Low-temperature heat-capacity and standard molar enthalpy of formation of the coordination compound Zn(Phe) 3(NO 3) 2·H 2O(s) [Phe= l-α-phenylalanine

Abstract

Low-temperature heat capacities of the coordination compound Zn(Phe) 3(NO 3) 2·H 2O(s) have been precisely measured over the temperature range between T=78 K and T=373 K through a small sample precision automated adiabatic calorimeter. Experimental heat capacities have been fitted into a polynomial equation of heat capacities ( C p,m) with the reduced temperatures ( x), [ x= f( T)]. The initial dehydration temperature of the complex ( T d) has been given from analysis of the heat-capacity curve, T d =327.25 K . Dissolution enthalpies of [ ZnSO 4·7 H 2 O( s)+2 NaNO 3( s)+3 L-Phe( s)] ( Δ d H m,1 0) and [ Zn( Phe) 3( NO 3) 2· H 2 O( s)+ Na 2 SO 4( s)] ( Δ d H m,2 0) in 100 mL of 2 molar HCl at T=298.15 K have been measured by means of an isoperibol solution calorimeter: Δ d H m,1 0=(115.430±2.797) kJ· mol −1 , Δ d H m,2 0=(67.211±2.147) kJ· mol −1 . The standard molar enthalpy of formation of the complex has been determined to be: Δ f H m 0=−(2292.44±11.12) kJ· mol −1 , through a Hess thermochemical cycle from the results of molar dissolution enthalpies and other auxiliary thermodynamic data.