Ionization of Water

Water ionizes to hydrogen ions and hydroxyl ions. While all ions are hydrated, the hydrogen ion is tiny and so strongly hydrated that it is sometimes called the hydronium ion HO. As with other weakly ionized molecules, the equilibrium expression is written as:

and

The small amount of ionization has so little effect that the concentration of water is constant at roughly 55 M and can be incorporated in a new Ki = [ H+ ][ OH¯ ]. Effect of temperature on the ionization of water

Explanation of pH

When a substance is partially ionized, its ionization constant is defined as:

Each ion concentration is raised to the power equal to the number formed by ionization as in these examples:


Acetic acid:

HAc = H+ + Ac¯

Phosphoric acid: H3PO4 = 3 H+ + PO---

Ionization constants can be expressed as pK that is analogous to pH. The pK is minus the logarithm of the ionization constant. The salts and the strong acids and bases are considered as completely ionized. To perform equilibrium calculations, concentrations of H+ and species of the weak acid and its ions are designated by symbols such as x and y. After accounting for any reactions such as neutralization, the known concentrations and the unknowns are substituted into the equilibrium expressions, and there should be enough equations for simultaneous solution. Often you must make simplifying assumptions about which concentrations are negligible compared to others.

Return to Equilibrium index

Wed Mar 22 11:09:21 EST 1995