Reaction of carbon dioxide and water

[Moogie]

Hi

I believe when you add carbon dioxide to water it will undergo hydrolysis to form carbonic acid

CO2 + H20 <=>H2CO3

But i believe the CO2 is only weakly soluble so what happens to the rest of the CO2 that doesn't undergo hydrolysis? Does it exist as a gas in the water?

thanks

 

[alxm]

Well, H2CO3 _is_ the solvated form of CO2, so there is no other CO2 there, really.

If you were to, say, lower the vapor pressure of CO2 over the water, changing the equilibrium, then the CO2 dissolved in the water would leave, forming bubbles. In other words, what happens when you open a soda bottle.

 

[Borek]

This is actually quite interesting problem.

Quantum calculations show that the H₂CO₃ molecule should be quite stable - when there is no water around. In the presence of water it decomposes very fast to CO₂ and water. However, what we observe and can easily measure are amount of CO₂ dissolved and pH of the solution.There are two reactions in between - one is H₂CO₃ creation, the other acid dissociation. As it is hard to observe each reaction separately, and they are both very fast, we can treat them as one reaction, with one equilibrium constant.

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methods

 

[Moogie]

I can see i have worded that question very badly. I really don't know what i was thinking when i wrote that.

what i meant to ask was this:

you see the reaction between carbon dioxide and water as an equilibrium reaction to create carbonic acid:

CO2 + H20 <=> H2CO3

Equilibrium means there is not net change and the forward and backward reactions are occurring at the same rate

But i don't understand how once CO2 has 'dissolved' in water how the reverse reaction can be happening. Does carbonic acid dissociate to give water and carbon dioxide gas which exists as a gas in the water? Doesn't seem likely! Or is it that when carbonic acid is in excess of water that equilibrium is so far to the right the reverse reaction doesn't effectively happen.

Borek, I don't understand your answer. It sounds to me as if you are saying that when you add carbonic acid to water it will rapidly decompose to CO2 and h20 but also undergo acid dissociation too.

 

[alxm]

This is actually quite interesting problem.

Quantum calculations show that the H₂CO₃ molecule should be quite stable - when there is no water around. In the presence of water it decomposes very fast to CO₂ and water.

Well, H₂O itself is also very stable - when there's no water around. When there is water around, it can decompose to H⁺ and OH^-.

Essentially the same thing with H₂CO₃. Breaking it into water + CO2 in gas phase would require moving a hydrogen to one oxygen in concert with that oxygen breaking its bond with carbon, which causes quite a distortion of the structure and leaves the proton relatively 'naked' in-between the two oxygens:

(transition state)

Whereas in water you can simply break an O-C bond creating an OH^-, while kicking out a proton, and let them recombine wherever.

 

[alxm]

Borek, I don't understand your answer. It sounds to me as if you are saying that when you add carbonic acid to water it will rapidly decompose to CO2 and h20 but also undergo acid dissociation too.

Yeah, I was wrong. CO2 (aq) and H2CO3 (aq) do exist at the same time.

What Borek is saying is that the they're in such close equilibrium (the difference in energy is tiny), and in water, interconvert so quickly, you don't need to take them into account as separate species. If you reduce the amount of one, the other will immediately compensate.

And yes, it can also undergo acid dissociation, depending on the pH of course.

 

[Borek]

i don't understand how once CO2 has 'dissolved' in water how the reverse reaction can be happening. Does carbonic acid dissociate to give water and carbon dioxide gas which exists as a gas in the water? Doesn't seem likely!

Why not? Think about two stages - first, CO₂ dissolves - and you have just water saturated with a gas. Then, it reacts with water creating acid. Each stage has its own equilibrium, each stage can proceed both forward and back. Sure, in fact they happen simultaneously, you can't say "this is water saturated with CO₂ and it doesn't contain carbonic acid, as the reaction has not started yet" - but it doesn't change the overall situation.

Borek, I don't understand your answer. It sounds to me as if you are saying that when you add carbonic acid to water it will rapidly decompose to CO2 and h20 but also undergo acid dissociation too.

More or less that's what I am saying - you may think about it as competing reactions, each with its own equilibrium (which means each proceeding to the left and to the right at the same time).

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[Moogie]

Ok, that's fine. I got confused as someone said there was no gas in the water.

Saturation is a term i probably use without really knowing what it means. What does it mean to say water is saturated with CO2? Does that mean as much carbon dioxide has dissolved as possible?

