Position of Equilibrium in Acid-Base Reactions
In any solution, the value of pKa indicates whether an acid is completely dissociated or not. A negative pKa corresponds to a stronger acid, whereas a positive pKa corresponds to a weaker acid. Consider the reaction between ammonia and an ethoxide ion. In this reaction, ethanol with a pKa of 15.9 is a stronger acid than ammonia with a pKa of 38. Recall that the strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base. Hence, the ethoxide ion is a weak base.