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

Question

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

Step-by-Step Solution

Answer

(a) HF - (s), (b) CH₃COOH - (q), (c) H₃BO₃ - (r), (d) H₃PO₂ - (t)

Step 1: Analyze HF

HF (Hydrofluoric acid) is known for its ability to form hydrogen bonds. This property makes it distinct among acids, and this indicates that HF corresponds to hydrogen bonding. Thus, HF matches with (s).

Step 2: Analyze CH_3COOH

Acetic acid (CH_3COOH) is categorized as a weak acid. It partially ionizes in solution, which is a characteristic of weak acids. Therefore, CH_3COOH matches with (q).

Step 3: Analyze H_3BO_3

Boric acid (H_3BO_3) is unique because its strength increases upon dilution. This property makes boric acid match with (r).

Step 4: Analyze H_3PO_2

Hypophosphorous acid (H_3PO_2) is known to be monobasic, meaning it donates only one proton per molecule. Therefore, H_3PO_2 matches with (t).

Step 5: Verify Matches for Remaining Element

There is only one option remaining; (p) relates to the increase in acid strength with concentration, which doesn't directly match any of the acids provided in the remaining context given.

Key Concepts

Hydrogen Bonding in Acids

Hydrogen bonding is a special intermolecular force that occurs when hydrogen is bonded to very electronegative atoms like fluorine, oxygen, or nitrogen. In acids, hydrogen bonding can significantly influence the properties of the acid. For example, hydrofluoric acid (HF) is known for its unique capability to form strong hydrogen bonds due to the highly electronegative fluorine atom. This makes HF molecules attract each other strongly, influencing its boiling point, solubility, and its overall behavior in chemical reactions.

Hydrogen bonding in HF makes it an acid with some unusual properties compared to other acids like HCl or HBr. In aqueous solutions, these hydrogen bonds also contribute to HF's ability to be more viscous and less volatile. This property not only shows the complex nature of acid behavior but also underscores how hydrogen bonding can alter an acid's interaction with other substances, making HF particularly aggressive in reacting with glass and metals.

Weak Acids

Weak acids are acids that do not completely dissociate in water, meaning they only partially release their hydrogen ions into solution. An example of a weak acid is acetic acid ( CH_3COOH). Unlike strong acids which dissociate nearly 100% in solution, weak acids reach an equilibrium where both the acid and its ions are present in the solution.

Common properties of weak acids include their typically higher pH compared to strong acids (at the same concentration), and they often have lower reactivity. This partial ionization can be described by an equilibrium constant, known as the acid dissociation constant ( K_a). The smaller the K_a, the weaker the acid.

• Weak acids typically have K_a values less than 1.
• They taste sour and can cause chemical burns, although usually less severe than strong acids at the same concentration.
• They react with bases to form salts and water.

The behavior and reactions of weak acids, like acetic acid, are crucial for buffering and pH balance in various processes, such as in biological systems.

Acid Strength and Dilution

Acid strength refers to an acid’s ability to donate protons or hydrogen ions. While this is generally straightforward, some acids behave counter-intuitively, such as boric acid ( H_3BO_3), which increases in strength with dilution. Typically, diluting an acid decreases the concentration of hydrogen ions, which should weaken the acid.

Boric acid stands out because it acts primarily as a Lewis acid rather than a Brønsted-Lowry acid. It accepts hydroxide ions from water, increasing H⁺ concentration when diluted. This unusual behavior makes boric acid stronger with dilution, as it becomes more efficient at lowering the pH of a solution.

• Understanding this principle can help predict and manipulate chemical reactions in fields like biochemistry and pharmacology.
• It emphasizes the importance of examining each acid individually to understand their unique properties.

Recognizing these exceptions is essential for properly applying acids in chemical reactions and industrial processes.

Monobasic Acid

Monobasic acids, like hypophosphorous acid ( H_3PO_2), are acids that can donate only one proton ( H^+ ) per molecule in an aqueous solution. This type of acid is relatively straightforward in its interactions because it only loses one hydrogen ion, making its reactions more predictable.

Monobasic acids are often involved in simple acid-base neutralization reactions. For example:

• They react with bases to form a salt and water, without complex or additional hydrogen donor behavior.
• Consistently exhibit the same stoichiometry in reactions, adding predictability for chemists.

These acids typically align with fundamental chemistry principles and are often used in educational settings to illustrate basic acid properties and reactions. Being familiar with monobasic acids helps students and chemists understand how acids work in both simple and complex reactions, setting a foundation for deeper chemical exploration.