Question

a Explain how (in procedures step format), the reaction of Ni(OH)2 with NH3 would enable you to separate a mixture of iron(III) and nickel(II) hydroxide.

I need help with question a but can you tell me if my answers are right for b and c Thank you

b Explain why you can dissolve Ni(OH)3 in aqueous ammonia.

The compound Ni(OH)2 is very much soluble in ammonia, since it will form a complex precipitate.

c. Could you use excess NaOH to separate a mixture of the precipitates of Fe(OH)3 and Ni(OH)2? Explain briefly why or why not.

You can not use excess NaOH to separate a mixture of the precipitates of Fe(OH)3 and Ni(OH)2 because both precipitates are not soluble in excess NaOH. They would not dissolve in the NaOH solution and both would stay precipitate.

Answer 1

a. [Fe(H₂O)₆]³⁺ + 3OH^- = Fe(OH)₃ + 6H₂O

[Fe(H₂O)₆]³⁺ + 6NH₃ = [Fe(NH₃)₆]³⁺ + 6H₂O

Fe(OH)₃ is insoluble in water and more stable than [Fe(H₂O)₆]³⁺ or [Fe(NH₃)₆]³⁺ so it is preferentially formed than the other two.

[Ni(H₂O)₆]²⁺ + 2OH^- = Ni(OH)₂ + 6H₂O

[Ni(H₂O)₆]²⁺ + 6NH₃ = [Ni(NH₃)₆]²⁺ + 6H₂O

[Ni(NH₃)₆]²⁺ is a very stable complex and it is soluble in water and is preferentially formed than Ni(OH)₂.

So when ammonia is added in a mixture of Fe(OH)₃ and Ni(OH)₂ , Ni(OH)₂ will readily form [Ni(NH₃)₆]²⁺ but Fe(OH)₃ will remain as a precipitate. The precipitate can be separated from the mixture.

b. [Ni(H₂O)₆]²⁺ + 2OH^- = Ni(OH)₂ + 6H₂O

[Ni(H₂O)₆]²⁺ + 6NH₃ = [Ni(NH₃)₆]²⁺ + 6H₂O

The second equilibrium is more fast forward that the first equilibrium. This makes it possible to dissolve Ni(OH)3 in aqueous ammonia.

c. Your answer is right.