[JaeKyung]

Hello chem folks,

Does carbonic acid always immediately separate into water and carbon dioxide?
I am writing down the reaction equation for HCl(aq) + NaHCO3(s) and HCl(aq) + Na2CO3(aq) that I tested in the lab.
My TA told me carbon dioxide wouldn't form but I don't know why that is, and I keep thinking H2CO3 would break up when it's formed.

Thank you in advance.

[Aeon]

It's an equilibrium.

To find out what form of the acid predominates in the equilibrium at a certain pH, you need to refer to the Henderson-Hasselbalch equation. What information does the pKa give you? What happens if pH>pKa? If pH<pKa? If pH=pKa?

When CO₂ dissolves in water, it lowers the pH. Why?
If you can answer this, you have the answer to your question.

[Borek]

As I read the question it was more about

H₂O + CO₂ <-> H₂CO₃

than about

H₂CO₃ <-> H⁺ + HCO₃^-

Thing is, reaction that we observe - and equilirbium constant of which we can easily measure - is

H₂O + CO₂ <-> H⁺ + HCO₃^-

so in reality we don't know what is concentration of the carbonic acid. I have seen different equilibrium constants reported for both processes.

Quantum calculations show that isolated molecule of the carbonic acid should be quite stable, but its decomposition in the presence of water is very fast. As decomposition produces water, this is an autocatalytic process, hence it is not possible to isolate pure carbonic acid, also concentration of the carbonic acid in the solution is probably very low.

Jae: whether carbon dioxide starts to evolve depends on the concentration of reagents and concentration of produced gas. In more concentrated solutions you will see evolving gas, in diluted it doesn't start to evolve.

[Cat in the Lab]

you can never collect a bottle of "pure carbonic acid",
becuz it's always a mixture of CO2, H2O and H2CO3.
I guess that's the meaning of your TA.

[AWK]

you can never collect a bottle of "pure carbonic acid",
becuz it's always a mixture of CO2, H2O and H2CO3.
I guess that's the meaning of your TA.

You are not right. H₂CO₃ is a solid and can be obtained in the reaction:
NaHCO₃(s) + HCl(g) = NaCl + H₂CO₃ in dry ether. Carbonic acid is soluble in ether and can be obtained as solid after evaporation. Solid exists to the moment of contact with water (liquid or vapor) which catalically decomposes the carbonic acid.

[Cat in the Lab]

you can never collect a bottle of "pure carbonic acid",
becuz it's always a mixture of CO2, H2O and H2CO3.
I guess that's the meaning of your TA.

You are not right. H₂CO₃ is a solid and can be obtained in the reaction:
NaHCO₃(s) + HCl(g) = NaCl + H₂CO₃ in dry ether. Carbonic acid is soluble in ether and can be obtained as solid after evaporation. Solid exists to the moment of contact with water (liquid or vapor) which catalically decomposes the carbonic acid.

Not really. H2CO3<-->H2O + CO2. When ether evaporates, a small amount of H2O evaporates and some CO2 goes with it as well. also, solid H2CO3 doesn't even exist above 0 degree celcius. so even if you dissolve Carbonic acid in ether, you won't obtained the solid carbonic acid after evaporation.

[AWK]

Taken from Angewandte Chemie International Edition
Volume 39, Issue 5, pages 891–894, March 3, 2000

A half-life of 0.18 million years for dry H2CO3 at 300 K clearly demonstrates that carbonic acid is not unstable