An updated version of this lesson is available at Visionlearning: Acids & Bases
The result of this proton exchange is that at any given moment 2 water
molecules out of every 1 billion are split into a positively charged H3O+
(called hydronium) ion and a negatively charge OH- (called hydroxide)
ion. In a sample of pure water, the concentration of hydronium ions
is equal to 1 x 10-7 moles per
liter (0.0000001 moles per liter). In pure water, the number of hydronium
ions equals the number of hydroxide ions, so the concentration of hydroxide
ions must also equal 1 x 10-7 moles per liter (moles per liter
can be abbreviated: M). This equilibrium between hydronium and hydroxide
ions can shift if we mix other substances with water.
When the compound HCl is
dissolved in water it separates into 2 ions: a positively charged hydrogen
proton and a negatively charged chlorine ion. The positively charged
hydrogen proton (H+) combines with water and increases the concentration
of H3O+ ions, shifting the equilibrium we discussed
earlier. Some of these H3O+ ions recombine
with the OH- ions, and our sample becomes acidic because it
contains more H3O+ ions than OH- ions.
The original compound that we added, HCl, is said to be an acid
because it donates protons (H+) to the mixture.
Shown below are 2 solutions.
On the left is a solution of pure water in which the concentration of hydronium
ions equals the concentration of hydroxide ions. On the right is
the same solution after addition of small amount of acid. The number
of hydronium ions now exceeds the number of hydroxide ions.
|
|
| Neutral Solution: [H3O+] = [OH-] | Acid Solution: [H3O+] > [OH-] |
An acid can be defined
as a proton donor, a chemical that increases the concentration of hydronium
ions in solution. Conversely, we can define a base as a proton
acceptor, a chemical that reduces the concentration of hydronium ions in
solution (and increases the concentration of hydroxide ions).
Acid-base chemistry is an
important part of everyday life. The excess hydronium ions in acids
give them interesting properties. Acids can react with metals and
other materials. The strong acid HCl is produced in your stomach
to help digest food. In dilute concentrations, acids are responsible
for the sour taste of lemons, limes, vinegar and other substances.
Bases are also very reactive. The strong base NaOH is used in many
household cleaning agents such as oven cleaner and drain clog-remover.
But how do we measure the concentration of an acid or base?
The acidity (or basicity)
of a solution is measured using the pH scale. The pH scale corresponds
to the concentration of hydronium ions in a solution. In fact, if
you take the exponent of the H3O+ concentration and
remove the negative sign, you have the pH of a solution. For example,
in pure water the concentration of hydronium ions is 1 x 10-7
M. Thus, the pH of a solution of pure water is 7. The pH scale
ranges from 0 to 14, where 7 is considered neutral ([H3O+]
= [OH-]), below 7 acidic and above 7 basic. The further
from 7 you are on the pH scale, the more acidic or basic the solution.
For example, a solution with a pH = 1 has a hydronium ion concentration
of 1 x 10-1 M (0.1 M). The table below further illustrates
the relationship between hydronium ion concentration and pH.
| [H3O+] | pH | [OH-] | Example |
| 1 X 100 | 0 | 1 X 10-14 | HCl (4%) |
| 1 X 10-1 | 1 | 1 X 10-13 | Stomach acid |
| 1 X 10-2 | 2 | 1 X 10-12 | Lemon juice |
| 1 X 10-3 | 3 | 1 X 10-11 | Vinegar |
| 1 X 10-4 | 4 | 1 X 10-10 | Soda |
| 1 X 10-5 | 5 | 1 X 10-9 | Rainwater (unpolluted) |
| 1 X 10-6 | 6 | 1 X 10-8 | Milk |
| 1 X 10-7 | 7 | 1 X 10-7 | Pure water |
| 1 X 10-8 | 8 | 1 X 10-6 | Egg whites |
| 1 X 10-9 | 9 | 1 X 10-5 | Baking Soda |
| 1 X 10-10 | 10 | 1 X 10-4 | Ammonia |
| 1 X 10-11 | 11 | 1 X 10-3 | |
| 1 X 10-12 | 12 | 1 X 10-2 | Drano® |
| 1 X 10-13 | 13 | 1 X 10-1 | NaOH (4%) |
| 1 X 10-14 | 14 | 1 X 100 |
