Low-temperature heat capacities and standard molar enthalpy of formation of the complex Zn(Val)SO4 center dot H2O(s) Val=L-alpha-valine)

摘要

Low-temperature heat capacities of a solid complex Zn(Val)SO4 center dot H2O(s) were measured by a precision automated adiabatic calorimeter over the temperature range between 78 and 373 K. The initial dehydration temperature of the coordination compound was determined to be, T (D)=327.05 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. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the complex relative to the standard reference temperature 298.15 K were given with the interval of 5 K. Enthalpies of dissolution of the [ZnSO4 center dot 7H(2)O(s)+Val(s)] (Delta H-sol(m,l)0) and the Zn(Val)SO4 center dot H2O(s) (Delta(sol) H (0)(m,2)) in 100.00 mL of 2 mol dm(-3) HCl(aq) at T=298.15 K were determined to be, Delta H-sol(m,l)0=(94.588 +/- 0.025) kJ mol(-1) and Delta H-sol(m,2)0=-(46.118 +/- 0.055) kJ 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-f(m)0 (Zn(Val)SO4 center dot H2O(s), 298.15 K)=-(1850.97 +/- 1.92) kJ 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 [ZnSO4 center dot 7H(2)O(s)+Val(s)] mixture in 2 mol dm(-3) hydrochloric acid) and solution A' (from dissolution of the complex Zn(Val)SO4 center dot H2O(s) in 2 mol dm(-3) hydrochloric acid).