Die Elektronenaffinität der Halogene ermittelt aus elektrochemischen und kalorimetrischen Messungen The Electron Affinity of the Halogens Determined from Electrochemical and Calorimetrical Measurements

Using a conception given in [1, 3] for single ionic solvation and redox reactions the electron affinities (EA) of halogens are calculated from calorimetric data of their reactions with methyleneblue radical giving the values 8.81 eV; 8.23 eV; and 7.16 eV for Cl, Br and I, respectively. For iodine EA is also obtained from the reaction enthalpy of the reaction between iodide and Mg-porphyrine cation radical. EA calculated from electrochemical standard potentials are only approximately 0.2 eV higher than found from calorimetric data except for iodine, the standard potential of which could be influenced by I3-. The disagreement of the EA values given in the literature and those presented in this work is mainly caused to the author's opinion by a different interpretation of single ionic solvation and the size of ionic radii in solutions. From experimental data [1,3] solvation of anions is regarded as an energy consuming process which is understood as the overall repulsion between the anionic charge and the outer sphere electrons of solvent molecules. For calculation of the solvation energy according to the Born equation different radii have to be used: in the case of anions, the radius is composed of the radius of anions in the crystals extended by the average radius of a solvent molecule, in the case of cations the solvation of which is an energy supplying process the radius of atoms must be taken. On this basis the ionisation energy of tetramethyl-jo-phenylenediamine is calculated from calorimetric data for the reaction with bromine. As other examples the reaction enthalpies of the reactions of chlorine with methyleneblue radical, Li, Na, K, Cs, Be, Ca, La, Al and Zr are calculated in the gas phase as well as in water and compared with experimental ZlG-values obtained from electrochemical standard-potentials. Excellent agreement for all selected examples is found. The discussion includes electron absorption energies of anions and solvation of ionic crystals in a solvent.