Consider the carbonate ion test that you might perform on a typical +2 metal car
ID: 956589 • Letter: C
Question
Consider the carbonate ion test that you might perform on a typical +2 metal carbonate as an equilibrium:
MCO3 + 2H3O+ 3H2O = M(H2O)6^2+ +CO2
A) based on this equilibrium and the observed (limited) solubility of CO2 in water, predict the outcome of bubbling CO2 gas through solutions of each of them three +1/+2 cations. Justifly tour prediction.
B) In light of your answer to part (a) of this question, explain why a solution of Zn(H2O)2(OH)4^2- upon treatment with CO2 gas, first gives a precipitate of ZNCO3; then the precipitate dissolves on continued addition of CO2. Write equations for both reactions involved.
Explanation / Answer
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)