Carbonic Acid Decomposition
Date: Around 1999
Why does H2CO3 decompose so quickly into CO2 and H2O?
What is its structure? Is it because of a particular bond, or is there
some other reason?
Good question! You would think that the answer to such an obvious question
would have been answered long ago, but that turns out not to be the case.
Energetically, it is quite favorable for carbonic acid (H2CO3) to decompose
into CO2 and H2O. However, that alone is not enough to make it happen. It
is also quite energetically favorable for wood to combine with atmospheric
oxygen to form H2O and CO2, but without an easy way for this to happen, wood
and the air can co-exist for a very long time. Carbonic acid is not
intrinsically any worse than a lot of other persistent compounds, but it
DOES have an easy way to break down.
The key is water. Carbonic acid by itself is actually a fairly stable
molecule. In the past few years, several chemists have managed to prepare
carbonic acid that does not also contain present, and found that it doesn't
immediately break down. However, if water is present, it will greatly
accelerate the decomposition of carbonic acid. Since water is also a
product of the decomposition, even miniscule traces of water will rapidly
destroy a sample of carbonic acid. In most cases, carbonic acid is produced
in water solution (the common baking soda + vinegar reaction, for example),
so the decomposition is a foregone conclusion.
Very recently, a group in Austria has reported quantum chemical calculations
of possible pathways for the decomposition reaction (T. Loerting, C.
Tautermann, R. T. Kroemer, I. Kohl, A. Hallbrucker, E. Mayer, K. R. Liedl,
Angewandte Chemie International Edition in English, v. 39, no. 5, pp 892-894
(2000)). They calculate that two water molecules together can increase the
rate of decomposition of carbonic acid by a factor of 50 billion.
So, what is this reaction through which water catalyzes the decomposition of
carbonic acid? It's hard to describe without drawing, but I'll try.
Carbonic acid is basically trigonal planar at the carbon atom, O=C(OH)2.
Picture now the two OH groups of the carbonic acid. Each oxygen is
connected to the carbon atom, and the hydrogen atoms are connected only to
their oxygens. Interposed between these two OH groups are two molecules of
water, in a hydrogen bonded chain. The chain is something like this:
where a hyphen represents a single covalent bond, (X) represents an X atom
connected to the atom written before it by a single covalent bond, and ".."
represents a hydrogen bond. The carbon atoms at both ends of the chain are
one and the same here. (It's hard to represent a cyclic structure with a
line of text.) This carbon atom is also doubly bonded to another oxygen
atom. In the decomposition, some of the hydrogen atoms move a bit, so that
some of the covalent and hydrogen bonds change places, making
C=O..H-O(H)..H-O(H)..H-O-H + C=O
Recognizing again that the two carbon atoms are one and the same, what we
now have is 0=C=O (carbon dioxide) and three water molecules. The overall
H2CO3 + 2 H2O --> CO2 + 3 H2O.
Related compounds, such as carboxylic acids, cannot produce a stable product
by such an easy pathway, so they persist in the presence of water.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012