3. Assuming the activity of Cu2+(aq) is controlled by equilibrium with the solid

Question

3. Assuming the activity of Cu2+(aq) is controlled by equilibrium with the solid phase

Cu(OH)2(s):

(i) Write the balanced chemical reaction.

(ii) Now write a second balanced chemical reaction for the formation of an aqueous complex

between Cu2+ (aq) and a Cl- (aq) ligand.

(iii) Given these two reactions, does the presence of the complexing ligand in solution affect the

free aquo metal concentration? Explain.

(iii) Does the presence of metal complexing ligands affect the total metal concentration in

equilibrium with a solid phase? Explain.

Explanation / Answer

Solution:

(i) The balanced chemical reaction is as follows:

Cu(OH)2 (s) --------------> Cu2+ (aq) + 2 OH- (aq)

(ii) The balanced chemical reaction of Cu2+(aq) and Cl(aq) is as follows:

            Cu2+ (aq) + 2 Cl- (aq) ----------> CuCl2 (aq)

(iii) Definitely there would be change in the free metal concentration, if some complexing ligand (eg. NH3) is there in the aqueous solution of Cu2+ depending on the concentration of the complexing ligand. This is because of the ligand which forms complex with free Cu2+(aq) ions in the solution resulting non-ionisable complex, which cannot furnish Cu2+(aq) ions further in the solution once it (complex) is formed.

(iv) No, there would be no affect in the total metal concentration in equilibrium in presence of the complexing ligand. This is because of the fact that the complexing ligand will form complex with that of free Cu2+(aq) ions only with respect to the concentration of the complexing ligand. If the concentration of complexing ligand is lower than that of the Cu2+(aq) ions than there would be some of free Cu2+(aq) ions left in the solution. Conversely, if concentration of ligand is more than that of the Cu2+(aq) ions, then all of the Cu2+(aq) ions would be consumed by the ligand to form the complex, leaving zero free Cu2+(aq) ions in the solution. Thus overall the total concentration of the metal remains unchanged in the equilibrium in presence of complexing ligand in the solution.

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