The Strength of Acids and Bases
You are most likely
familiar with the terms acids
and bases. Hydrochloric acid
is a solution that many
students have worked with in
high school. Sodium
hydroxide is a common base. You have also studied chemical reactions and
equations. No doubt you are familiar with acid-base (neutralization) reactions, in
which an acid and a base are combined, producing water and a salt. However,
have you ever wondered what an acid and a base really are? Do you know why
they react with other compounds in the ways that they do? Protons are the
positive hydrogen ions associated with the acids, or the H+ ions. These protons
may react with the hydroxide ions (OH-) of the base producing water as a result.
When acids and bases react with other compounds, what you are observing in
those reactions is simply a transfer of protons.
Acids and bases have specific properties based on their chemical
structures and arrangements. In the beginning, acids and bases were categorized
by taste. Acids had a sour taste, and bases had a bitter taste. In 1884, a scientist
named Svante Arrhenius discovered that all acids contained H+ ions and that all
bases contained OH- ions. Therefore, he concluded that an acid was any solution
that contained more H+ than OH- ions. A base was defined as any solution that
contained more OH- ions than H+ ions. This became the Arrhenius rule for
distinguishing acids from bases.
In 1923, this definition was modified when two scientists, Johannes
Brønsted and Thomas Lowry, independently made the same discovery. Their tests
found that during an acid base reaction, a proton was actually transferred from
the acid to the base. They defined an acid as a proton donor and a base as a
proton acceptor. This meant that any compound that donated a proton could be
considered an acid. Any compound that accepted a proton could be a base. This
became the Brønsted-Lowry rule for distinguishing acids from bases.
Acids and bases may be strong or weak. What does this mean? There are
very few true strong acids. A strong acid is an acid that completely releases its
hydrogen ions in solution, and a strong base is a base that completely ionizes in
solution. Conversely, there are many weak acids. A weak acid is an acid that
does not release all of its hydrogen ions in solution. A weak base is a base that
does not completely ionize in solution. How strong or weak these compounds are
can be determined, in part, by the pH level of the solution. The pH scale is a scale
ranging from 1–14 that measures the concentration of hydrogen ions in a solution.
A strong acid will have a pH reading of 1. A strong base will have a pH reading of
14. A neutral solution will have a pH reading of 7.
If we use the Brønsted-Lowry definition of acids (and bases), a strong acid
will more easily lose its protons than a weak acid. Furthermore, a strong base will
more likely attract protons than a weak base. Strong acids include compounds
such as hydrochloric acid, or HCl. This acid will completely dissociate in solution,
leaving H+ ions and Cl- ions. This acid is “happier” when it is dissociated into ions
in solution. A common weak acid is acetic acid, the acid found in vinegar. This
acid, HC2H3O2, will only partially dissociate in solution, leaving some H+ ions and
some C2H3O2- ions. This acid is “happier” when it is an acid in solution. Therefore,
in solution, strong acids tend to be ions, and weak acids tend to be acids.
So, what does this have to do with strong and weak bases? Each acid has
something called a conjugate base. The conjugate base of HCl is Cl-. The
conjugate base of HC2H3O2 is C2H3O2-. The strong acid HCl has a weak conjugate
base of Cl-. Why? Remember that HCl prefers to be dissociated into ions in
solution. So, each time a Cl- ion is present in solution, it is very unlikely that it
will accept an H+ ion. Therefore, Cl- has a weak attraction for protons, making it
a weak base. On the other hand, the C2H3O2- ion is a strong base, as its conjugate
acid prefers to be an acid in solution. This means that the C2H3O2- ion has a
strong attraction for protons, making this ion a strong base. In general, the
stronger the acid, the weaker its conjugate base, and vice versa.