Using Hess's Law to Calculate an Unknown heat ^ H Nitrogen oxides undergo many interesting reactions in the environment and in industry. Given the following information, calculate H for the overall equation: 2 NO2(g) + 1/2 O2(g) N2O5(s). 4 NO(g) + 3 O2(g) 2 N2O5(s), H = -447.4 kJ 2 NO(g) + O2(g) 2 NO2(g), H = -114.2 kJ heat