Thermodynamics of the coordination compound Zn(Met)(NO3)2·1/2H2O(s) (Met = l-α-methionine)

Abstract

Low temperature heat capacities of the solid coordination compound, Zn(Met)(NO3)2·1/2H2O(s), have been measured by a precision automated adiabatic calorimeter over the temperature range 78–371 K. The initial dehydration temperature of the coordination compound is determined to be, TD = 325.05 K, by analysis of the heat-capacity curve. The experimental values of molar heat capacities in the temperature region have been fitted to a polynomial equation of heat capacities with the reduced temperature (X), [X = f(T)], by a least squares method. Smoothed heat capacities and thermodynamic functions relative to the standard reference temperature 298.15 K of the compound are derived from the fitted polynomial equation and listed at 5 K internals. Enthalpies of dissolution of {ZnSO4·7H2O(s) + 2NaNO3(s) + l-Met (s)} and {Zn(Met)(NO3)2·1/2H2O(s) + Na2SO4(s)} in 100 cm3 of 2 mol dm−3 HCl(aq) at T = 298.15 K have been determined to be \( \Updelta_{\rm{d}} H_{m,1}^{0} \) = (56.929 ± 0.051) kJ mol−1 and \( \Updelta_{\rm{d}} H_{m,2}^{0} \) = (37.337 ± 0.029) kJ mol−1, respectively, with an isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound is determined to be \( \Updelta_{\rm{f}} H_{\rm{m}}^{0} \) [Zn(Met)(NO3)2·1/2H2O, s] = −(1327.08 ± 0.75) kJ mol−1 from the enthalpies of dissolution of the reactants and products and other thermodynamic data by a Hess’ thermochemical cycle.