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Reaction of carbon dioxide and water

  1. Aug 21, 2010 #1

    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?

  2. jcsd
  3. Aug 21, 2010 #2


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    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.
  4. Aug 21, 2010 #3


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    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.

  5. Aug 21, 2010 #4
    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 occuring 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 doesnt 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.
  6. Aug 21, 2010 #5


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    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.

    Attached Files:

  7. Aug 21, 2010 #6


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    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.
  8. Aug 22, 2010 #7


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    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.

    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).

  9. Aug 22, 2010 #8
    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?
  10. Aug 22, 2010 #9
    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?
  11. Aug 22, 2010 #10


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

    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 :smile:

  12. Aug 22, 2010 #11
    That's brilliant. You've been very helpul. I really appreciate it.
  13. Aug 22, 2010 #12
    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.
  14. Aug 22, 2010 #13


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    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" [Broken]). 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 gonna have to take a look around if this is published yet. I might get a small paper out of this...
    Last edited by a moderator: May 4, 2017
  15. Jun 9, 2011 #14
  16. Jun 9, 2011 #15
    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.

    Tom Richardson
    Gaia Water
  17. Jun 9, 2011 #16


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    Gold Member

    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.
    Last edited: Jun 10, 2011
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