Given: N2(g) + O2(g) → 2NO(g), ΔH = +180.6 kJ, and 2NO2(g) → 2NO(g) + O2(g), ΔH = +114.2 kJ. What is ΔH for N2(g) + 2O2(g) → 2NO2(g)?

Core Concept

The second equation must be reversed so NO2 lands on the product side, changing its ΔH to -114.2 kJ. Adding the reversed step to the first equation cancels 2NO and one O2, leaving exactly the target equation. Hess's law then gives ΔH = 180.6 - 114.2 = +66.4 kJ. The positive value agrees with the positive formation enthalpies of the nitrogen oxides.