Hydronium ion

In chemistry, hydronium is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). In chemistry, hydronium is the common name for the aqueous cation H3O+, the type of oxonium ion produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O). The ratio of hydronium ions to hydroxide ions determines a solution's pH. The molecules in pure water auto-dissociate (i.e.: react with each other) into hydronium and hydroxide ions in the following equilibrium: In pure water, there is an equal number of hydroxide and hydronium ions, so it is a neutral solution. At 25 °C (77 °F), water has a pH of 7 (this varies when the temperature changes: see self-ionization of water). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution. According to IUPAC nomenclature of organic chemistry, the hydronium ion should be referred to as oxonium. Hydroxonium may also be used unambiguously to identify it. A draft IUPAC proposal also recommends the use of oxonium and oxidanium in organic and inorganic chemistry contexts, respectively. An oxonium ion is any ion with a trivalent oxygen cation. For example, a protonated hydroxyl group is an oxonium ion, but not a hydronium ion. Since O+ and N have the same number of electrons, H3O+ is isoelectronic with ammonia. As shown in the images above, H3O+ has a trigonal pyramidal molecular geometry with the oxygen atom at its apex. The H–O–H bond angle is approximately 113°, and the center of mass is very close to the oxygen atom. Because the base of the pyramid is made up of three identical hydrogen atoms, the H3O+ molecule's symmetric top configuration is such that it belongs to the C3v point group. Because of this symmetry and the fact that it has a dipole moment, the rotational selection rules are ΔJ = ±1 and ΔK = 0. The transition dipole lies along the c-axis and, because the negative charge is localized near the oxygen atom, the dipole moment points to the apex, perpendicular to the base plane. Hydronium is the cation that forms from water in the presence of hydrogen ions. These hydrons do not exist in a free state - they are extremely reactive and are solvated by water. An acidic solute is generally the source of the hydrons, but hydronium ions exist even in pure water. This special case of water reacting with water to produce hydronium (and hydroxide) ions is commonly known as the self-ionization of water. The resulting hydronium ions are few and short-lived. pH is a measure of the relative activity of hydronium and hydroxide ions in aqueous solutions. In acidic solutions, hydronium is the more active, its excess proton being readily available for reaction with basic species. The hydronium ion is very acidic: at 25 °C, its pKa is 0. It is the most acidic species that can exist in water (assuming sufficient water for dissolution): any stronger acid will ionize and protonate a water molecule to form hydronium. The acidity of hydronium is the implicit standard used to judge the strength of an acid in water: strong acids must be better proton donors than hydronium, otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike hydronium in neutral solutions that result from water's autodissociation, hydronium ions in acidic solutions are long-lasting and concentrated, in proportion to the strength of the dissolved acid. pH was originally conceived to be a measure of the hydrogen ion concentration of aqueous solution. We now know that virtually all such free protons quickly react with water to form hydronium; acidity of an aqueous solution is therefore more accurately characterized by its hydronium concentration. In organic syntheses, such as acid catalyzed reactions, the hydronium ion (H3O+) can be used interchangeably with the H+ ion; choosing one over the other has no significant effect on the mechanism of reaction.