Thermochemistry of Gaseous Ammonium Nitrate, NH4NO3(g)

Abstract

From the measured molecular weight of vapor over NH₄NO₃(c) by the simultaneous torsion-effusion/mass loss method, the partial pressure of NH₄NO₃(g) was evaluated over the range 321−360 K; in addition, the presence of the gaseous nitrate was verified by effusion-beam mass spectrometry. Third-law calculations of the sublimation enthalpy of NH₄NO₃ were made with estimated molecular constants, yielding ΔH₂₉₈^o(sub) = 20.9 ± 2 kcal mol⁻¹, as compared to a less-reliable second-law value of 23.2 ± 3 kcal mol⁻¹. We prefer the third-law value, which leads to the enthalpy of formation Δ_fH₂₉₈^o (NH₄NO₃(g)) = −66.5 ± 2.1 kcal mol⁻¹ when combined with the known Δ_fH₂₉₈^o value of NH₄NO₃(c).