Low-temperature heat capacities and standard molar enthalpy of formation of the coordination compound Zn(His)SO 4·H 2O(s) ( [formula omitted

Abstract

Low-temperature heat capacities of the solid coordination compound Zn(His)SO 4·H 2O(s) were precisely measured by a precision automated adiabatic calorimeter over the temperature range between T=78 and 373 K. The initial dehydration temperature of the coordination compound was determined to be, T d=324.20 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,m) with the reduced temperatures, x ( x= f( T)), by least square method. Enthalpies of dissolution of the ZnSO 4·7H 2O(s)+His(s) (Δ d H° m,1) and the Zn(His)SO 4·H 2O(s) (Δ d H° m,2) in 100.00 ml of 2 mol l −3 HCl(aq) at T=298.15 K were determined to be, Δ d H° m,1=−(13.129±0.013) kJ mol −1 and Δ d H° m,2=−(11.271±0.012) kJ mol −1, by means of a homemade isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as: Δ f H° m ( Zn( His) SO 4· H 2 O, s, 298.15 K)=−(1827.61±2.31) kJ mol −1 from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle by using the homemade isoperibol calorimeter. Furthermore, the reliability of the designed Hess thermochemical cycle was verified by comparing between UV/Vis spectra and the refractive indexes of Solution A (from dissolution of the [ZnSO 4·7H 2O(s)+His(s)] mixture in 2 mol l −3 hydrochloric acid) and Solution A′ (from dissolution of the complex Zn(His)SO 4·H 2O(s) in 2 mol l −3 hydrochloric acid).