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Enthalpy of Reaction
[2ΔHf(NO2 (g))] - [2ΔHf(NO (g)) + 1ΔHf(O2 (g))]
[2(33.18)] - [2(90.25) + 1(0)] = -114.14 kJ
-114.14 kJ (exothermic)
[2ΔSf(NO2 (g))] - [2ΔSf(NO (g)) + 1ΔSf(O2 (g))]
[2(239.95)] - [2(210.65) + 1(205.03)] = -146.43 J/K
-146.43 J/K (decrease in entropy)
Free Energy of Reaction (at 298.15 K)
From ΔGf° values:
[2ΔGf(NO2 (g))] - [2ΔGf(NO (g)) + 1ΔGf(O2 (g))]
[2(51.3)] - [2(86.57) + 1(0)] = -70.54 kJ
-70.54 kJ (spontaneous)
From ΔG = ΔH - TΔS:
-70.48 kJ (spontaneous)
Equilibrium Constant, K (at 298.15 K)
This process is favorable at 25°C.
Ebbing, Darrell D. General Chemistry 3rd ed.; Houghton Mifflin Company: Boston, MA, 1990; pp 219, 302.