Thermodynamic properties of metal amides determined by ammonia pressure-composition isotherms

Thermodynamic properties of Mg(NH2)2 and LiNH2 were investigated by measurements of NH3 pressure-composition isotherms (PCI). Van’t Hoff plot of plateau pressures of PCI for decomposition of Mg(NH2)2 indicated the standard enthalpy and entropy change of the reactions were ΔH° = (120 ± 11) kJ · mol−1 (per unit amount of NH3) and ΔS° = (182 ± 19) J · mol−1 · K−1 for the reaction: Mg(NH2)2 → MgNH + NH3, and ΔH° = 112 kJ · mol−1 and ΔSo = 157 J · mol−1 · K−1 for the reaction: MgNH → (1/3)Mg3N2 + (1/3)NH3. PCI measurements for formation of LiNH2 were carried out, and temperature dependence of plateau pressures indicated ΔH° = (−108 ± 15) kJ · mol−1 and ΔS° = (−143 ± 25) J · mol−1 · K−1 for the reaction: Li2NH + NH3 → 2LiNH2.