Strong and Weak Electrolytes; Degree of Ionization

Substances whose aqueous solutions conduct electricity are called electrolytes, and they do so

because they exist, at least partly, as freely mobile ions in solution. The extent to which a given

electrolyte actually breaks up into ions distinguishes two broad classes.

A strong electrolyte ionizes essentially completely in dilute aqueous solution — for practical

purposes, 100% of the dissolved formula units exist as separated ions, with no meaningful equilibrium

between the un-ionized and ionized forms. Common strong acids ($\text{HCl}$, $\text{HNO}_3$,

$\text{H}_2\text{SO}_4$), strong bases ($\text{NaOH}$, $\text{KOH}$) and most soluble ionic salts

(such as $\text{NaCl}$) fall into this category. Because ionization is essentially total, a strong

electrolyte's own "ionization equilibrium" is not treated as a genuine chemical equilibrium at all —

writing $\text{HCl} \to \text{H}^+ + \text{Cl}^-$ with a single arrow, rather than

$\rightleftharpoons$, correctly signals that the reverse reaction is negligible.

A weak electrolyte, in contrast, ionizes only partially: at any instant, a genuine dynamic

equilibrium exists between the un-ionized molecules and the ions they have produced. Acetic acid,

$\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$, and aqueous

ammonia, $\text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-$, are classic weak

electrolytes: only a small fraction of the dissolved molecules are ionized at any given moment, and

this fraction is characteristic of the substance, its concentration, and the temperature.

This fraction is formalized as the degree of ionization, $\alpha$, defined as the ratio of the

number of moles of the electrolyte that have ionized to the total number of moles originally

dissolved:

$\alpha$ is a pure (unitless) fraction between 0 and 1, often expressed as a percentage. For a strong

electrolyte, $\alpha \approx 1$ (100%) at all reasonable dilutions. For a weak electrolyte, $\alpha$ …