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Thread: Carbonic acid.

  1. #1 Carbonic acid. 
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    Have a question in my mind, and it's probably a stupid question but I hope you bear with me.

    If carbonic acid rapidly decomposes into carbon dioxide and water, then how can it have time to dissociate into a bicarbonate ion and a hydrogen ion?


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    There is a series of equilibria

    CO2 + H2O = H2CO3 = H+ + HCO3-= 2H++ CO32-


    There will always be some CO2, H2CO3, H+ + HCO3- and CO32-

    the relative amounts depending on the equlibrium constants for each of the equilibria under the conditions you are working under.

    If you need exampes have a play with this thermodynamic (Pitzer) model: http://www.aim.env.uea.ac.uk/aim/accent4/model.php

    I haven't used this version myself (I normally use the E-AIM inorganic salt version) but if you put in a H+ = 2 and CO32- = 1 (i.e. 1 mole per m3 of carbonic acid) it will return the equlibrium molality and activity coefficients of each ion (and CO2) in the solution, the partial pressure of CO2 in the gas phase at equilibrium at 298 K and 1 bar pressure).


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  4. #3  
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    Ah ok thanks. When carbonic acid decomposes into co2 and h2o does the co2 immediately diffuse into the environment or still persist in the water itself?
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  5. #4  
    Bullshit Intolerant PhDemon's Avatar
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    CO2 is reasonably soluble (look up it's Henry's Law constant) so some will be dissolved in the aqueous phase but most will come out into the gas phase.

    It says here:http://en.wikipedia.org/wiki/Carboni...al_equilibrium

    "The hydration equilibrium constant at 25C is called Kh, which in the case of carbonic acid is [H2CO3]/[CO2] ≈ 1.710−3 in pure water[2] and ≈ 1.210−3 in seawater.[3] Hence, the majority of the carbon dioxide is not converted into carbonic acid, remaining as CO2 molecules." So the majority of CO2 in solution is as CO2 molecules, i.e CO2(aq) rather than H2CO3...
    Last edited by PhDemon; February 19th, 2014 at 07:04 AM.
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