Thermochemistry of ammonium and rubidium perrhenates, and the effect of hydrogen bonding on the solubilities of ammonium salts

Abstract

The standard enthalpies of formation of ammonium and rubidium perrhenate have been determined, and the difference between them, 132.8 ± 1.5 kJ mol^–1 is, as previously predicted, consistent with the observed barrier to ammonium ion rotation of 9–10 kJ mol^–1. New thermodynamic properties are proposed for the perrhenate ion, including S_m^⊖(ReO₄^–, aq)= 198.1 ± 3 J K^–1 mol^–1, and derived values consistent with these are Δ_fH_m^⊖(NH₄ReO₄, s)=–975 kJ mol^–1, Δ_fH_m^⊖(RbReO₄, s)=–1 108 kJ mol^–1, and S_m^⊖(RbReO₄, s)= 182 J K^–1 mol^–1. As with several other anions which provide relatively little resistance to ammonium ion rotation, the solubility of the ammonium salt exceeds that of the rubidium compound. This may be attributable to hydrogen bonding of the ammonium ion in aqueous solution.