Re: why does carbonic acid decompose when it is created with acetic acid?

why does carbonic acid decompose when it is created with acetic acid?

I recently did an experiment in class, where I placed an egg (hard boiled) into a small beaker of vinegar (acetic acid). according to the chemical equation the reaction of the acetic acid and calcium carbonate of the egg shell produces calcium acetate and carbonic acid, which then decomposes into water and carbon dioxide.

what I have been trying to prove is why the acid decomposes. my theory is that the energy given off by the spontaneous reaction of the calcium carbonate and acetic acid is enough to raise the produced carbonic acid to its activation energy for decomposing. first, is this correct? second, is this incorrect because the hydrogen in the carbonic acid is actually in the form of hyronium ions? thirdly, if that is incorrect am I even close? thank you for your time.

Ben Trubits

Carbonic acid is a weak acid in aqueous solution, which means that it is only partly dissociated: The equilibrium constant for this reaction is about 10^-5. So really very little of the hydrogen in the carbonic acid is actually in the form of hyronium ions; dissolved in vinegar (pH about 2.4), only 0.4% of the carbonic acid will be dissociated.

First, as long as the dissociation of carbonic acid is reversible, the extent to which it is dissociated in water is irrelevent and we can treat all carbonic acid (not CO₂, that's another matter) as H₂CO₃.

Second, the decomposition of carbonic acid is a rapid equilibrium at room temperature:

The amount of CO₂ which remains in solution depends on its solubility: Since the solubility of CO₂ is only 1.45 g/L (about 0.03 mol/L) at 25° C, most of the carbonic acid formed by dissolving an eggshell in vinegar will escape into the atmosphere. Ultimately, this drives the process of dissolving the eggshell by removing carbonic acid as it is formed!

I hope this answered your question; if not, feel free to contact me directly.