7.2B: The Self Ionization of Water

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Solvated H⁺ ions

The hydron (a completely free or "naked" hydrogen atomic nucleus) is far too reactive to exist in isolation and readily hydrates in aqueous solution. The simplest hydrated form of the hydrogen cation, the hydronium (hydroxonium) ion H₃O⁺ (aq), is a key object of Arrhenius' definition of acid. Other "simple" hydrated forms include the Zundel cation H₅O₂⁺ which is formed from a hydron and two water molecules, and the Eigen cation H₉O₄⁺, formed from a hydronium ion and three water molecules. The hydron itself is crucial in the more general Brønsted-Lowry acid-base theory, which extends the concept of acid-base chemistry beyond aqueous solutions. Both of these complexes represent ideal structures in a more general hydrogen bonded network defect. A freezing-point depression study determined that the mean hydration ion in cold water is on average approximately H₃O⁺(H₂O)₆: where each hydronium ion is solvated by 6 water molecules. Some hydration structures are quite large: the H₃O⁺.20H₂O magic ion number structure (called magic because of its increased stability with respect to hydration structures involving a comparable number of water molecules).

Structure from Lancaster site

In 1806 Theodor Grotthuss proposed a theory of water conductivity. He envisioned the electrolytic reaction as a sort of "bucket line" where each oxygen atom simultaneously passes and receives a single hydrogen atom. It was an astonishing theory to propose at the time, since the water molecule was thought to be OH not H₂O and the existence of ions was not fully understood. The theory became known as the Grotthuss mechanism. The transport mechanism is now thought to involve the inter-conversion between the Eigen and Zundel solvation structures, Eigen to Zundel to Eigen (E-Z-E).