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 correct option is (d): \((\text{HCl}, \text{Cl}^-)\) and \((\text{CH}_3\text{COOH}_2^+, \text{CH}_3\text{COOH})\).

Step 1: Identify the Acid and Base

In the reaction, \( \text{HCl} \) donates a proton \( (\text{H}^+) \) while acetic acid \( (\text{CH}_3\text{COOH}) \) accepts it to form \( \text{CH}_3\text{COOH}_2^+ \). Therefore, \( \text{HCl} \) is acting as an acid, and \( \text{CH}_3\text{COOH} \) is acting as a base.

Step 2: Identify Conjugate Pairs

In the context of the equilibrium, \( \text{HCl} \) and \( \text{Cl}^- \) form a conjugate acid-base pair because \( \text{Cl}^- \) is the conjugate base of \( \text{HCl} \). Similarly, \( \text{CH}_3\text{COOH} \) and \( \text{CH}_3\text{COOH}_2^+ \) form another conjugate acid-base pair because \( \text{CH}_3\text{COOH}_2^+ \) is the conjugate acid of \( \text{CH}_3\text{COOH} \).

Step 3: Match Conjugate Pairs with Choices

Now, match the identified conjugate pairs with the given options. Option (d): \( (\text{HCl}, \text{Cl}^-) \) and \( (\text{CH}_3\text{COOH}_2^+, \text{CH}_3\text{COOH}) \) correctly represents the conjugate acid-base pairs.

Key Concepts

Equilibrium in Acid-Base Reactions

In acid-base reactions, equilibrium is a state where both reactants and products are present in concentrations that have no further tendency to change with time. This balance occurs because the forward and reverse reactions happen at the same rate.
For instance, when hydrogen chloride (HCl) is mixed with acetic acid (CH₃COOH), an equilibrium is established between these compounds and their ionized forms. In this scenario, HCl reacts with CH₃COOH to form Cl⁻ and CH₃COOH₂⁺. Here, the equilibrium can shift depending on external conditions like concentration and temperature, demonstrating the dynamic nature of these systems.
Understanding equilibrium helps chemists predict how the reaction will behave when subjected to different conditions. This is crucial in industrial and laboratory settings where precise control over chemical reactions is needed.

Acid-Base Equilibrium

Acid-base equilibrium involves the balancing of acid and base species within a reaction medium. This concept centers around the ability of acids and bases to donate or accept protons (H⁺ ions).
In our example, HCl acts as an acid by donating a proton, while acetic acid (CH₃COOH) acts as a base by accepting the proton. The equilibrium is defined by the coexistence of the original acid and base with their conjugate forms: Cl⁻ (conjugate base of HCl) and CH₃COOH₂⁺ (conjugate acid of CH₃COOH).

• Equilibrium in acid-base reactions is fundamentally about proton exchanges.
• It highlights the dynamic balance between proton donors and acceptors in a closed system.
• The equilibrium position can be affected by various factors, including changes in concentration, temperature, and pressure.

By understanding acid-base equilibrium, chemists can predict reaction behavior, which is essential for tasks ranging from drug development to designing new materials.

Proton Transfer Reactions

Proton transfer reactions are the backbone of acid-base chemistry. These reactions involve the movement of protons between molecules, reshaping their structure and properties.
In the equation \[\mathrm{HCl} + \mathrm{CH}_3\mathrm{COOH} \leftrightarrow \mathrm{Cl}^- + \mathrm{CH}_3\mathrm{COOH}_2^+\]Hydrochloric acid (HCl) donates a proton to acetic acid (CH₃COOH). This results in the formation of chloride ions (Cl⁻) and the protonated form of acetic acid (CH₃COOH₂⁺).

• The proton donation by HCl demonstrates how acids release protons to form their conjugate base.
• Conversely, acetic acid accepts the proton, demonstrating base behavior and forming its conjugate acid.
• The movement of protons in such reactions is indicative of their equilibrium state and provides insights into reaction dynamics.

Understanding proton transfer reactions helps in recognizing the roles of different molecules in chemical processes, which is fundamental in fields as diverse as biochemistry and environmental science.