Chemistry
State why the blue ppt. formed on addition of NH4OH to CuSO4 soln. dissolves to give a deep blue solution with excess of NH4OH. Give an equation for the reaction. State why Zn(OH)2 is soluble in excess of NH4OH.
Ammonia
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Answer
Ammonium hydroxide if first added in small quantity and then in excess to a solution of copper sulphate, a pale blue ppt. of copper hydroxide is formed which dissolves in excess of ammonium hydroxide forming a soluble complex salt.
CuSO4 + 2NH4OH ⟶ (NH4)2SO4 + Cu(OH)2 ↓
Cu(OH)2 + (NH4)2SO4 + 2NH4OH ⟶ [Cu(NH3)4]SO4 + 4H2O
The pale blue ppt. of copper hydroxide is soluble in excess of a NH4OH, since a soluble complex [Cu(NH3)4]SO4 [tetrammine copper [II] sulphate] is formed.
Zinc salt forms a white gelatinous ppt. of Zn(OH)2, which dissolves when excess of ammonium hydroxide is added due to the formation of [Zn(NH3)4]SO4
ZnSO4 + 2NH4OH ⟶ (NH4)2SO4 + Zn(OH)2 ↓
Zn(OH)2 + (NH4)2SO4 + 2NH4OH ⟶ [Zn(NH3)4]SO4 + 4H2O
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Related Questions
Give balanced equations for the reducing reactions of ammonia with
(i) copper (II) oxide,
(ii) lead (II) oxide,
(iii) chlorine using (a) excess NH3 (b) excess Cl2
State how NH4OH is used for identifying:
- Fe2+
- Fe3+
- Pb2+
- Zn2+
- Cu2+ cations.
Give also a balanced equation in each case for a known example.
State five tests for ammonia where a colour change is involved.
State why an aqueous solution of ammonia (NH4OH) is used for identifying cations.