Re: CO2 as an acid

[Bill Warner <lww at ictech_net>]

> Date: Fri, 10 May 2002 15:36:36 -0700
> From: jgarden2000 at juno_com
> Subject: Re: Planted tank for biology lessons (and my own question at
> the end!)
>
> My understanding is that an acid is a proton donor; a molecule that
> releases an H+ ion in solution. In that case, how can CO2 be an acid,
> since it doesn't contain H?

Carbon dioxide reacts with water like so:

	CO2 + H2O = H+ + [HCO3]-	[1]

This is sometimes written as a two step reaction showing the formation of
carbonic acid which then dissociates:

	CO2 + H2O = H2CO3		[2]
	H2CO3 = H+ + [HCO3]-		[3]

However, while this may help you to visualize where the H+ comes from, it
is largely a notational fiction needed to shoehorn CO2 into the Arrhenius
definition of an acid (HX = H+ + X-).  The species H2CO3 does not exist in
solution to any significant degree (less than 1% of the dissolved CO2). 
Reaction [1] is a better representation of what is actually in solution,
that is, hydrated CO2 in equilibrium with H+ and [HCO3-].

--Bill