Question

What is the cause of the color change observed when `HCl` is added to the complex ion `[Cu(NH_3)_4]^(2+)`?

Answer 1

I have explained a similar ligand displacement reaction to this in the answer below:

Explanation:

https://socratic.org/questions/hydrochloric-acid-copper-ll#415291

In this case the complex we have is tetraammine copper(II):

If excess chloride ions are added the ammonia ligands are displaced and the green, tetrachloro copper(II) complex forms:

`sf(color(blue)([Cu(NH_3)_4(H_2O)_2]^(2+))+4Cl^(-)rarrcolor(green)([CuCl_4]^(2-))+4NH_3+2H_2O)`

The solution looks like this: