Why Does Zinc Hydroxide Dissolve in Ammonium Chloride Solution?

[cyt91]

Zn(II) ions form a white ppt. zinc hydroxide with an aqueous solution of ammonia. The white ppt. dissolves when a ammonium chloride solution is added to it. Which statement explains this observation?

A. a soluble zinc chloride compound is formed
B. ammonia molecule is a stronger ligand than hydroxide ion
C. ammonium ions increase the solubility product of zinc hydroxide
D. ammonium ions decrease the concentration of hydroxide ions in the solution

The answer given is B.
Why not D?

 

[Borek]

Give your reasoning. Just because question doesn't fit template doesn't mean you can show no effort.

 

[cyt91]

Zn(OH)2 <----> 2OH^- + Zn^2+ (equilibrium 1)

When NH4Cl is added, NH4+ undergoes hydrolysis.
NH4+ + H2O ----> NH3 + H3O+

H3O+ removes OH^- in equilibrium 1 and the equilibrium shifts to the right. Therefore, zinc hydroxide dissolves.

This is my reasoning for choosing D.

Or is it the NH3 formed complexes with zinc hydroxide to form the soluble complex
[Zn(NH3)4]^2+ and this formation of soluble complex is more significant towards the solubility of zinc hydroxide?

It seems to me both cases are possible.

 

[Borek]

There is something wrong with B. When you add ammonium chloride you don't increase substantially concentration of ammonia present in the solution. Quite the opposite - you are diluting it (ammonium chloride was added in the form of solution). So pH goes down (think in terms of buffer solution, you add conjugate acid) and concentration of ammonia goes down - B can't be correct, D must be correct.

Also I wonder how A is different from B, after all, zinc complexation means creation of a soluble zinc compound.

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[DeeNos]

Both B and D could potentially explain the observation, but B is a more likely explanation. This is because ammonia is a stronger ligand than hydroxide ion, meaning it forms stronger bonds with the zinc ions. This makes it more likely that the ammonia molecules will replace the hydroxide ions and dissolve the white precipitate. In contrast, D suggests that the presence of ammonium ions is decreasing the concentration of hydroxide ions, which would make it less likely for the precipitate to dissolve. However, it is possible that the decrease in hydroxide ion concentration is not significant enough to explain the complete dissolution of the precipitate. Therefore, B is a more plausible explanation for the observation.