Hydrogen and Hydroxide Ions
If you refer to Table 1 from Ions in Solution (Electrolytes), you will see that pure water does conduct some electrical current, albeit much less than-even the weak electrolytes listed there. This is because water itself is a very weak electrolyte. It ionizes to hydrogen ions and hydroxide ions to an extremely small extent:
H2O(l) H+(aq) + OH–(aq) (1a)
Careful measurements show that at 25°C the concentrations of H+(aq) and OH–(aq) are each 1.005 × 107 mol dm–3. At higher temperatures more H+(aq) and OH–(aq) are produced while at lower temperatures less ionizationA process in which an atom, molecule, or negative ion loses an electron; a process in which a covalent molecule reacts with a solvent to form positive and negative ions; for example, a weak acid reacting with water to form its conjugate base (an anion) and a hydrogen (hydronium) ion. of water occurs. Nevertheless, in pure water the concentration of H+(aq) always equals the concentration of OH–(aq). Dissolving acids or bases in water can change the concentrations of both H+(aq) and OH–(aq), causing them to differ from one another. The special case of a solution in which these two concentrations remain equal is called a neutral solution.
A hydrogen ion, H+, is a hydrogen atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. which has lost its single electron; that is, a hydrogen ion is just a protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom.. Because a proton is only about one ten-thousandth as big as an average atom or ion, water dipoles can approach very close to a hydrogen ion in solution. Consequently the proton can exert a very strong attractive force on a lone pairIn a covalently bonded molecule or ion, a pair of electrons not shared between two atoms and hence not involved in a bond. of electrons in a water molecule—strong enough to form a coordinate covalent bondA bond between two atoms in which the shared electrons are considered to be contributed by only one of the atoms.:
The H3O+ formed in this way is called a hydronium ion. All three of its O―H bonds are exactly the same, and the ion has a pyramidal structure as predicted by VSEPR theory (1a). To emphasize the fact that a proton cannot exist by itself in aqueous solution, Eq. (1a) is often rewritten as
2H2O(l) H3O+(aq) + OH–(aq) (1b)
Like other ions in aqueous solution, both hydronium and hydroxide ions are hydrated. Moreover, hydrogen bonds are involved in attracting water molecules to hydronium and hydroxide ions.
In both cases three water molecules appear to be rather tightly held, giving formulas H3O(H2O)3+ (or H9O4+) and HO(H2O)3– (or H7O4–). Possible structures for the hydrated hydronium and hydroxide ions are shown in Fig. 1.
Hydrogen bonding of hydronium and hydroxide ions to water molecules accounts rather nicely for the unusually large electrical currents observed for some electrolytes containing H and OH. The case of the hydronium ion is illustrated in Fig. 2. When a hydronium ion collides with one end of a hydrogen-bonded chain of water molecules, a different hydronium ion can be released at the other end. Only a slight movement of six protons and a rearrangement of covalent and hydrogen bonds is needed. In effect a hydronium ion can almost instantaneously “jump” the length of several water molecules. It need not elbow its way through a crowd as other ions must. The same is true of aqueous hydroxide ions. Since both ions move faster, they can transfer more electrical charge per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. time, that is, more current.