Question

What happens when ammonia solution is added
(a) dropwise and then
(b) in excess to the following solutions:
(i) CuSO₄
(ii) ZnSO₄
(iii) FeCl₃
(c) write balanced reactions.

Answer 1

Salt solution	Addition of ammonia solution
	In drop	In excess
(i) CuSO4 (blue solution)	It forms pale blue precipitate of cupric hydroxide. $$\begin{gathered} {\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \mathrm{Cu}(\mathrm{OH}{)}_2(\mathrm{s})+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \\ \left(\mathrm{Blue}\right)\mathrm{Cupric}\mathrm{hydroxide} \\ (\mathrm{Pale}\mathrm{blue}\mathrm{ppt}) \end{gathered}$$	Formed precipitate dissolves in excess ammonia solution to form deep blue complex tetrammine copper(II) sulphate. $$\begin{gathered} \mathrm{Cu}(\mathrm{OH}{)}_2\left(\mathrm{s}\right)+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \left[\mathrm{Cu}\right({\mathrm{NH}}_3{)}_4]{\mathrm{SO}}_4(\mathrm{aq})+4{\mathrm{H}}_2\mathrm{O}(\mathrm{l}) \\ (\mathrm{Pale}\mathrm{blue}\mathrm{ppt}.)(\mathrm{In}\mathrm{excess})\mathrm{Tetrammine}\mathrm{copper}(\mathrm{II}) \\ \mathrm{sulphate}(\mathrm{deep}\mathrm{blue}) \end{gathered}$$
(ii)ZnSO4 (colourless solution)	It forms white gelatinous precipitate of zinc hydroxide. $$\begin{gathered} {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \mathrm{Zn}(\mathrm{OH}{)}_2(\mathrm{s})+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \\ \left(\mathrm{Colourless}\right)\mathrm{Zinc}\mathrm{hydroxide} \\ (\mathrm{White}\mathrm{gelatinous}\mathrm{ppt}) \end{gathered}$$	Formed precipitate dissolves in excess ammonia solution to form colourless complex tetrammine zinc(II) sulphate. $$\begin{gathered} \mathrm{Zn}(\mathrm{OH}{)}_2\left(\mathrm{s}\right)+({\mathrm{NH}}_4{)}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \left[\mathrm{Zn}\right({\mathrm{NH}}_3{)}_4]{\mathrm{SO}}_4(\mathrm{aq})+4{\mathrm{H}}_2\mathrm{O}(\mathrm{l}) \\ \mathrm{White}\mathrm{gelatanous}\mathrm{ppt}(\mathrm{In}\mathrm{excess})\mathrm{Tetrammine}\mathrm{Zinc}(\mathrm{II}) \\ \mathrm{sulphate}(\mathrm{colourless}) \end{gathered}$$
(iii) FeCl3 (yellow solution)	It forms reddish brown precipitate of ferric hydroxide. $$\begin{gathered} {\mathrm{FeCl}}_3\left(\mathrm{aq}\right)+3{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)\to \mathrm{Fe}(\mathrm{OH}{)}_3(\mathrm{s})+3{\mathrm{NH}}_4\mathrm{Cl}(\mathrm{aq}) \\ \mathrm{Yellow}\mathrm{Ferric}\mathrm{hydroxide} \\ (\mathrm{reddish}\mathrm{brown}\mathrm{ppt}) \end{gathered}$$	Formed precipitate is insoluble in excess ammonium hydroxide solution.