Strong and Weak Acids

Let us consider the strengths of acids first. A small number of acids ionize completely in aqueous solution. For example, when HCl dissolves in water, every molecule of HCl separates into a hydronium ion and a chloride ion:

$$\text{HCl }+{\text{ H}}_{\text{2}}\text{O}(\ell )\stackrel{~100\%}{\to }{\text{ H}}_{\text{3}}{\text{O}}^{+}\text{(aq) }+{\text{ Cl}}^{-}\text{(aq)}$$

HCl(aq) is one example of a strong acidAn acid that is 100% ionized in aqueous solution., which is a compound that is essentially 100% ionized in aqueous solution. There are very few strong acids. The important ones are listed in Table 10.2 "Strong Acids and Bases (All in Aqueous Solution)".

By analogy, a strong baseA base that is 100% ionized in aqueous solution. is a compound that is essentially 100% ionized in aqueous solution. As with acids, there are only a few strong bases, which are also listed in Table 10.2 "Strong Acids and Bases (All in Aqueous Solution)".

If an acid is not listed in Table 10.2 "Strong Acids and Bases (All in Aqueous Solution)", it is likely a weak acidAn acid that is less than 100% ionized in aqueous solution., which is a compound that is not 100% ionized in aqueous solution. Similarly, a weak baseA base that is less than 100% ionized in aqueous solution. is a compound that is not 100% ionized in aqueous solution. For example, acetic acid (HC₂H₃O₂) is a weak acid. The ionization reaction for acetic acid is as follows:

HC₂H₃O₂(aq) + H₂O(ℓ) → H₃O⁺(aq) + C₂H₃O₂⁻(aq)

Depending on the concentration of HC₂H₃O₂, the ionization reaction may occur only for 1%–5% of the acetic acid molecules.

Chemical Equilibrium

The behavior of weak acids and bases illustrates a key concept in chemistry. Does the chemical reaction describing the ionization of a weak acid or base just stop when the acid or base is done ionizing? Actually, no. Rather, the reverse process—the reformation of the molecular form of the acid or base—occurs, ultimately at the same rate as the ionization process. For example, the ionization of the weak acid HC₂H₃O₂ (aq) is as follows:

HC₂H₃O₂(aq) + H₂O(ℓ) → H₃O⁺(aq) + C₂H₃O₂⁻(aq)

The reverse process also begins to occur:

H₃O⁺(aq) + C₂H₃O₂⁻(aq) → HC₂H₃O₂(aq) + H₂O(ℓ)

Eventually, there is a balance between the two opposing processes, and no additional change occurs. The chemical reaction is better represented at this point with a double arrow:

HC₂H₃O₂(aq) + H₂O(ℓ) ⇆ H₃O⁺(aq) + C₂H₃O₂⁻(aq)

The ⇆ implies that both the forward and reverse reactions are occurring, and their effects cancel each other out. A process at this point is considered to be at chemical equilibrium (or equilibrium)The condition in which the extent of a chemical reaction does not change any further.. It is important to note that the processes do not stop. They balance out each other so that there is no further net change; that is, chemical equilibrium is a dynamic equilibrium.

Finally, you may realize that the autoionization of water is actually an equilibrium process, so it is more properly written with the double arrow:

H₂O(ℓ) + H₂O(ℓ) ⇆ H₃O⁺(aq) + OH⁻(aq)

The pH Scale

One qualitative measure of the strength of an acid or a base solution is the pH scaleA logarithmic scale that relates the concentration of the hydrogen ion in solution., which is based on the concentration of the hydronium (or hydrogen) ion in aqueous solution. A neutral (neither acidic nor basic) solution, one that has the same concentration of hydrogen and hydroxide ions, has a pH of 7. A pH below 7 means that a solution is acidic, with lower values of pH corresponding to increasingly acidic solutions. A pH greater than 7 indicates a basic solution, with higher values of pH corresponding to increasingly basic solutions. Thus, given the pH of several solutions, you can state which ones are acidic, which ones are basic, and which are more acidic or basic than others. Table 10.3 "The pH Values of Some Common Solutions" lists the pH of several common solutions. Notice that some biological fluids are nowhere near neutral.

Weak acids and bases are relatively common. You may notice from Table 10.3 "The pH Values of Some Common Solutions" that many food products are slightly acidic. They are acidic because they contain solutions of weak acids. If the acid components of these foods were strong acids, the food would likely be inedible.