Polyprotic Acids

Classifying Acids

• Acids can be classified by the number of protons per molecule they can donate.

  • Monoprotic: Contains one ionizable hydrogen atom per molecule.

  • Not all hydrogens in an acid are acidic (e.g., acetic acid; only the hydrogen bound to oxygen is acidic).

Diprotic Acids

• Contain two ionizable hydrogen atoms per molecule.

• Ionization occurs in two steps, with the first ionization being more extensive than the second.

• Example: Sulfuric acid (H2SO4) and Carbonic acid.

• The $K_a$ value decreases significantly from the first to the second ionization (e.g., by a factor of $10^{-4}$).

Approximation for Weak Diprotic Acids

• If the first ionization constant is larger than the second by a factor of at least 20, treat the first ionization separately.

• Calculate concentrations from the first ionization and use those values as the starting point for the second ionization equilibrium calculation.

Triprotic Acids

• Acids with three ionizable hydrogens.

• Each successive ionization is less extensive, reflected in decreasing $K_a$ values.

• The successive ionization constant differs by a factor of about $10^5$ to $10^6$.

• Example: Phosphoric acid (H3PO4), found in cola.

Polyprotic Bases

• Concepts for polyprotic acids also apply to bases.

• Diprotic bases can accept two protons.

• Example: Carbonate ion, accepts a proton to form a bicarbonate ion, then accepts a second proton to form carbonic acid.

• $K_b$ values decrease significantly from the first to the second protonation.