Problem 209 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is true? (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{SO}_{3}\) (b) in aqueous medium HF is a stronger acid than \(\mathrm{HCl}\) (c) \(\mathrm{HClO}_{4}\) is a weaker acid than \(\mathrm{HClO}_{3}\) (d) \(\mathrm{HNO}_{3}\) is a stronger acid than \(\mathrm{HNO}_{2}\)

Step-by-Step Solution

Answer

Statement (d) is true: \( \mathrm{HNO}_{3} \) is a stronger acid than \( \mathrm{HNO}_{2} \).

Step 1: Identify Acid Types

Determine if the acids in question are binary acids or oxyacids. Binary acids are generally stronger as you move down a group on the periodic table, while oxyacids' strength is generally determined by the number of oxygen atoms.

Step 2: Analyze Statement (a)

Compare the acidity of \( \mathrm{H}_{3} \mathrm{PO}_{3} \) and \( \mathrm{H}_{2} \mathrm{SO}_{3} \). \( \mathrm{H}_{2} \mathrm{SO}_{3} \) (sulfurous acid) typically is a stronger acid than \( \mathrm{H}_{3} \mathrm{PO}_{3} \) (phosphorous acid) due to the higher electronegativity and lower hydrogen bonding of sulfur.

Step 3: Analyze Statement (b)

Compare the acidity of HF and \( \mathrm{HCl} \). HF is a weaker acid compared to \( \mathrm{HCl} \), because HF forms strong hydrogen bonds, while \( \mathrm{HCl} \) dissociates completely in water.

Step 4: Analyze Statement (c)

Compare the acidity of \( \mathrm{HClO}_{4} \) and \( \mathrm{HClO}_{3} \). \( \mathrm{HClO}_{4} \) (perchloric acid) is a stronger acid than \( \mathrm{HClO}_{3} \) (chloric acid) due to having more oxygen atoms, which stabilize the acid through resonance.

Step 5: Analyze Statement (d)

Compare the acidity of \( \mathrm{HNO}_{3} \) and \( \mathrm{HNO}_{2} \). \( \mathrm{HNO}_{3} \) (nitric acid) is a stronger acid than \( \mathrm{HNO}_{2} \) (nitrous acid) because it has more oxygen atoms that contribute to better resonance stabilization.

Step 6: Conclusion

The only true statement is (d): \( \mathrm{HNO}_{3} \) is a stronger acid than \( \mathrm{HNO}_{2} \).

Key Concepts

Oxyacids

Oxyacids are acids that contain oxygen in addition to hydrogen and another element. These acids play a crucial role in acid strength determination due to the presence of oxygen, which affects their ability to dissociate in water. The strength of an oxyacid is generally dependent on the number of oxygen atoms it contains. More oxygen atoms usually mean a stronger acid. This is because additional oxygen atoms help stabilize the negative charge after the acid dissociates by resonance.
For instance, consider the comparison between perchloric acid ( HClO_4) and chloric acid (HClO_3). Perchloric acid is stronger due to its additional oxygen atom. This extra oxygen contributes to greater electron-withdrawing power, enhancing its ability to stabilize the negative charge through resonance.

• Additional oxygen atoms lead to higher acid strength.
• Resonance stabilization is enhanced with more oxygen atoms.

Binary Acids

Binary acids are composed of hydrogen and one other element, typically a non-metal. Their acidity increases down a group in the periodic table as electronegativity decreases and the bond strength between hydrogen and the other element weakens.
For example, comparing hydrobromic acid (HBr) and hydroiodic acid (HI), HI is stronger because the HI bond is weaker, allowing it to dissociate more readily in water.

• Binary acids consist of hydrogen and one other element.
• Acid strength increases with weaker hydrogen bonds, seen lower down the periodic table.

Resonance Stabilization

Resonance stabilization plays a significant role in the strength of oxyacids. When an oxyacid dissociates, it releases a proton (H^+) and leaves behind a negatively charged conjugate base. The more stable the conjugate base, the stronger the acid.
Resonance allows the negative charge to be delocalized over several atoms, reducing the energy and increasing stability of the conjugate base.
Take nitric acid (HNO_3) as an example: it has resonance structures that allow for the effective distribution of charges, making it a stronger acid than nitrous acid (HNO_2), which has fewer resonance structures.

• Resonance increases stability by delocalizing charge.
• More resonance structures result in a stronger acid.

Electronegativity Effect

The electronegativity of the atoms involved in an acid's structure significantly affects its strength. In general, higher electronegativity means better ability to stabilize the negative charge through resonance or inductive effects, ultimately leading to a stronger acid.
Oxyacids with high electronegativity central atoms, such as H_2SO_3 and H_2SO_4, show significant stabilization effects. When comparing HF and HCl, despite fluorine's high electronegativity, HCl is a stronger acid because chlorine's larger atomic size results in weaker H-Cl bonding and thus easier dissociation.

• Higher electronegativity enhances acid strength through better charge stabilization.
• The atomic size can also play a role by affecting bond strength.