What is the chemical reaction for the equilibrium of CO2 and rain water?

Answer 1

CO2 is physicaly solutive :

CO2 + H2O <-> H2CO3 pk 2,6
H2CO3 + H2O <-> H2O+ + HCO3(-) pk 3,8
HCO3(-) + H2O <-> H3O(+) + CO3(2-)pk 10,3

complete: CO2 + 2(H2O) <-> H3O(+) + HCO3(-) pk 6,4

for further investigation look at: http://en.wikipedia.org/wiki/H2CO3

Answer 2

H2O + CO2 <-----> H2CO3

2H2O + CO2 <-----> H3O^ 1 + HCO3^ -1