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Name: Meagan
Status: student
Age: 18
Location: N/A
Country: N/A
Date: Around 1999 


Question:
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?


Replies:
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:

C-O-H..O(H)-H..O(H)-H..O(H)-C=O

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 reaction was

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.
Assistant Director
PG Research Foundation, Darien, Illinois



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