Reaction Exothermicity from Enthalpies of Formation

Steps:

• Apply ΔH_rxn = Σ(nΔH_f products) - Σ(mΔH_f reactants) to assess energy change.
• Identify reactants: CH₃ (positive ΔH_f as radical) and NO₂ (positive ΔH_f ≈ +34 kJ/mol).
• Identify products: 3N₂ (ΔH_f = 0), 4CO₂ (ΔH_f = -394 kJ/mol each), 6H₂O (ΔH_f = -286 kJ/mol each, assuming liquid).
• Sum shows large negative contribution from CO₂ and H₂O outweighs positive reactants, yielding ΔH_rxn < 0.

Why C is correct:

• CO₂ and H₂O as products have negative ΔH_f values; per Hess's law, this dominates in ΔH_rxn formula, making overall enthalpy change negative (exothermic).

Why the others are wrong:

• A: Incorrectly labels reaction endothermic; chemical-to-heat conversion indicates exothermic release.
• B: Wrongly states endothermic; high N-O bond energy suggests breaking it absorbs energy, but reaction is exothermic.
• D: Flawed reason; NO₂ has N=O bonds (resonance), but breaking them is endothermic, not explaining exothermicity.

Final answer: C