Aqueous Zinc chloride (ZnCl2) reacts with ammonia (NH3) and produce a white colour precipitate, zinc hydroxide (Zn(OH)2). If excess NH3 is added, Zn(OH)2 precipitate dissolves and colourless tetraamminezinc(ii) ion complex is given as the product. In this tutorial, we will study more about these reactions.
Reaction of zinc chloride and ammonia
According to the added ammonia amount, you will get zinc hydroxide or tetraamminezinc(ii) ion complex.
Formation of zinc hydroxide precipitate by ammonia
Aqueous ammonia solution contains hydroxyl ions (OH-) due to partial dissociation as below. Therefore, aqueous ammonia solution show higher pH values than 7.
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
When concentration of OH- ion is enough to make the zinc hydroxide precipitate as solubility product value suggests, you will see the white precipitate generation.
Zn2+(aq) + OH-(aq) ⇌ Zn(OH)2(s)
- Equilibrium point of this equilibrium reaction is usually shifted to the right side (Ksp = 1.46 * 10-10 mol2 dm-6).
Formation of tetraamminezinc(ii) ion complex
If you add more aqueous ammonia soltion to the solution which contains zinc hydroxide precipitate, colourless solution is formed by dissolving the white precipitate.
Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]+(aq) + 2OH-(aq)
Adding concentrated ammonia solution to zinc chloride solution
If concentrated ammonia solution is added to zinc chloride solution, probably you will not see the formation of white precipitate as mentioned above because due to higher concentration of ammonia, tetraamminezinc(ii) ion coordination complex is immediately formed.
https://www.chemistryscl.com/reactions/ZnCl2+NH3/index.php
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