When hydrochloric acid (HCl), which exists in its pure state as a gas, is dissolved
in water, the following reaction takes place:
HCl + H2O <====> H3O + Cl-
The hydronium ion can easily react with other groups as a proton donor. The
hydrogen ion itself is made up of one proton.
The terms hydronium ion, hydrogen ion, and proton donor may be used
interchangeably. However, we will simply refer to the hydrogen ion in future
course work, but you should understand that this actually implies the
presence of the hydronium ion.
One of the primary drawbacks of the Arrhenius concept is that it defines only
aqueous solutions as acids and bases. Other theories allow for nonaqueous solutions.
a. Examples of Arrhenius Acids.
HCl -- hydrochloric acid.
H2SO4 -- sulfuric acid.
H3PO4 -- phosphoric acid.
b. Examples of Arrhenius Bases.
NaOH -- sodium hydroxide.
Ca(OH)2 -- calcium hydroxide.
KOH -- potassium hydroxide.
The Bronsted-Lowry theory presents a broader definition of the terms acid and
base. An acid is a proton donor and a base is a proton acceptor. Hydrochloric acid in
solution is a Bronsted-Lowry use it forms hydrogen ions but because it is able to give up
that hydrogen ion to another substance, such as to a molecule of water to form
hydronium ions. For HCl, the difference between the two definitions seems minor.
However, it becomes important when the solvent is not water. For example, when HCl
reacts with NH3 (ammonia), the proton is given up by the HCl and accepted by the NH3,
making the HCl an acid and NH3 a base.
HCl + NH3 <====> NH4 + Cl