Re: CO2/H2CO3 equilibrium
To: Aquatic-Plants at actwin_com
Subject: Re: CO2/H2CO3 equilibrium
From: psears at NRCan_gc.ca (Paul Sears)
Date: Thu, 30 May 1996 09:56:51 -0400 (EDT)
In-Reply-To: <199605300739.DAA22112 at looney_actwin.com> from "Aquatic-Plants-Owner at ActWin_com" at May 30, 96 03:39:03 am
> From: Bill Warner <lww at ictech_com>
> There is a difference between "dissolved CO2" and carbonic acid. When CO2 is
> added to water it initially forms a loosely hydrated species denoted CO2(aq).
> This dissolved molecular CO2 reacts _slowly_ with water to form carbonic acid,
> CO2(aq) + H2O = H2CO3(aq)
> Furthermore, this kinetically slow reaction does not go to completion. At
> equilibrium, only a small fraction (ca. 0.2%) is actually converted to
> carbonic acid. Most of the CO2 remains as solvated molecular CO2. In fact,
> the pKa most often reported for carbonic acid (pK1 = 6.38) is not really the
> true pKa of carbonic acid. Rather, it is the pKa of the equilibrium mixture
> of CO2(aq) and carbonic acid. Carbonic acid itself is actually a much
> stronger acid than this, with a true pK1 value of 3.58.
Right on! I think I should point out, though, that the reaction of
CO2 with water to give H2CO3 is considered slow by chemists, because it
takes seconds. Many of the other reactions that interest us are orders of
Remember too that equilibrium reactions don't just go to the
equilibrium point and stop. The forward and reverse reactions continue,
but at equal rates. The systems are dynamic, and _very_ fast. The ammonia/
ammonium equilibrium has been mentioned here a lot. A given ammonia
molecule will change to ammonium and back many times a second. The higher
the pH, the bigger fraction of the time it will be ammonia, and
the higher the number of ammonia molecules present at any given instant.
My apologies to the chemists here, but I don't think the last point
is _generally_ appreciated.
Paul Sears Ottawa, Canada