Problem 219 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following Column-I (a) HF (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) ( \(\mathrm{COOH}\) (c) \(\mathrm{H}_{3} \mathrm{BO}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) Column-II (p) Strength of acid increases with concentration (q) Weak acid (r) Strength of acid increases with dilution (s) Acid shows hydrogen bonding (t) Monobasic acid.

Step-by-Step Solution

Answer

(a) Matches: q, s; (b) Matches: q, s; (c) Matches: q, r; (d) Matches: t.

Step 1: Analyze each compound in Column I

First, we need to understand the chemical properties of each compound in Column I: - (a) HF (Hydrofluoric acid) is a weak acid known for hydrogen bonding. - (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (Acetic Acid) is a weak acid and also exhibits hydrogen bonding. - (c) \(\mathrm{H}_{3} \mathrm{BO}_{3}\) (Boric Acid) is an acid that becomes stronger upon dilution and is considered a weak acid. - (d) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (Hypophosphorous Acid) is a monobasic acid, meaning it donates only one proton (H⁺).

Step 2: Match properties from Column I to Column II

Using the analysis from Step 1, match each compound:- (a) HF: Matches with (q) Weak acid and (s) Acid shows hydrogen bonding.- (b) \(\mathrm{CH}_{3} \mathrm{COOH}\): Matches with (q) Weak acid and (s) Acid shows hydrogen bonding.- (c) \(\mathrm{H}_{3} \mathrm{BO}_{3}\): Matches with (r) Strength of acid increases with dilution and (q) Weak acid.- (d) \(\mathrm{H}_{3} \mathrm{PO}_{2}\): Matches with (t) Monobasic acid.

Key Concepts

Weak Acids

Weak acids are a fascinating category in acid-base chemistry. Unlike their strong acid counterparts, weak acids do not fully dissociate into their ions in water. This characteristic means that only a small percentage of their molecules release hydrogen ions, which are responsible for the acidic properties of the solution. As a result, the equilibrium between the undissociated acid and the ions remains significant.

In the exercise, both HF (hydrofluoric acid) and CH₃COOH (acetic acid) are identified as weak acids. This is because when dissolved in water, they do not release all their hydrogen ions. Instead, a balance is maintained between the ionized and unionized forms. This equilibrium can be represented by the formula:

\[ ext{HA} ightleftharpoons ext{H}^+ + ext{A}^- \]

• Where HA is the weak acid, H⁺ is the hydrogen ion, and A⁻ is the conjugate base.
• The position of this equilibrium varies from one weak acid to another, often measured as the acid dissociation constant (Ka).

Understanding weak acids is crucial to appreciate how mild acids behave differently compared to strong acids, particularly their reactivity and how they affect pH levels in solutions.

Hydrogen Bonding

Hydrogen bonding is an essential concept in understanding the properties of certain acids. This type of bonding is a strong intermolecular force that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom like fluorine, oxygen, or nitrogen.

For example, both HF and CH₃COOH display hydrogen bonding due to their molecular structures:

• In HF, hydrogen forms a bond with highly electronegative fluorine, leading to strong hydrogen bonding connections between molecules.
• In the case of acetic acid (CH₃COOH), hydrogen bonding occurs because of the presence of the highly electronegative oxygen atoms in the carboxyl group (COOH).

This bonding affects their physical properties significantly, such as boiling and melting points, as hydrogen bonds require additional energy to break compared to other intermolecular forces. Consequently, even though HF is a weak acid, its hydrogen bonding grants it unique properties, including its etching capability on glass and metals.

Monobasic Acids

Monobasic acids are acids that donate only one hydrogen ion per molecule when dissolved in water. This characteristic makes them simpler than polybasic acids which can donate more than one hydrogen ion.

In the problem, H₃PO₂ (hypophosphorous acid) is identified as a monobasic acid. Despite having multiple hydrogen atoms in its structure, hypophosphorous acid only actively donates one hydrogen ion. This can be illustrated by the equation:

• \[ ext{H}_3 ext{PO}_2 ightleftharpoons ext{H}^+ + ext{H}_2 ext{PO}_2^- \]

This distinction is crucial as it determines how the acid behaves in reactions, including its reactivity, pH level influence, and the types of salts it forms. Monobasic acids like hypophosphorous acid can participate in typical acid-base reactions but will contribute less to changes in the solution’s acidity compared to polybasic acids.

Acid Strength and Concentration

Acid strength and concentration are key factors in understanding an acid's behavior in a solution. Acid strength refers to an acid's ability to donate protons (H⁺ ions) in a solution. In contrast, concentration describes the amount of acid present in a given volume of solution.

For instance, H₃BO₃ (boric acid) demonstrates an unusual characteristic where its strength increases with dilution. This behavior is atypical since most acids get weaker as they are diluted. However, boric acid acts differently because it becomes more effective at releasing protons as water molecules interact more with its structure:

• In a diluted solution, an increase in water concentration facilitates the hydrolysis reaction, enhancing its ability to donate protons.
• This unique behavior is a hallmark of certain substrates like boric acid, where binding with water molecules boosts ionization.

Understanding both the intrinsic strength and concentration of an acid is essential in applications involving pH adjustments, buffer solutions, and chemical reactions where precise control of acidity is required.