Question

[cu(nh3)4]2 forms a blue solution. when concentrated hcl is added to this solution, what color will the solution change to

Answer 1

[Cu(NH3)4]2+ forms a blue solution. When concentrated HCl is added to this solution, what color will the solution change to yellow.

Reason: 
When conc. HCl is added to the solution containing [Cu(NH3)4]2+, Cl- ions will replace NH3 and form [Cu(Cl)4]2- complex. This can be understood for the following reaction:

[Cu(NH3)4]2+     +          Cl-          ↔         [Cu(Cl)4]2-      +        NH3

The Cl- ion is a weak field ligand, while NH3 is a strong field ligand. Hence, Cl- will in less splitting of d-orbitals, as compared to NH3. Due to this, photons of different energies will be absorbed by these complexes, and hence they display different colours.

Answer 2

Answer:

Green.

Explanation:

Hello,

The tetra amine copper (II) cation is naturally blue due to the presence of copper and nitrogen into the solution. On the other hand, when chloride anions are added due to the hydrochloric acid addition, the formed species is $[Cu(Cl)_4]^{-2}$ whose color is green which is characteristic for copper and chlorine based salts (for example cupric chloride).

Best regards.