Thermochemistry on the coordination compounds of zinc sulphate with several l-α-amino acids

Molar enthalpies of dissolution of {ZnSO4 · 7H2O(s) + l-AA(s)} (in which AA are valine, methionine, histidine and threonine, respectively) in 100 cm3 of 2 mol · dm−3 HCl at T = (298.15 ± 0.001) K have been measured to be, ΔdHm,1∘=(94.588±0.025)kJ·mol-1, (−70.619 ± 0.051) kJ · mol−1, −(13.129 ± 0.013) kJ · mol−1, and (72.001 ± 1.602) kJ · mol−1, respectively, by means of an isoperibol solution-reaction calorimeter. Molar enthalpies of dissolution of Zn(AA)SO4 · H2O(s) in 100 cm3 of 2 mol · dm−3 HCl at T = (298.15 ± 0.001) K have been determined to be ΔdHm,2∘=-(46.118±0.055)kJ·mol-1, (58.415 ± 0.046) kJ · mol−1, −(11.271 ± 0.012) kJ · mol−1, and (6.9 ± 0.8) kJ · mol−1, respectively, by the calorimeter. The standard molar enthalpies of formation of the complexes Zn(AA)SO4 · H2O(s) have been derived to be, ΔfHm∘[Zn(Val)SO4·H2O,s,298.15K]=-(1850.97±1.92)kJ·mol-1, ΔfHm∘[Zn(Met)SO4·H2O,s,298.15K]=-(2069.304±0.743)kJ·mol-1, ΔfHm∘[Zn(His)SO4·H2O,s,298.15K]=-(1827.61±2.31)kJ·mol-1 and ΔfHm∘[Zn(Thr)SO4·H2O,s,298.15K]=-(2111.9±3.40)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.