Determination of Standard Molar Enthalpy of Formation for the Coordination Compound Zn(His)Cl₂·/₂H₂O(s) by an Isoperibol Solution−Reaction Calorimeter

Abstract

An isoperibol solution−reaction calorimeter suitable for measuring molar enthalpies of solid−liquid and liquid−liquid interactions was constructed. The volume of calorimetric vessel made by transparent hard glass is about 100 cm3. Its energy equivalent is 0.48 kJ·K-1 when containing 100 cm3 of double distilled water. The thermal leakage modulus is 2.7 × 10-4 s-1. The time constant of the calorimeter is about 2.9 s. The precision of controlling and measuring the temperature is within ±0.001 K and ±0.0001 K, respectively. The performance of the calorimeter was tested by measuring the enthalpy of dissolution of KCl in double distilled water, Δs = (17 547 ± 13 J·mol-1) at T = (298.15 ± 0.001) K, and the measuring uncertainty was estimated to be within ±0.30% by comparing our measured value with the reference. Molar enthalpies of dissolution of the {ZnCl2(s) + l-His(s)} mixture and the complex Zn(His)Cl2·1/2H2O(s) in 100 cm3 of 2 mol·L-1 HCl at T = (298.15 ± 0.001) K have been measured to be Δd = (26.419 ± 0.019) kJ·mol-1 and Δd = (11.862 ± 0.009) kJ·mol-1, respectively, by means of the isoperibol solution−reaction calorimeter. The standard molar enthalpy of formation of the complex Zn(His)Cl2·1/2H2O(s) has been derived to be Δf [Zn(His)Cl2·1/2H2O, s, 298.15 K] = − (1010.1 ± 2.3) kJ·mol-1 by the combination of the experimental values of enthalpies of dissolution with some auxiliary thermodynamic data through a designed Hess thermochemical cycle on the basis of a supposed chemical reaction.