Predicting Acid–Base Reactions from pKa Values

Compilations of pKa values like those in Table 1 is useful for predicting whether a given acid–base reaction will take place, because H⁺ will always go from the stronger acid to the stronger base.

 That is, an acid will donate a proton to the conjugate base of a weaker acid, and the conjugate base of a weaker acid will remove a proton from a stronger acid. Since water (pKa = 15.74) is a weaker acid than acetic acid (pKa = 4.76), for example, hydroxide ion holds a proton more tightly than acetate ion does. Hydroxide ion will therefore react to a large extent with acetic acid, CH₃CO₂H, to yield acetate ion and H₂O.

Another way to predict acid–base reactivity is to remember that the product conjugate acid in an acid–base reaction must be weaker and less reactive than the starting acid and the product conjugate base must be weaker and less reactive than the starting base. In the reaction of acetic acid with hydroxide ion, for example, the product conjugate acid (H₂O) is weaker than the starting acid (CH₃CO₂H), and the product conjugate base (CH₃CO₂^-) is weaker than the starting base (OH^-).