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

Abstract

The thermodynamic constants ΔH⁰_n−1,n, ΔG⁰_n−1,n, and ΔS⁰_n−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 ΔH⁰_n−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 ΔS⁰_0,1 calculated on basis of statistical mechanics and energies obtained with the electrostatic model is in very good agreement with the experimental result.