Question

5. Consider the carbonate ion test that you might perform on a typical +2 metal carbonate as an equilibrium: MH20 +CO a) Based on this equilibrium and the observed (limited) solubility of C02 in water, predict the outcome of bubbling C02 gas through solutions of each of the three +1/+ 2 cations studied in this experiment. Justify your prediction In light of your answer to part (a) of this question, explain why a solution of Zn (H20)2 (0H)4 upon treatment with C02 gas, first gives a precipitate of Znc03 then the precipitate dissolves on continued addition of CO2. Write equations for both reactions involved. Equation: Equation:

Answer 1

a) For +1 cation like Na⁺ bubbling with CO₂ gives bicarbonate as below

Na₂CO₃ (saturated) + H₂O + CO₂ = 2 NaHCO₃? .

For +1 cation like Li⁺ bubbling with CO₂ gives bicarbonate as below

Li₂CO₃ (solid) + H₂O + CO₂ ? 2LiHCO₃ (solution)

For +1 cation like K⁺ bubbling with CO₂ gives bicarbonate as below

K₂CO₃ (solid) + H₂O + CO₂ ? 2KHCO₃ (solution)

For +2 cation like Ca²⁺ bubbling with CO₂ gives bicarbonate as below

CaCO₃ (s) + H₂O (l) + CO₂ (aq) ? Ca(HCO₃)₂ (aq)

b) [Zn(H₂O)₂(OH)₄]^2- upon treatment with CO₂ gas, first gives a precipitate of ZnCO₃

[Zn(H₂O)₂(OH)₄]^2- (aq) + CO₂ (g) ? ZnCO₃ (s) + 3H₂O (l) + 2OH^- (aq)

On continued addition of CO₂ we get bicarbonate precipitate, wherein bicarbonate being unstable dissolves in solution.

ZnCO₃(s) + CO₂(g) + H₂O(l) ? Zn(HCO₃)₂ (aq) (soluble)