Question

Write the equilibrium equation for the reaction of CO₂ with H₂O creating carbonic acid and the carbonic acid dissociating into hydrogen ion and bicarbonate

Answer 1

The reaction between CO₂ and H₂O can be written as below.

CO₂ (g) + H₂O (l) <=====> H₂CO₃ (aq) ……(1)

The equilibrium constant is given as

K = [H₂CO₃]/P_CO2 where P_CO2 is the partial pressure of CO₂ (we choose partial pressure because gases are best described in terms of partial pressure). The concentration of H₂O is not included since H₂O is present in large excess.

Again, H₂CO₃ dissociates as below.

H₂CO₃ (aq) <=====> H⁺ (aq) + HCO₃^- (aq) ……(2)

The acid dissociation constant is given as

K_a1 = [H⁺][HCO₃^-]/[H₂CO₃]

Combine equations (1) and (2) to get the equilibrium equation as

CO₂ (g) + H₂O (l) + H₂CO₃ (aq) <=====> H₂CO₃ (aq) + H⁺ (aq) + HCO₃^- (aq)

Cancel out the common terms and write

CO₂ (g) + H₂O (l) <=====> H⁺ (aq) + HCO₃^- (aq)