Question

How is the tetraamminecopper(II) hydroxide complex formed in the reaction of copper salt with NH₄OH?

Answer 1

When ammonium hydroxide (NH₄OH) is added to a copper(II) salt solution, a pale blue precipitate of copper(II) hydroxide, Cu(OH)₂, is first formed.

\[\ce{CuSO4​ + 2NH4​OH → Cu(OH)2​↓ + (NH4​)2​SO4​}\]

On adding excess ammonium hydroxide, the pale blue precipitate dissolves and forms a deep blue solution of tetraamminecopper(II) hydroxide.

\[\ce{Cu(OH)2​ + 4NH4​OH → [Cu(NH3​)4​](OH)2​ + 4H2​O}\]

Thus, the tetraamminecopper(II) hydroxide complex is formed by the reaction of copper(II) hydroxide with excess ammonium hydroxide, producing the characteristic deep blue colour.