example. Hydrochloric is a strong acid. It gives up protons readily. It follows that
the Cl–ion has little tendency to attract and retain a proton. Consequently, the Cl–
ion is an extremely weak base.
This observation leads to an important conclusion: the stronger an acid is,
the weaker its conjugate base; the stronger a base is, the weaker its conjugate
acid. This concept allows strengths of different acids and bases to be compared to
predict the outcome of a reaction. As an example, consider the reaction of
perchloric acid, HClO 4 , and water.
Another important general conclusion is that proton-transfer reactions favor
the production of the weaker acid and the weaker base. For a reaction to
approach completion, the reactants must be much stronger as an acid and as a
base than the products.
The Lewis Theory (1916) defines acids and bases in terms of the electron-
pair concept, which is probably the most generally useful concept of acids and
bases. According to the Lewis definition, an acid is an electron-pair acceptor;
and a base is an electron-pair donor. An example is the formation of ammonium
ions from ammonia gas and hydrogen ions.
Notice that the hydrogen ion is in fact accepting the electron pair of the
ammonia, so it is a Lewis acid. The ammonia is donating its electron pair, so it is
a Lewis base.
Another example is boron trifluoride. It is an excellent Lewis acid. It forms a
fourth covalent bond with many molecules and ions. Its reaction with a fluoride
ion is shown below.