 

[Moogie]

also, when the reverse reaction happens from carbonic acid to h2o and co2, what form does the CO2 take? Is it just gaseous carbon dioxide molecules interspersed with the water molecules?

 

[Borek]

What does it mean to say water is saturated with CO2? Does that mean as much carbon dioxide has dissolved as possible?

Yes (under given pressure/temperature). Same hold for any other solution of gas/solid.

also, when the reverse reaction happens from carbonic acid to h2o and co2, what form does the CO2 take? Is it just gaseous carbon dioxide molecules interspersed with the water molecules?

Something like that. Details can be more complicated (not that I know anything more), but that will work perfectly as the first approximation. Just don't call these molecules gaseous any longer

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[Moogie]

That's brilliant. You've been very helpul. I really appreciate it.

 

[Antiphon]

Borek, the wikipedia entry for Carbonic acid has a table entry for solubility in water that reads "exists only in solution". Does this mean you can't make a pure quantity of it or that it isn't done? I'm trying to reconcile this solubility with what you had said about it being quite stable when water is not present.

 

[alxm]

Borek, the wikipedia entry for Carbonic acid has a table entry for solubility in water that reads "exists only in solution". Does this mean you can't make a pure quantity of it or that it isn't done? I'm trying to reconcile this solubility with what you had said about it being quite stable when water is not present.

Well, having read the paper (and re-done a few of the calcs, see above).. the case is that yes, it's stable in the sense that a single H₂CO₃ molecule in vacuum is very stable. They claim a '0.18 million year half-life' which the wikipedia article cites. However, given the methods used, that number is give-or-take an order of magnitude. It's still very stable either way though.

However, even a single water molecule will catalyze the decomposition dramatically (as they showed), so you'd have a chain reaction and any significant amount of carbonic acid would likely decompose rapidly.

They don't seem to have taken into account the possibility that H₂CO₃ decomposition might be able to occur autocatalytically (coordinated proton transfer in the dimer). I suspect this can happen as well. (and I'm running a calculation on the side to see if I'm right.*)

Anyway, carbonic acid doesn't exist only in solution (http://www.sciencemag.org/cgi/content/abstract/279/5355/1332" ). It's just damn hard to keep it from decomposing. Not because it's an unstable compound, but because it's decomposition is catalyzed by its own decomposition products (and possibly by itself).

*Edit/update: Yes, I was correct. H₂CO₃ can catalyze its own decomposition, and in fact does so better than one or two water molecules do. I'm going to have to take a look around if this is published yet. I might get a small paper out of this...

 

[geotommy]

Hi

I believe when you add carbon dioxide to water it will undergo hydrolysis to form carbonic acid

CO2 + H20 <=>H2CO3

But i believe the CO2 is only weakly soluble so what happens to the rest of the CO2 that doesn't undergo hydrolysis? Does it exist as a gas in the water?

Hello, we have a unique system where we can mix gas into liquids. One of the industries we work in is agriculture. We infuse Co2 in water to over 950ppm. When kept in a 50 gallon barrel there is no loss of CO2 for weeks. If we pour it into a tray in a hot greenhouse, it slowly degasses the CO2 near the plants, allowing for rapid growth enhancement. Or sprayed onto plants it slowly releases and gets absorbed. I have scoured the Internet looking for anyone else who has infused vast quantities of CO2 dissolved into water but cannot find anything at all. Could you please let me know if you have heard of anyone able to do this?

Thank you,
Tom Richardson Gaia Water

 

[geotommy]

Well, H2CO3 _is_ the solvated form of CO2, so there is no other CO2 there, really.

If you were to, say, lower the vapor pressure of CO2 over the water, changing the equilibrium, then the CO2 dissolved in the water would leave, forming bubbles. In other words, what happens when you open a soda bottle.

We dissolve CO2 in water, up to 950ppm saturation. After 7 days it shows no loss of CO2 when contained in a 50 gal barrel. It is used for enhancing plant growth in commercial greenhouses. Do you know of anyone else who diffuses CO2 in water at extremely high levels? I've checked everywhere but can't seem to find any info online.

Thanks!
Tom Richardson
Gaia Water

 

[morrobay]

Regarding whether molecular CO₂ exists in water like O₂
Is it correct to say that " salting out a gas" in the case of carbon dioxide does refer to CO₂ alone,
and not dissolved carbonic acid.
For example when salt , NaCl, is added to beer, carbon dioxide bubbles up.
Its my understanding that the polar water molecules that were holding the carbon dioxide gas, physically by way of inter molecular hydrogen bonding,
are now preferentially attracted to the salt. Thus releasing the gas.