Question

onsider the insoluble compound nickel(II) carbonate, NiCO3. The nickel ion also forms a complex with cyanide ions. Write a net ionic equation to show why the solubility of NiCO3(s) increases in the presence of cyanideand calculate the equilibrium constant for this reaction. Solubility product constant data is found in the Chemistry References. For Ni(CN)42-, Kf = 2.0×1031. Use the pull-down boxes to specify states such as (aq) or (s).

Answer 1

First write the following two equations:

Ni²⁺ + 4CN^- ----> Ni(CN)₄^2-

K_f = [Ni(CN)₄^2-]/([Ni²⁺]*[CN^-]⁴) = 2*10³¹

And,

NiCO₃(s) ----> Ni²⁺ + CO₃^2- (aq)

K_sp = [Ni²⁺]*[CO₃^2-] = 6.6*10^-9

Now,

The balanced equation for the reaction is as shown below:
NiCO₃(s) + 4CN^-(aq) -----> Ni(CN)₄^2-(aq) + CO₃^2-(aq)

For this reaction:
K_eq = ([Ni(CN)₄^2-]*[CO₃^2-]) / ([CN^-]⁴) = K_f*K_sp = 1.32*10²³

Addition of cyanide ion leads to a very rapid reaction with Ni²⁺, due to which Ni²⁺ gets consumed quickly and hence the NiCO₃ is forced to decompose more, owing to the Le-Chatlier's principle.