Weak Bases
Weak bases (bases without OH−) react with water to produce a hydroxide ion. Common examples
of weak bases are ammonia (NH3), methylamine (CH3NH2), and ethylamine (C2H5NH2). The lone
pair on N forms a bond with a H+. Most weak bases involve N.
The equilibrium expression for bases is known as the Kb.
for weak base reactions: B + H2O HB+ + OH−
Notice that Ka and Kb expressions look very similar. The difference is that a base produces the
hydroxide ion in solution, while the acid produces the hydronium ion in solution.
Another note on this point: H+ and H3O+ are both equivalent terms here.
Often water is left completely out of the equation since it does not appear in the equilibrium. This
has become an accepted practice. (* However, water is very important in causing the acid to
dissociate.)
polyprotic bases--accept more than one H+; anions with −2 and −3 charges (ex. PO43− ; HPO42−)
Amphiprotic or amphoteric --molecules or ions that can behave as EITHER acids or bases; water,
anions of weak acids (look at the examples above—sometimes water was an acid, sometimes it acted
as a base)
