WAEC - Chemistry (2021 - No. 18)
What would be observed when aqueous ammonia is added in drops and then in excess to a solution of copper(II) ions?
blue precipitate is formed which is soluble in excess ammonia
brick red precipitate is produced which is insoluble in excess ammonia
white precipitate is formed which is excess in ammonia
green precipitate is formed which is insoluble in excess ammonia
Giải thích
When a few drops of ammonia solution is added to copper(II) ions, a blue precipitate of copper(II) hydroxide is formed.
CuSO₄(aq) + 2NH₃(aq) + 2H₂O(ℓ) ⇌ Cu(OH)₂(s) + (NH₄)₂SO₄(aq)
However, when ammonia solution is added in excess, the blue precipitate re-dissolves to form a deep blue solution of tetraamine copper(II) hydroxide.
Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄](OH)₂(aq). Therefore, the correct answer is option A.