Question

What would happen if `"HCl"` is added to a solution of cobalt (II) ions?

Answer 1

The green complex `sf([CuCl_4]^(2-))` is formed.

Explanation:

I am assuming we are in aqueous conditions.

The aqueous copper(II) ion consists of a central `sf(Cu^(2+))` ion surrounded by 6 `sf(H_2O)` ligands:

It has the formula `sf([Cu(H_2O)_6]^(2+))` and is blue in colour. In solution it looks like this:

If a large excess of chloride ions is added the water ligands are displaced as the following equilibrium is established:

`sf([Cu(H_2O)_6]^(2+)+4Cl^(-)rightleftharpoonsCuCl_4^(2-)+6H_2O)`

`stackrel(color(white)(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx))(color(blue)(rarr)`

`sf(" "color(blue)(blue)" "color(green)(green))`

Concentrated hydrochloric acid contains a large amount of chloride ions so Le Chatelier's Principle tells us that adding this will cause the position of equilibrium to shift to the right producing green `sf(CuCl_4^(2-))` ions.

They have a tetrahedral structure:

The solution looks like this:

If excess water is now added the position of equilibrium is driven back to the left and the blue colour returns.

`sf([Cu(H_2O)_6]^(2+)+4Cl^(-)rightleftharpoonsCuCl_4^(2-)+6H_2O)`

`stackrel(color(white)(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx))(color(blue)(larr)`

`sf(" "color(blue)(blue)" "color(green)(green))`