Oxidation behavior of iodide ions

Steps:

• Recall halogen reactions with NaOH: Cl₂ with cold dilute NaOH forms NaCl + NaClO; cold concentrated forms NaClO₃ with hot conditions.
• Assess HX thermal stability: decreases down group (HCl > HBr) due to weakening H-X bond energies, not electronegativity differences.
• Examine H₂SO₄ with halides: conc. H₂SO₄ oxidizes I⁻ to I₂ (stronger reducing agent than Cl⁻/Br⁻), but dehydrates HCl/HBr.
• Check AgX solubility in NH₃: AgCl dissolves in dilute; AgBr needs concentrated; AgI insoluble.

Why C is correct:

• Concentrated H₂SO₄ acts as an oxidizing agent (E° > I₂/I⁻), oxidizing I⁻ to I₂ via 2I⁻ + H₂SO₄ → I₂ + SO₂ + ... (redox reaction).

Why the others are wrong:

• A: Cl₂ with cold concentrated NaOH forms NaCl + NaClO₃, not NaOCl (requires cold dilute NaOH).
• B: HCl stability > HBr due to stronger H-Cl bond energy; Cl is more electronegative than Br.
• D: AgI insoluble in dilute NH₃ (requires complex formation, ineffective for AgI).

Final answer: C