Why does carbonic acid decompose spontaneously at room temperature ?

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Offline blueskies

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and does it have anything to do with bond enthalpies?

thanks to those who will answer [:)]


Offline yor_on

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Okay I'll take a shot on it, and then Mr. Chem, or C4 (Chemistry4me) as he sometimes also goes under and BoredChemist can correct me :)

The main reason it breaks down is known as chemical decomposition

--Diverse quotes--

They are the opposite of synthesis reactions, with the format AB → A + B. Decomposition reactions "take things apart". Just as synthesis reactions can only form one product, decomposition reactions can only start with one reactant. Compounds that are unstable will decompose quickly without outside assistance. And the details of decomposition processes are generally not well defined, as a molecule may break up into a host of smaller fragments. There are broadly three types of decomposition reactions: Thermal, electrolytic, and catalytic.

Chemical decomposition is often an undesired chemical reaction. However, chemical decomposition is exploited in several analytical techniques, notably mass spectrometry, traditional gravimetric analysis, and thermogravimetric analysis.

--End of quotes---

Okay, so now we know what that was. So what is carbonic acid? Well, "The salts of carbonic acids are called bicarbonates (or hydrogen carbonates) and carbonates." And we use them for those bubbly soft drinks, everything we want to be bubbly in fact. We also use it for our red blood cells which "contain carbonic anhydrase which both increases the reaction rate and dissociates a hydrogen ion (H+) from the resulting carbonic acid, leaving bicarbonate (HCO3-) dissolved in the blood plasma.

This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO2 and allows it to be expelled." So it seems a very energy smart way to catalyze air into its compounds (as I understands it, you'll see later why I think it's energy efficient). And it seems also to play a role for our oceans acidification causing "the ocean's average surface pH to shift by about 0.1 unit from pre-industrial levels. This process is known as ocean acidification."

But how does it catalyze and decompose?


The carbonation leading to the lovely bubbles in our champagne and soda water is usually attributed to carbonic acid. However, this is not entirely accurate. It is actually carbon dioxide (CO2) that was formed during fermentation or forced into the beverage. A mere 0.003% of the dissolved gas is present in the form of carbonic acid (H2CO3).

According to the textbooks, pure carbonic acid does not exist because it is extremely unstable. This has since been revealed to be a mistake. In recent years, various research groups have been able to isolate and characterize carbonic acid as a pure compound. Its stability has been proven in both the solid and gas phases, a result that could not be properly explained for a long time. What causes the discrepancy with earlier observations?

Innsbruck researchers working with Klaus R. Liedl, Erwin Mayer and Andreas Hallbrucker have now revealed the secret. By performing some theoretical calculations, they found out, under which conditions carbonic acid is stable, and why this stability went unrecognized for so long.

The result obtained by Liedl and his colleagues is both amazing and simple: water plays the crucial role! Absolutely water-free carbonic acid is very stable. Liedl calculated a half-life of 180,000 years for it. "However, carbonic acid decomposes immediately if it comes in contact with water. A single water molecule is enough to speed up the decomposition of a molecule of carbonic acid a billion times."

Why is carbonic acid a subject of research at all? Some interested parties are scientists who are trying to determine whether carbonic acid is present in the universe. For this they need the spectra of pure, gaseous H2CO3 from the laboratory for comparison.

However, carbonic acid is also vital for us: it is an essential intermediate in the transfer of CO2 between tissue and blood, in particular, between the lungs and blood. The transformation of carbon dioxide into carbonic acid is accelerated by an enzyme, carboanhydrase. In solution, thus also in blood, carbonic acid itself mostly dissociates into a bicarbonate anion (HCO3-) and a proton (H+). This equilibrium is necessary to maintain a constant pH level in blood.

---End of quote-

And as to if it have any connection to Bond enthalpy?

I don't know? It's apparently water that are the 'active' ingredients in the breakdown (decomposition). And as "bond enthalpy is a term used in the study of thermodynamics that refers to the amount of energy required to break the chemical bond forged between any two atoms within a molecule." it do seems to have a general mathematical/chemical relevance to me? But as the reaction(s) are activated through water, and we have a natural humidity in our air, as well as in our blood it must be natures way to be really energy efficient, as a guess?

"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."