Vapor-Phase Equilibrium of NO, NO2, H2O, and HNO2

Abstract

The thermodynamic equilibrium constant for the vapor‐phase reaction NO+NO₂+H₂O⇌2HNO₂ has been determined at reduced pressures by evaluating the equilibrium distribution of known quantities of the reactant gases by measurements of light absorption by nitrogen dioxide. The value of the equilibrium constant at 25°C is K₁=2.38 atm^—1, which corresponds to ΔF°=—0.512 kcal/mole. The free energy of formation of HNO₂ vapor at 25°C and at isomeric equilibrium is ΔF_f°=—11.019 kcal/mole, based on the equilibrium measurements and the free energies of formation of NO, NO₂, and H₂O.