Consider the insoluble compound cobalt(II) hydroxide , Co(OH)2 . The cobalt(II) ion also forms a complex with ammonia . Write a balanced net ionic equation to show why the solubility of Co(OH)2 (s) increases in the presence of ammonia and calculate the equilibrium constant for this reaction.

4.56×10^-10

Explanation:

Co(OH)2(s)<>Co^2+(aq) + 2OH^-(aq)

Co^2+(aq) + 6NH3(aq) >[Co(NH3)6]^2+(aq)

Overall equation:

Co(OH)2(s)+ 6NH3(aq) >[Co(NH3)6]^2+(aq) + 2OH^-(aq)

The increased solubility of the cobalt salt in the presence of ammonia is as a result of the formation of the hexammine cobalt II complex shown above.

Knet= Ksp×Kf

Ksp of Co(OH)2= 5.92 × 10^-15.

Kf= 7.7 x 10^4

Knet= 5.92 × 10^-15 × 7.7 x 10^4

Knet= 4.56×10^-10

Note Knet is the equilibrium constant of the overall reaction.

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