Using Acid – Base principles, (a) Explain why hydrofluoric acid is NOT a strong acid, while the other acid halides are strong acids. (B)

(A) Why Hydrofluoric Acid (HF) is Not a Strong Acid

Despite being a halogen acid like HCl, HBr, and HI, HF is not a strong acid. Here’s why:

• Bond Strength: The H–F bond is very strong (due to the small size and high electronegativity of fluorine). This makes it difficult to dissociate fully in water.
• Hydration Energy: Although HF forms strong hydrogen bonds with water, the dissociation is limited compared to other halogen acids.
• Electronegativity vs Bond Dissociation: While fluorine is the most electronegative element, its strong bond to hydrogen counteracts its ability to release H⁺ ions.

Therefore, HF only partially ionizes in aqueous solution, making it a weak acid, unlike HCl, HBr, and HI which completely dissociate.

(B) Relative Acid Strengths: HClO₂, HClO₃, HClO₄

These are oxoacids and their acidity increases with the number of oxygen atoms attached to the central chlorine atom:

• HClO₂ (weakest)
• HClO₃ (moderate)
• HClO₄ (strongest)

Explanation:

• More oxygen atoms increase the electron-withdrawing effect, stabilizing the conjugate base (ClO_n^–).
• Increased delocalization of the negative charge on the conjugate base enhances acid strength.

Lewis Dot Structures:

Conclusion: The more electronegative oxygen atoms around Cl pull electron density away, making the proton more easily lost (i.e., stronger acid).

(C) Effect of Varying Atom “A” in A–O–H Structure

In the general structure A–O–H, the nature of atom A greatly affects whether the compound behaves as an acid or base:

• If A is highly electronegative (e.g., Cl, S, N): It pulls electrons away from the O–H bond, weakening it and promoting the release of H⁺ → acidic behavior.
• If A is electropositive (e.g., Na, Ca, metal ions): The O–H bond is more basic, and the compound tends to accept H⁺ → basic behavior.

Examples:

• H–O–Cl → acidic (electronegative Cl pulls electron density)
• Na–O–H → basic (Na is electropositive, stabilizes OH^– formation)

Conclusion: The more electronegative the atom A, the stronger the acid; the more electropositive A is, the more basic the compound becomes in water.