Question

h2co3 +heat------------------------

Answer 1

H2CO3 --heat--> H2O + CO2 
CO2 + H2O <==> H3O+ + HCO3- Ka = 4.3 x 10^-7
                                                  as  H2CO3 does not exist as a discrete molecule. There is probably no H2CO3 in aqueous solution. Carbonic acid, H2CO3, simply represents carbon dioxide gas dissolved in water, which exists as the following equilibrium. 

Answer 2

H2CO3 --heat--> H2O + CO2 
CO2 + H2O <==> H3O+ + HCO3- Ka = 4.3 x 10^-7