Question

(a) Using dissociation constants from Appendix D, determine the value for the equilibrium constant for each of the following reactions.

(i) HCO₃^-(aq) + OH^-(aq) CO₃^2-(aq) + H₂O(l)
(ii) NH₄⁺(aq) + CO₃^2-(aq) NH₃(aq) + HCO₃^-(aq)

(b) We usually use single arrows for reactions when the forward reaction is appreciable (K much greater than 1) or when products escape from the system, so that equilibrium is never established. If we follow this convention, which of these equilibria might be written with a single arrow?

and be derived as 1/K_b for carbonate ion:

The K_b for carbonate ion is calculated from the K_a2 for carbonic acid, as it is a conjugate base of HCO₃^– ion.

The equilibrium constant K is

b) The equilibrium constant for the reaction: NH₄⁺(aq) + CO₃^2-(aq) NH₃(aq) + HCO₃^-(aq) is

and can be derived from K_b for ammonia and K_b for carbonate ion.

NH₃ + H₂O NH₄⁺(aq) + OH^-(aq)

CO₃^2– + H₂O HCO₃^– + OH^–

The K_b for carbonate ion is calculated from the K_a2 for carbonic acid, as it is a conjugate base of HCO₃^– ion.

The equilibrium constant K is

b) Both equations might be written with single arrows as both equilibrium constants are larger than 1.