Broensted Acids and Bases: They are not Substances but Molecules or Ions!

Abstract

For chemistry education we are discussing mainly two concepts of acids and bases: theories of Arrhenius and Broensted. For the first theory, dissociation into ions is generally discussed: acidic solutions contain H⁺(aq) ions, alkaline solutions contain OH^-(aq) ions. This theory therefore deals with substances, which are referred toas acids and bases – it would be even better to take the logical names "acidic and alkaline solutions". If both solutions are mixed in equivalent quantities, the H⁺(aq) ions react with OH^-(aq) ions to form H₂O molecules, while the other ions remain in solution (in exceptional cases an insoluble salt may precipitate: sulfuric acid solution reacts with barium chloride solution to solid white barium sulfate and water). The Broensted theory defines proton transfer: a molecule or an ion transfers a proton to another molecule or ion, two conjugated acid-base pairs are involved. Thus, Broensted acids and bases are no more substances, but individual types of particles. Due to the autoionization of H₂O molecules (not "autoionization of water"), the following equilibrium exists:

H₂O + H₂O ⇌ H₃O⁺(aq) + OH^-(aq)

Through this protolysis it is more advantageous to argue rather with H₃O⁺(aq) ions than with H⁺(aq) ions. In this theory there are still ampholyte particles which react as acid or as base particles depending on the reaction partners: H₂O molecules, NH₃molecules , HSO₄^-ions – water, ammonia or sodium hydrogen sulfate cannot be regarded asampholytes. The original publication of Broensted [1] from 1927 makes clear that acids and bases should be molecules or ions, but not substances.