formulas. This theory did explain the majority of the compounds known at the time, but
there were some exceptions. Chemists knew, for example, that metal oxides (MgO,
CaO, etc.) dissolved in water exhibited base-like properties. Also, ammonia (NH3) in
solution exhibited the properties of a base. The attempts to explain these exceptions
led to new definitions of acids and bases.
b. Modern Acid-Base Theory. In 1923, Bronsted and Lowry, two chemists in
different countries, independently derived new definitions of acids and bases to explain
the exceptions to the classical theory. The new theory they developed was
named, appropriately, the Bronsted-Lowry theory. This theory differs from the classical
theory in that the dissociation of water is considered as well as the dissociation of the
(1) Dissociation of water. Even though we often think of water as merely
being an inert solvent, it does dissociate into ions.
H2O <-------- H+ + OH -
This is an equilibrium type reaction as indicated by the double arrow. Actually, very few
ions exist at any time since they rapidly recombine to form molecular water. If we put
numbers in this reaction, there are 500 million molecules of water for each hydrogen or
(2) Bronsted-Lowry acid. By the Bronsted-Lowry theory, an acid
is any compound (charged or uncharged) capable of donating a proton. This is
essentially the same as the classical definition.
(3) Bronsted-Lowry base. The real value of the Bronsted-Lowry theory is in
the definition of a base. A base is defined as a charged or uncharged substance
capable of accepting a proton. Generally, the proton a base accepts comes from the
dissociation of water.
Consider, for example, ammonia dissolved in water:
NH3 + H2O <------- NH3 + H + OH <------- NH4 + + OH -
By accepting a proton from water, ammonia has effectively increased the concentration
of hydroxyl ions in the solution. This would account for the properties like those of a