Low-temperature heat capacities and standard molar enthalpy of formation of the solid-state coordination compound trans-Cu(Ala) 2(s) (Ala = l-α-alanine)

Abstract

Low-temperature heat capacities of the solid coordination compound trans-Cu(Ala) 2(s) have been measured by a precision automated adiabatic calorimeter over the temperature range from T = 78 K to 390 K. The experimental values of the molar heat capacities in the temperature region were fitted to a polynomial equation of heat capacities ( C p,m ) with the reduced temperatures ( X), [ X = f ( T)], by a least square method. The smoothed molar heat capacities and thermodynamic functions of the complex trans-Cu(Ala) 2(s) were calculated based on the fitted polynomial. The smoothed values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were tabulated with an interval of 5 K. Enthalpies of dissolution of {Cu(Ac) 2·H 2O(s) + 2Ala (s)} and 2:1 HAc (aq) in 100 ml of 2 mol dm −3 HCl, respectively, and trans-Cu(Ala) 2(s) in the solvent [2:1 HAc (aq) + 2 mol dm −3 HCl] at T = 298.15 K were determined to be Δ S H m ° [ Cu ( Ac ) 2 ⋅ H 2 O ( s ) + 2 Ala ( s ) ] = ( 6.75 ± 0.04 ) kJ mo l − 1 , Δ S H m ° [ 2 : 1 HAc ( aq ) ] = − ( 1.63 ± 0.01 ) kJ mo l − 1 , and Δ S H m ° [ t r a n s -Cu ( Ala ) 2 ( s ) ] = − ( 10.24 ± 0.08 ) kJ mo l − 1 by means of an isoperibol solution-reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as Δ f H m ° ( t r a n s -Cu ( Ala ) 2 ( s ) , 298.15 K ) = − ( 1038.6 ± 3.5 ) kJ mo l − 1 from the enthalpies of dissolution and other auxiliary thermodynamic data using a Hess thermochemical cycle.