Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

Select the correct statement regarding the behaviour of HF as non-aqueous solvent (a) HCl behaves as an acid and HF as a base (b) \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) behave as base and \(\mathrm{HF}\) as an acid (c) HClO \(_{4}\) behaves as a base and HF as an acid (d) all the statements are correct

Step-by-Step Solution

Answer

None of the statements (a), (b), (c), or (d) are correct.

Step 1: Analyze HF as a Non-Aqueous Solvent

HF, being a polar molecule with strong hydrogen bonding, can participate in acid-base reactions differently compared to its behavior in aqueous solutions. In many non-aqueous cases, HF acts as a stronger acid than in water.

Step 2: Evaluate Statement (a)

Statement (a) claims that HCl behaves as an acid and HF as a base. In non-aqueous solvents, HCl is typically a stronger acid than HF, so HF will not act as a base to the primary strong acid, HCl. This statement is incorrect.

Step 3: Evaluate Statement (b)

Statement (b) suggests that \(\mathrm{HNO}_3\) and \(\mathrm{H}_2\mathrm{SO}_4\) behave as bases while HF acts as an acid. Both \(\mathrm{HNO}_3\) and \(\mathrm{H}_2\mathrm{SO}_4\) are strong acids, and HF can act as an acid as well, not inducing base behavior in the others. This statement is incorrect.

Step 4: Evaluate Statement (c)

Statement (c) implies that \(\mathrm{HClO}_4\) behaves as a base and HF as an acid. \(\mathrm{HClO}_4\) is one of the strongest acids known, and it will not act as a base. HF acting as an acid in non-aqueous media can be true, but not in relation to \(\mathrm{HClO}_4\) acting as a base, making this statement incorrect.

Step 5: Evaluate Statement (d)

Statement (d) claims all the above statements are correct. Since none of (a), (b), or (c) are correct, this statement must also be incorrect.

Key Concepts

Acid-Base Behavior

Understanding acid-base behavior is critical when exploring non-aqueous solvents. In simple terms, acids are substances that can donate a proton (or H⁺ ion), while bases accept a proton. However, this behavior can change significantly in non-aqueous environments.

In non-aqueous solvents, the acidity or basicity of a substance can be quite different compared to aqueous solutions (water-based). The solvent's polarity, hydrogen bonding, and its own ability to donate or accept protons can all influence the acid-base behavior.

• **Stronger Acids vs Weaker Solvents:** In solvents weaker than water, even weak acids may demonstrate stronger acidic properties.
• **Proton Transfer:** Non-aqueous solvents can change how easily a proton is transferred between the acid and the base.
• **Unique Interactions:** New solvent-specific interactions may come into play that influence strength and behavior.

Understanding these differences is key to predicting which substances will behave as acids or bases under various conditions.

Hydrofluoric Acid (HF)

Hydrofluoric acid (HF) is notorious for being a dangerous chemical. Yet, it's fascinating in terms of its chemical properties. HF is a small and highly polar molecule which allows it to form strong hydrogen bonds, both in aqueous and non-aqueous solutions.

In water, HF is considered a weak acid because it does not fully dissociate into ions. However, in non-aqueous solvents, its behavior can change:

• **Polar Nature:** The high polarity of HF can lead to strong interactions with other polar molecules.
• **Strength in Non-Aqueous Solvents:** In many non-aqueous environments, HF acts as a stronger acid compared to its presence in water. This might lead to more vigorous reactions than expected.
• **Hydrogen Bonding:** HF's ability to form strong hydrogen bonds can also influence its interactions with other substances, impacting its reactivity.

These unique characteristics make HF particularly interesting when exploring acid-base reactions in different chemical settings.

Strong Acids

Strong acids are distinguished by their ability to completely dissociate in solution, releasing all their hydrogen ions. This characteristic leads them to fully exhibit their acidic properties, usually rendering pH to lower values and making them crucial in many chemical processes.

Common strong acids include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), and perchloric acid (HClO₄). When dealing with non-aqueous solvents, their behavior can differ in terms of proton donation and interaction:

• **Complete Dissociation:** Strong acids are known for almost entirely releasing their protons in solution, but the extent to which this occurs can change in different solvents.
• **Competing Strengths:** In a non-aqueous solvent, relative strength versus other acids comes into play. This can lead some strong acids to act differently than expected.
• **Solvent Influence:** The acidity might reduce or increase based on how the solvent interacts with the acid molecules, predominantly through hydrogen bonding or other intermolecular forces.

These behaviors underline why recognizing the nature of strong acids and their interactions is pivotal for deeper chemical understanding.