Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

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

The conjugate acid-base pairs are given by option (d).

Step 1: Identify the Acid and Base

First, let's identify the roles of the chemicals in the given equilibrium reaction \( \mathrm{HCl} + \mathrm{CH}_{3} \mathrm{COOH} \leftrightarrow \mathrm{Cl}^{-} + \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \). Here, \( \mathrm{HCl} \) is acting as an acid because it donates a proton (\( \mathrm{H}^+ \)), and \( \mathrm{CH}_{3} \mathrm{COOH} \) acts as a base because it accepts a proton to form \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \).

Step 2: Determine Conjugate Pairs

A conjugate acid-base pair consists of two substances that transform into each other by gaining or losing a proton. From the equilibrium, \( \mathrm{HCl} \) and \( \mathrm{Cl}^{-} \) form a conjugate acid-base pair. Similarly, \( \mathrm{CH}_{3} \mathrm{COOH} \) and \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \) also form a conjugate acid-base pair because \( \mathrm{CH}_{3} \mathrm{COOH} \) becomes \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \) after accepting a proton.

Step 3: Match with Given Options

Now, let's look at the options and match them with the conjugate acid-base pairs we identified: \( \mathrm{HCl} \) and \( \mathrm{Cl}^{-} \); \( \mathrm{CH}_{3} \mathrm{COOH} \) and \( \mathrm{CH}_{3} \mathrm{COOH}_{2}^{+} \). The set of conjugate pairs in (d) \( (\mathrm{HCl}, \mathrm{Cl}^{-}) \) and \( (\mathrm{CH}_{3} \mathrm{COOH}_{2}^{+}, \mathrm{CH}_{3} \mathrm{COOH}) \) matches.

Key Concepts

Bronsted-Lowry Theory

The Bronsted-Lowry Theory is a fundamental concept in understanding acids and bases. According to this theory, an acid is a substance that donates a proton (\( ext{H}^+ \)), while a base is a substance that accepts a proton. In the example provided, Hydrogen Chloride (\( ext{HCl} \)) acts as a Bronsted-Lowry acid by donating a proton. This proton is transferred to Acetic Acid (\( ext{CH}_3 ext{COOH} \)), which functions as a Bronsted-Lowry base due to its acceptance of the proton. Thus, the Bronsted-Lowry theory provides a more flexible definition of acids and bases, allowing a substance to act as either depending on the situation, facilitating discussions of a wide range of chemical reactions involving proton transfer.

Acid-Base Equilibrium

The concept of Acid-Base Equilibrium is central in understanding chemical reactions involving acids and bases. In an equilibrium, the reaction can move in both directions, and the concentrations of reactants and products remain stable over time.In the given chemical reaction, when Hydrogen Chloride (\( ext{HCl} \)) is mixed with Acetic Acid (\( ext{CH}_3 ext{COOH} \)), an equilibrium is established. This means that the forward reaction, where \( ext{HCl} \) and \( ext{CH}_3 ext{COOH} \) react to form Chloride (\( ext{Cl}^- \)) and \( ext{CH}_3 ext{COOH}_2^+ \)), occurs at the same rate as the reverse reaction, in which \( ext{Cl}^- \) and \( ext{CH}_3 ext{COOH}_2^+ \)) reform the original reactants.Under equilibrium conditions, the system is dynamic; reactions continue to occur, but the concentrations of the participating species do not change. Understanding this balance is crucial for predicting the outcome of reactions and the effect of different conditions like temperature and concentration on the system.

Proton Transfer

Proton transfer is a key process in many acid-base reactions. It involves the movement of a proton (\( ext{H}^+ \)) from an acid to a base. When \( ext{HCl} \)) dissolves in \( ext{CH}_3 ext{COOH} \)), this transfer occurs: the \( ext{HCl} \)) donates a proton to \( ext{CH}_3 ext{COOH} \)), resulting in the formation of Chloride \( ext{Cl}^- \)) and \( ext{CH}_3 ext{COOH}_2^+ \)). The result of this transfer is the generation of conjugate acid-base pairs. - \( ext{HCl} \)) becomes its conjugate base, \( ext{Cl}^- \)).- \( ext{CH}_3 ext{COOH} \)) becomes its conjugate acid, \( ext{CH}_3 ext{COOH}_2^+ \)).Proton transfer is not only crucial in establishing equilibrium but also in distinguishing between different conjugate pairs. It provides insight into the acidity or basicity of the solution, influenced by the direction of the proton transfer. Understanding these processes at a molecular level aids in grasping the larger picture of chemical reactivity and behavior.