Question

1. Explain in terms of ion concentrations and K_sp why a precipitate forms for some solutions.
2. When ammonia is added to a solution that has Cu(OH)₂(s) precipitate in it, the precipitate dissolves. Use Le Châtelier’s principle to explain this.
3. Based on your observations, when a colour change occurs but no phase change occurs, what kind of reaction might have occurred?

Answer 1

AB <...........> A+ + B-

Ksp = [A+][B-]

when ionic product is greater than Ksp then compound is precipitated from solution.

and

Due to common ion effect also dissolved ions precipitated more.

Reaction:

4 NH₃ (aq) + Cu(OH)₂ (s) ......................>  [Cu(NH3)4]²⁺ (aq) + 2 OH- (aq)

Here

Cu(OH)2 react with NH3 to form soluble complex [Cu(NH3)4]²⁺

Hence the Cu((OH)2 is precipitated.

Thus

complex formation reaction occurs.

Due to higher stability of [Cu(NH3)4]²⁺ complex, equilibrium shift in right direction.

Blue color changes to violet color in the solution and precipitate dissolves.