Low temperature heat capacities and standard molar enthalpy of formation of the coordination compound Zn(His)(NO₃)₂.1/2H₂O(s) (His = L-⍺- Histidine)

Abstract

Low temperature heat capacities of the solid coordination compound Zn(His)(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, <i style="">T</i>_D= 326.75 K, by analysis of the heat-capacity curve. The experimental values of the molar heat capacities in the temperature region have been fitted to a polynomial equation of heat capacities with the reduced temperature (<i style="">X</i>), [<i style="">X</i> = <i style="">f </i>(<i style="">T</i>)], by least squares method. Enthalpies of dissolution of {ZnSO₄.7H₂O(s) + 2NaNO₃(s) + L-His(s)} and {Zn(His)(NO₃)₂.1/2H₂O(s) + Na₂SO₄(s)} in 100 ml of 2 mol dm^-3 HCl(aq) at <i style="">T</i> = 298.15 K have been determined to be <i style=""></i>Δ_d<i>H</i>⁰_m,1= 79.830 ± 0.058 kJ mol^-1 and <i style=""></i>Δ_d<i>H</i>⁰_m,2= 51.146 ± 0.041 kJmol^-1 respectively, with a isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound is determined as <b style=""></b>Δ_f<i>H</i>⁰_m(Zn(His)(NO₃)₂.1/2H₂O, s, 298.15 K) = - 1207.19 ± 2.82 kJ mol^-1 from the enthalpies of dissolution of the reactants and products and other thermodynamic data by a Hess thermochemical cycle. Furthermore, the reliability of the Hess thermochemical cycle has been verified by comparing UV/vis spectra and the refractive indices of solutions obtained from dissolution of the {ZnSO₄.7H₂O(_S) + 2NaNO₃(s) + L-His(s)} and from dissolution of the {Zn(His)(NO₃)₂.1/2H₂O(s) + Na₂SO₄(s)} mixtures.