The kinetics and thermodynamics of the reaction H+NH3⇄NH2+H2 by the flash photolysis–shock tube technique: Determination of the equilibrium constant, the rate constant for the back reaction, and the enthalpy of formation of the amidogen radical

Abstract

Equilibrium constants K₁ for the reaction H+NH₃⇄NH₂+H₂ were measured over the temperature range 900 to 1600 K using a flash photolysis–shock tube apparatus. The experimental values of K₁ ranged from 1.0 at 900 K to 2.2 at 1620 K with an estimated experimental error of about ±10%. The value obtained from the third law analysis for the enthalpy of formation of the amidogen radical, ΔH⁰_f298 (NH₂), is 45.3 kcal/mol (ΔH⁰_f0 =46.0). The corresponding value for the bond dissociation energy, D⁰(NH₂–H), is 107 kcal/mol. These values are in good agreement with the data tabulated in the revised JANAF tables (1982), with those derived from new measurements of the photoionization threshold for the amidogen radical, and with those from independent measurements of the rate constants of the forward and back reactions. The Arrhenius rate expression derived for reaction (−1), NH₂+H₂→NH₃+H, is k₋₁(T)=5.38×10⁻¹¹ exp(−6492/T) cm³ molecule⁻¹ s⁻¹ for the temperature range 900–1620 K. The estimated error in k₋₁(T) is about ±25%.