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 \(\left(\mathrm{CH}_{3} \mathrm{COOH}^{+}, \mathrm{CH}_{3} \mathrm{COOH}\right)\)

Short answer

Option (d) characterizes the conjugate acid-base pair correctly.

Step-by-step solution

Identify Acid-Base Pairs

In the reaction, identify the acid and base on the reactant side and their corresponding conjugate pairs on the product side. Here, \(\mathrm{HCl}\) donates a proton to get \(\mathrm{Cl}^{-}\), making \(\mathrm{HCl}\) the acid and \(\mathrm{Cl}^{-}\) its conjugate base. \(\mathrm{CH}_{3}\mathrm{COOH}\) accepts a proton to become \(\mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}\), hence, \(\mathrm{CH}_{3}\mathrm{COOH}\) is the base and \(\mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}\) is its conjugate acid.

Match Conjugate Pairs with Options

Go through each option and identify which correctly matches the pairs identified. The correct conjugate pairs according to the equation are \(\left(\mathrm{HCl}, \mathrm{Cl}^{-}\right)\) and \(\left(\mathrm{CH}_{3}\mathrm{COOH}, \mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}\right)\).

Analyze Provided Options

Check each option to see which one fits the acid-base pair identified:- Option (a) and (b) have incorrect pairs not matching with the identified ones.- Option (c) incorrectly places \(\mathrm{HCl}\) with \(\mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}\).- Option (d) correctly lists the pairs as \(\left(\mathrm{HCl}, \mathrm{Cl}^{-}\right)\) and \(\left(\mathrm{CH}_{3}\mathrm{COOH}, \mathrm{CH}_{3}\mathrm{COOH}_{2}^{+}\right)\).

Key concepts

Bronsted-Lowry Theory

Understanding the Bronsted-Lowry Theory is key to grasping acid-base reactions. According to this theory, an acid is a substance that can donate a proton (\(\text{H}^+ \)), while a base is one that can accept a proton. This framework helps explain how different substances behave in reactions, depending on their ability to donate or accept protons.
In the case of hydrogen chloride (\(\text{HCl} \)) and acetic acid (\(\text{CH}_3 ext{COOH} \)), \(\text{HCl} \) acts as the proton donor, becoming \(\text{Cl}^- \) after losing a hydrogen ion. Meanwhile, \(\text{CH}_3 ext{COOH} \) acts as the base by accepting a proton to form \(\text{CH}_3 ext{COOH}_2^+ \). This reaction showcases the ability of substances to change their role as acids or bases depending on the environment. It emphasizes the dynamic nature of proton exchanges and how these interactions underlie many chemical reactions in our world.
In summary, the Bronsted-Lowry model gives a clear roadmap for predicting the direction and outcome of acid-base reactions by following the proton transfers.

Acid-Base Equilibrium

Acid-base equilibrium refers to the balance that exists in a chemical reaction involving acids and bases where each side of the equation maintains a balance of proton donation and acceptance. In reversible reactions like the one between \(\text{HCl}\) and \(\text{CH}_3\text{COOH}\), an equilibrium state occurs where the reactions in both forward and reverse directions happen at the same rate.
The equation \(\mathrm{HCl} + \mathrm{CH}_3\mathrm{COOH} \leftrightarrow \mathrm{Cl}^- + \mathrm{CH}_3\mathrm{COOH}_2^+\) demonstrates this equilibrium. In this state, the concentration of the reactants \(\mathrm{HCl}\) and \(\mathrm{CH}_3\mathrm{COOH}\) remain constant due to the continuous transfer of protons.
Maintaining equilibrium is crucial because it indicates stability within the chemical system. It helps chemists understand how much of each reactant and product will be present at any given time once equilibrium is reached. In practical terms, it allows us to predict the efficiency of reactions and the yields of products, which is vital in industrial and laboratory settings.

Proton Transfer Reactions

Proton transfer reactions are central to the understanding of acid-base chemistry. These reactions involve the movement of protons from one molecule (an acid) to another molecule (a base). They are the essence of what happens in the exchange between acids and bases.
Take the reaction involving \(\mathrm{HCl}\) and \(\mathrm{CH}_3\mathrm{COOH}\) as an example. \(\mathrm{HCl}\), acting as an acid, donates its proton to \(\mathrm{CH}_3\mathrm{COOH}\), creating \(\mathrm{Cl}^-\) and \(\mathrm{CH}_3\mathrm{COOH}_2^+\) as products. In doing so, the \(\text{H}^+\) ion is transferred, resulting in the formation of the conjugate acid-base pairs.
The significance of proton transfer reactions lies in their ubiquity across chemical processes. They govern not only simple acid-base reactions but also complex biological processes in living organisms. By understanding these reactions, students can comprehend how energy is transferred, how metabolic pathways work, and how chemical reactions are balanced within systems. The concept is crucial across many fields, from medicine to environmental science, highlighting its fundamental role in healthy functioning systems.