Thermal stability trends in hydrogen halides

Steps:

• Bond dissociation energy decreases down group 17: H-F > H-Cl > H-Br > H-I.
• Weaker H-I bond leads to lower thermal stability for HI compared to HBr.
• Evaluate options: A reverses reducing strength; B misstates volatility cause; C matches bond trend; D confuses reactivity factors.
• Confirm C aligns with bond energy data showing H-Br (366 kJ/mol) > H-I (298 kJ/mol).

Why C is correct:

• Thermal stability of H-X increases with H-X bond strength, per bond dissociation energy formula, making HBr more stable than HI.

Why the others are wrong:

• A: Iodide ion is stronger reducing agent than chloride due to easier I- to I2 oxidation.
• B: Chlorine is more volatile than bromine from weaker London dispersion forces, not stronger.
• D: Iodine's lower reactivity stems from weaker I2 oxidizing power, not just covalent bond weakness.

Final answer: C