Potentiometric and Calorimetric Studies on the Formation of Ethylenediamine Complexes of Copper(II) Ion in Water and Aqueous Dioxane Solutions

Abstract

Abstract Gibbs energies (or formation constants), enthalpies and entropies of formation of ethylenediamine complexes of copper(II) ions have been determined by means of potentiometric and calorimetric titrations at 25 °C in an aqueous solution and dioxane–water mixtures (dioxane contents are 0.1 and 0.2 mole fractions, which correspond to 35.2 and 55.0% w/w, respectively), all containing 3 mol dm−3 LiClO4 as a constant ionic medium. The thermodynamic quantities of the reactions CuLn−12++L=CuLn2+ (L=ethylenediamine and n=1 and 2) determined in the solutions were as follows: ΔG1°=−65.0 kJ mol−1 (dioxane content: 0.0 mole fraction), −66.8 kJ mol−1 (0.1 mole fraction), and −68.8 kJ mol−1 (0.2 mole fraction); ΔG2°=−56.9 kJ mol−1 (0.0), −58.8 kJ mol−1 (0.1), and −61.1 kJ mol−1 (0.2); ΔH1°=−67.7 kJ mol−1 (0.0), −68.6 kJ mol−1 (0.1), and −66.7 kJ mol−1 (0.2); ΔH2°=−71.0 kJ mol−1 (0.0), −70.2 kJ mol−1 (0.1), and −68.8 kJ mol−1 (0.2); TΔS1°=−2.7 kJ mol−1 (0.0), −1.8 kJ mol−1(0.1), and 2.1 kJ mol−1 (0.2); TΔS2°=−14.1 kJ mol−1 (0.0), −11.4 kJ mol−1(0.1), and −7.7 kJ mol−1 (0.2). In any solvent examined the enthalpies contribute predominantly to the Gibbs energies. On the other hand, for a given complex with varying solvent compositions, the entropies mainly contribute to the change in the Gibbs energies of the complex formation reactions. With an increase in the dioxane content of the solvent, the Gibbs energies of the complex formation reactions become more negative, while the enthalpies of the reactions remain practically unchanged or even become less negative. Consequently, the entropies of the reactions increase with increasing dioxane content of the solvent, which is explained in terms of weakening of the hydrogen-bonded structure of water in the bulk solvent by the addition of dioxane.