Hydration of the potassium ion in the gas phase: enthalpies and entropies of hydration reactions

Abstract

The thermodynamic constants ΔH0n−1,n, ΔG0n−1,n, and ΔS0n−1,n for the reaction [n − 1, n]: K+(H2O)n−1 + H2O = K+(H2O)n proceeding in the gas phase were measured for n = 1 to n = 6. The equilibria were studied in a reaction chamber containing water vapor at pressures from 0.3 to 6 Torr and at temperatures from −11 to 390 °C. The potassium ions were produced by thermionic emission in a filament chamber and then were equilibrated with the water vapor in the reaction chamber. The relative ionic concentrations were detected with a mass spectrometer. The experimental ΔH0n−1,n values are compared with potential energy differences for the reactions [n − 1, n] based on simple electrostatic model calculations. The electrostatic calculations produce values in fair agreement with the experimental results. The ΔS00,1 calculated on basis of statistical mechanics and energies obtained with the electrostatic model is in very good agreement with the experimental result.