Classification of Acids
Date: July 2008
I've been perusing the myriad of acids known to man, and I
have become flabbergasted at the dizzying number of varieties.
Furthermore, I cannot understand how acids are classified. Could one
of your knowledgeable scientists please explain to me what
categories acids fall into? If possible, I would like to know every
category and subcategory of the acid family tree. Thank you in
earnest for your efforts. This is a marvelous site, and I hope it
will continue to flourish as it nourishes eager minds.
I do not tend to think of acids as belonging to some kind of classification
"tree". Rather, I tend to differentiate acids depending on what they do or
what molecular structure they might contain.
The most well-known and often used differentiation is that of strong versus weak
acids. Strong acids are those that fully dissociate in water: HA --> H(+) + A(-);
where A(-) is the counterion of the the proton - versus those acids that form an
equilibrium with their ions (that is, some HA still exists in solution when
HA <=> H(+) + A(-).
Another distinction are acids that contain oxygen in their molecular formula
(HClO4, HCOOH, etc.) as opposed to those that do not (HCl, HCN, etc.) This is
often done since acid strengths can often be trended depending on how the oxygen
is structured within the molecule.
There is also the distinction between organic acids versus inorganic acids.
Inorganic acids are derived from minerals whereas organic acids are derived
from living organisms.
I hope that helped.
Greg (Roberto Gregorius)
You clearly are interested in Chemistry. I congratulate you on your good taste.
If you do not already have a copy of Zumdahl's "College Chemistry" test book, get
one. Go to Barnes and Noble, or Amazon and look up choices on their "Used and
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help you answer many of your chemistry questions.
Now about acids. There are 5 common acids: hydrochloric, nitric, sulfuric,
phosphoric and acetic. Sulfuric acid is the most important chemical ingredient
in the world. Its use, volume and price is sometimes a good indicator of economic
health of the USA and world.
Sulfuric, nitric and hydrochloric acids are considered strong mineral acids. Their
sources are sulfate, nitrate and chloride salts/minerals. They are strong because
each one will easily release a proton in an aqueous solution.
Phosphoric acid is a weak mineral acid. While it has 3 protons, it does not give
up the first proton 100% of the time when in water. Here is where the ideas of pH
You have acetic acid at home. It is called vinegar. Acetic acid is a simple organic
acid. Its structure is built on the two carbon atoms bonded to each other. One of
the carbons has 3 hydrogen atoms bonded to it. These hydrogens are not the protons
that make acetic acid an acid. The second carbon has 2 oxygen atoms bonded to it.
One of the carbon-oxygen bonds is a double bond; the other carbon-oxygen bond is a
single bond. This singly-bonded oxygen atom is also bonded to a hydrogen atom. This
hydrogen is the ACIDIC PROTON. It is the proton that dissociates from the molecule
and makes vinegar sour. The organic acid group is sometimes written as COOH. All
organic acids have this functional group. When an organic chemist wants to add an
acid group to a molecule, he runs a carboxylation reaction. When he wants to remove
a acid group from an organic acid, he runs a decarboxylation reaction.
The collection of organic acids is vast. Thousands of new ones are synthesized every
year. One important subset of organic acids is a group called amino acids. You should
have heard of them in biology. They have at least one -COOH group in their structure;
they also have a second functional group called the amino group. Its represented by
-NH2. The nitrogen is bonded to the 2 hydrogens and a third bond is usually to a
carbon atom. Google "Amino acid." You should see diagrams of the amino acids,
especially the 20 (or so essential) amino acids. Recall that the amino acids bond
to one another to produce proteins.
Read some more, then call us with more questions.
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Update: June 2012