Strengths of Acids and Bases

The equilibria of Eqs. (1-1) and (1-2) correspond to the equilibrium constant expressions:

                                      (1.1)

                    (1.2)

                                                                               (1.3)

                                                                          (1.4)

These expressions are often written in logarithmic form:

                                                                (1.5)

                                                              (1.6)

We can also think of NH₄⁺ as an acid, so that in solutions of NH₄Cl, we have the  equilibrium

                                                                    (1.7)

Adding eqs (1-5) and (1-7), we have

              (1.8)

Equation (1-8) is a general result relating pKa of an acid with pKb of the conjugate base.  The value of Ka or Kb (pKa or pKb) determines the extent of ionization of an acid or base. We think of strong acids or bases as completely ionized (or nearly so). This generally implies Ka or Kb greater than 1 (negative pKa or pKb). The pKa values for several acids are given in Table 1.1.

 Many common acids are oxoacids, i.e., molecules with the central atom surrounded by =O (oxo) and –OH (hydroxo) groups. Perchloric (HClO₄), chloric (HClO₃), chlorous (HClO₂), and hypochlorous (HClO) acids can be written ClO₃OH), ClO₂(OH), ClO(OH), and ClOH, respectively, to emphasize the distinction between oxo and hydroxo oxygens. The acid strength increases with the number of oxo O's. The increased acidity results from the delocalization of negative charge in the anions as shown in Figure 1.1 for the Cl oxoanions.  The average formal charge on oxygen is -1/4 in ClO₄^-, -1/3 in ClO₃^-, -1/2 in ClO₂^-, and -1 in ClO^-.

Table 1.1. pKa's of Some Common Acids

Figure 1.1. Resonance structures of the perchlorate, chlorate, chlorite, hypochlorite, formate, and nitrite ions.

Five of the acids listed in Table 1.1 are polyprotic acids, i.e., acids with more than one acidic H. Forexample, phosphoric acid ionizes in three steps:

The acidity of the neutral acid is in the range expected for an oxoacid with one oxo O. In the second and third steps, phosphoric acid is a much weaker acid. In general, we find that pKa increases by about 5 for each successive ionization step for a polyprotic acid. An exception to this rule is oxalic acid, H₂C₂O₄ = HOOCCOOH, where the acidic -OH groups are bound to different C atoms and behave more nearly independently.