Weak Bases

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 14:21

By analogy with weak acids, weak bases are not strong enough proton acceptors to react completely with water. A typical example is ammonia, which reacts only to a limited extent:

NH3 + H2O \rightleftharpoons NH4+ + OH

As in the case of acetic acid, the current conducted by 0.001 M NH3 (Table 1 from Ions in Solution) is slightly above 10 percent that of 0.001 M NaOH or KOH, indicating that somewhat more than one-tenth of the NH3 molecules have been converted to NH4+ and OH ions.

Weak bases fall into two main categories.

Ammonia and amines      Amines may be derived from ammonia by replacing one or more hydrogens with alkyl groups. They react with water in the same way as ammonia. Trimethyl amine behaves as follows:

(CH3)3N + H2O \rightleftharpoons (CH3)3NH+ + OH

Anions of weak acids      The fact that the molecules of a weak acid are not entirely converted into hydronium ions and anions implies that those anions must have considerable affinity for protons. For example, the anion of acetic acid, acetate ion, can accept protons as follows:

C2H3O2 + H2O \rightleftharpoons HC2H3O2 + OH

Thus when a compound like sodium acetate,dissolves, some hydroxide ions are produced and the solution becomes slightly basic. (Sodium ions do not react with water at all, and so they have no effect on acidity or basicity.) Other examples of weak bases which are anions of weak acids are CN, CO32–, PO43–.