Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of the Solid-State Coordination Compound trans-Cu(Gly)2·H2O(S) (Gly = l-α-Glycine)

Abstract

Low-temperature heat capacities of the solid coordination compound trans-Cu(Gly)2·H2O(s) were measured by a precision automated adiabatic calorimeter over the temperature range from T = 78 K to T = 381 K. The initial dehydration temperature of the coordination compound was determined to be TD = 329.50 K, by analysis of the heat-capacity curve. The experimental values of the molar heat capacities in the temperature region of 78−328 K were fitted to a polynomial equation of heat capacities (Cp,m) with the reduced temperatures (X), [X = f(T)], by a least-squares method. The smoothed molar heat capacities and thermodynamic functions of the complex trans-Cu(Gly)2·H2O(s) were calculated based on the fitted polynomial. The smoothed values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were tabulated with an interval of 5 K. Enthalpies of dissolution of {Cu(Ac)2·H2O(s) + 2Gly(s)} [ΔsHmϑ (1)] and HAc(l) [ΔsHmϑ (2)] in 100.00 mL of 2 mol·dm-3 HCl and trans-Cu(Gly)2·H2O(s) [ΔsHmϑ (3)] in the solvent [HAc(l) + 2 mol·dm-3 HCl] at T = 298.15 K were determined to be ΔSHmϑ (1) = (13.59 ± 0.03) kJ·mol-1, ΔSHmϑ (2) = −(0.93 ± 0.03) kJ·mol-1, and ΔSHmϑ (3) = −(14.64 ± 0.02) kJ·mol-1 by means of an isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as: ΔfHm0(trans-Cu(Gly)2·H2O, s, 298.15 K) = −(1251.6 ± 1.0) kJ·mol-1, from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle.