The equilibrium constant for N2 O5 ⇄NO2 +NO3 [reaction (1)], has been determined through direct concentration measurements of N2 O5, NO2, and NO3 in a temperature controlled long path cell, using absorption spectroscopy in the infrared and visible regions from 243 to 397 K. The results give K1C =1.30×1026  exp(−21 490/RT) molecules cm3 with an estimated overall uncertainty of 30% at the 95% confidence level. This results in an equilibrium constant near room temperature approximately one‐half that of previously accepted values. Using the temperature dependence of the equilibrium constant and new data for the enthalpy of formation of gaseous N2 O5 yields an enthalpy of formation for NO3 of 15.39±0.72 kcal mol1. In additional experiments the temperature dependent first order decay rates of N2 O5 were determined from which a value for the product of the equilibrium constant (K1C ) and the rate constant for reaction (2): NO2 +NO3 →NO+NO2 +O2 was obtained for temperatures from 298 to 396 K: K1k2 =1.23×1013  exp(−24 300/RT) s.1 When this is combined with the equilibrium constant of the present study the following expression is obtained for k2 : 9.5×10−14  exp(−2810/RT) cm3  molecule1 s.1

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