Question

Why is `ZnCl_2` is an acid though it does not have a `H^+` or a `OH^-`?

Answer 1

ZnCl2 is a Lewis acid because of the following reasons

Zn+2 is a Lewis acid

the chlorine does not hydrolyze so the equation would be like this

`["Zn"("H"_ 2"O")_ 6]_ ((aq))^(2+) + "H"_ 2"O"_ ((l)) rightleftharpoons ["Zn"("H"_ 2"O")_ 5("OH")]_ ((aq))^(+) + "H"_ 3"O"_ ((aq))^(+)`

`H_3O^+` indicates that something is acidic

Another way to determine ZnCl2 is acidic is this

`ZnCl_2 + 2H_2O = Zn(OH)_2 + 2HCl`

`2HCl + Zn(OH)_2` = acidic solution because of HCl is a strong acid so ZnCl2 is acidic.

6M of ZnCl2 has a pH of 3 - 4

Ksp of ZnCl2 cannot be found on the internet so i think to solve from ZnCl2 solubility in water

Solubility of ZnCl2 = `(430g)/(100ml)`

First convert 100ml to 1000ml or 1litre

`(430g)/(100ml) = (4300g)/(1000ml)`

4300g must converted to moles

`"Moles" = "weight"/"molar mass "`

`"4300g"/"136.315g"` = 31.55moles

= 31.55mol/1000ml

Whenever ZnCl2 dissolves it dissolves like this

`ZnCl_2 rightleftharpoons Zn^(+2) + 2Cl^-`

the Ksp expresssion = `[Zn^(+2)] [Cl^-]^2`

The molar ration of 2Cl- to ZnCl2
= 2 : 1

Therefore concentration of 2Cl- is 31.55 * 2 = 63.10mol/L

Ksp = `["31.55mol"/L] ["63.10mol"/L]^2`

= 125619.7955

Answer 2

`"ZnCl"_2` is an acid because `"Zn"^"2+"` ions are Lewis acids.

Explanation:

The zinc ion can accept six electron pairs to form a hydrated zinc ion.

`"Zn"^"2+""(aq)" + "6H"_2"O""(l)" → underbrace(["Zn"("H"_2"O")_6]^"2+""(aq)")_color(red)("hexaaquazinc(II) ion")`

The structure of the hydrated ion is

The electronegative `"Zn"^"2+"` ion removes electron density from the `"O-H"` bonds.

The bonds become weaker, so the water ligands become more acidic.

The aqueous solution forms hydronium ions through reactions like this:

`["Zn"("H"_2"O")_6]^"2+""(aq)" + "H"_2"O(l)" ⇌ underbrace(["Zn"("H"_2"O")_5("OH")]^"+""(aq)")_color(red)("pentaaquahydroxozinc(II) ion") + "H"_3"O"^"+""(aq)"`

I asked a chemist friend to prepare a 6 mol/L solution of `"ZnCl"_2`. The pH was 2.5.