Bulk gas-phase acidity

摘要

The capability of a gaseous Br?nsted acid HB to deliver protons to a base is usually described by the gas-phase acidity (GA) value of the acid. However, GA values are standard Gibbs energy differences and refer to individual gas pressures of 1 bar for acid HB, base B ~-, and proton H ~+. We show that the GA value is not suited to describe the bulk acidity of a gaseous acid. Here the pressure dependence of the activities of HB, H(HB) _n ~+, and B(HB) _m ~- that result from gaseous autoprotolysis have to be considered. In this work, the pressure-dependent absolute chemical potential of the proton in the representative gaseous proton acids CH _4, NH _3, H _2O, HF, and HCl was worked out and the general theory to describe bulk gas phase acidity-that can directly be compared with solution acidity-was developed. Br?nsted "gacids": Br?nsted acidity in the gas phase has been systematically extended from the established gas-phase acidity values to the pressure-dependent bulk acidity. Analogies to protolysis in solution are illustrated on the basis of a unified pH _(abs) scale. For most gaseous acids, the pressure-dependent speciation of ions in the gas phase must be considered (see figure) to obtain meaningful thermodynamic acidities of bulk gases.