Rainwater carbon dioxide equilibrium?

Question

An equilibrium between carbon dioxide and water leads to rainwater being natually acidic. What is the equilibrium chemical reaction for this process?

Answer 1

When Carbon Dioxide dissolves in water (be it rainwater, in soda, ...) it forms Carbonic acid, H2CO3.

Carbonic acid is a weak acid, meaning that it does not completely disassociate into ions in solution, an equilibrium exists.

In solution, the Carbonic acid equilibrium is,
H2CO3 (aq) --> H+ (aq) + HCO3- (aq)
The equilibrium lies far to the "left", toward the disassociated acid.

The pKa value for the first Hydrogen ion is H2CO3 is 3.60 which is low...the higher the value, the farther to the "right" the equilibrium lies.

Answer 2

I'm not positive, and I don't know the k value or anything (very low), but i assume its something like:

H2O + CO2 <> H2CO3

or

H2O + CO2 <> HCO3- + H+

or perhaps even inversly

H2CO3 + H2O <> HCO3- + H3O+