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.
According to the added ammonia amount, you will get zinc hydroxide or tetraamminezinc(ii) ion complex.
Aqueous ammonia solution contains hydroxyl ions (OH-) due to partial dissociation as below. Therefore, aqueous ammonia solution show higher pH values than 7.
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.
If you add more aqueous ammonia soltion to the solution which contains zinc hydroxide precipitate, colourless solution is formed by dissolving the white precipitate.
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.
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