A. Weak and Strong Acids
Acids differ enormously in the extent to which they dissociate into ions in aqueous solution. Some acids, such as hydrochloric and nitric acids, are strong electrolytes, completely dissociated into ions; these acids are known as strong acids. Others, such as acetic and nitrous acids, are only partially dissociated in solution. These acids are weak electrolytes and are known as weak acids. The solution of a strong acid contains no acid molecules; the solution of a weak acid contains both molecules and ions (see Figure 12.1).

FIGURE 12.1 Strong and weak acids in solution: (a) the solution of a strong acid contains only ions; (b) the solution of a weak acid contains both molecules and ions.

The ions of a weak acid tend to recombine to re-form molecules of the acid. We can show this recombination in the ionization equation with double arrows, which mean that the reaction is reversible and goes both ways. Some molecules are dissociating into ions; some ions are recombining to form molecules. Acetic acid is a weak acid; we show its ionization with double arrows to indicate that a solution of acetic acid contains both molecules and ions. When using double arrows, it is customary to show a longer or heavier arrow in the direction of the predominant reaction. Thus, in the ionization of a weak acid, in which there are fewer ions than molecules, the longer arrow points toward the molecules. (The topic of weak acids is discussed again in Section 12.6B.)

HC₂H₃O₂ H⁺ + C₂H₃O₂^-

Because the ions of a strong acid do not recombine, we show its ionization with a single arrow, meaning that only the ions of the acid are present in its aqueous solution. For the strong acid nitric acid, the ionization would be shown as:

HNO₃ H⁺ + NO₃^-

In the Brønsted-Lowry system, a strong acid is one that easily donates a proton. The conjugate base of a strong Brønsted-Lowry acid is a weak base, one that has little affinity for a proton. Thus, for hydrochloric acid,

A weak acid does not readily donate a proton. Its conjugate base (or anion) is a relatively strong base, one with considerable attraction for a proton. Thus, for acetic acid,

B. Polyprotic Acids
Molecules of some acids - such as sulfuric acid, H₂SO₄, and phosphoric acid, H₃PO₄ - have more than one ionizable hydrogen; these acids are called polyprotic acids. The ionization of these acids occurs in steps, with the molecule losing one proton at a time. The ionization of sulfuric acid, a diprotic acid, occurs as follows:

H₂SO₄ H⁺ + HSO₄^-

HSO₄^- H⁺ + SO₄^2-

Notice that, although the first ionization is that of a strong acid, the second is that of a weak acid.

The ionization of a phosphoric acid, a triprotic acid, occurs as follows:

H₃PO₄ H⁺ + H₂PO₄^-

H₂PO₄^- H⁺ + HPO₄^2-

HPO₄^-2 H⁺ + PO₄^3-

For sulfuric acid, and indeed for all polyprotic acids, the first ionization is much more complete than the subsequent ionizations.