Question

How does zinc chloride solution react with sodium hydroxide solution, drop wise then to excess?

Answer 1

Zinc chloride is very soluble in water and dissociates:

`ZnCl_(2(s))rarrZn_((aq))^(2+)+2Cl_((aq))^-`

Hydrated zinc ions are quite acidic:

`[Zn(H_2O)_6]^(2+)+H_2Orightleftharpoons[Zn(H_2O)_5(OH)]^++H_3O^+`

The +ve charge on the zinc ions weakens the O - H bond on the water ligand causing some to break to form ions.

This is common for many hydrated metal ions and is referred to as salt hydrolysis .

At low pH the equilibrium lies to the left so the `Zn_((aq))^(2+)` predominates.

If the pH is raised by adding alkali another proton can be removed:

`[Zn(H_2O)_5(OH)]^++OH^(-)rightleftharpoons[Zn(H_2O)_4(OH)_2]darr+H_2O`

You will note that the species `[Zn(H_2O)_4(OH)_2]` is neutral and has little attraction for the polar water molecules so drops out of solution as a white, insoluble precipitate.

If further alkali is added another proton is removed:

`[Zn(H_2O)_4(OH)_2]darr+OH^(-)rightleftharpoons[Zn(H_2O)_3(OH)_3]^(-)+H_2O`

And finally:

`[Zn(H_2O)_3(OH)_3]^(-)+OH^(-)rightleftharpoons[Zn(H_2O)_2(OH)_4]^(2-)+H_2O`

So the white precipitate dissolves in excess alkali to give a soluble species referred to as the zincate ion.

The ability for zinc to form soluble ions in both acid and basic conditions means it is amphoteric.

n.b. some references may also describe `Zn_((aq))^(2+)` as `[Zn(H_2O)_4]^(2+)`. The chemistry is still the same.