.

Proton Transfer

Proton transfer is a central concept in acid-base chemistry, showcasing how acids donate protons and bases accept them. In the equilibrium between HCl and acetic acid:

• HCl acts as a proton donor (acid), transferring an H⁺ ion to CH₃COOH.
• CH₃COOH, the proton acceptor (base), becomes CH₃COOH₂⁺ after gaining the proton.

This transfer of a proton from HCl to acetic acid results in the formation of a conjugate acid-base pair. Conjugate pairs differ by one proton, and understanding these pairs is crucial in predicting the behavior of acids and bases in various conditions. For example, calculating the pH or shifting an equilibrium can depend on how readily these protons are exchanged.

Acetic Acid Chemistry

Acetic acid is an important weak acid that plays a significant role in many chemical processes. Its structure allows it to participate in proton transfer reactions effectively. In the equilibrium with hydrochloric acid (HCl), acetic acid (CH₃COOH) accepts a proton to form CH₃COOH₂⁺.

• Initially, acetic acid acts as a proton acceptor from HCl.
• The protonated form, CH₃COOH₂⁺, can easily give back the proton to revert to its original state.

The versatility of acetic acid in accepting and donating protons makes it a significant component in many organic reactions. Its behavior is essential for understanding more complex acid-base interactions, making it a key component in studying chemical equilibria and proton transfer dynamics.