Low-Temperature Heat Capacities and Standard Molar Enthalpy of Formation of the Coordination Compound Zn(Leu)SO4·1/2H2O(s) (Leu = l-α-Leucine)

Abstract

Low-temperature heat capacities of the solid coordination compound Zn(Leu)SO(4)center dot 1/2H(2)O(s) were measured by a precision automated adiabatic calorimeter over the temperature range between T = 78 K and T = 371 K. The initial dehydration temperature of the coordination compound was determined to be T-D = 326.50 K by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation of heat capacities (C-p,C-m) with the reduced temperatures (x), [x = f (T)] by least-squares method. Enthalpies of dissolution of the [ZnSO(4)center dot 7H(2)O(s) + Leu(s)] ( Delta(sol)H degrees(m,1)) and the Zn(Leu)SO(4)center dot 1/2H(2)O(s) (Delta(sol)H degrees(m,2)) in 100.00 mL of 2 mol center dot dm(-3) HCl(aq) at T = 298.15 K were determined to be Delta solH degrees(m,1) = -(85.80 +/- 0.09) kJ center dot mol(-1) and Delta solH(m,2) = (55.97 +/- 0.06) kJ center dot mol(-1) by means of a homemade isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as Delta H degrees(m)(Zn(Leu)SO(4)center dot 1/2H(2)O, s, 298.15 K) = -(1999.0 +/- 0.9) kJ center dot mol(-1) from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle. Furthermore, the reliability of the Hess thermochemical cycle was verified by comparing UV/Vis spectra and the refractive indexes of solution A (from dissolution of the [ZnSO(4)center dot 7H(2)O(s) + Leu(s)] mixture in 2 mol center dot dm(-3) hydrochloric acid) and solution A' (from dissolution of the complex Zn(Leu)SO(4)center dot 1/2H(2)O(S) in 2 mol center dot dm(-3) hydrochloric acid).