Study of Compounds — Ammonia — Question 24
Back to all questionsQuestion 24
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.
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