When excess ammonium hydroxide is added to ferrous sulfate (FeSO₄), a series of reactions occur resulting in the formation of a green precipitate followed by a reddish-brown precipitate. Here's a breakdown:

Initial Reaction:

* FeSO₄ (aq) + 2NH₄OH (aq) → Fe(OH)₂ (s) + (NH₄)₂SO₄ (aq)

* Ferrous sulfate reacts with ammonium hydroxide to form a pale green precipitate of ferrous hydroxide (Fe(OH)₂).

* Ammonium sulfate ((NH₄)₂SO₄) remains in solution.

Excess Ammonium Hydroxide:

* Fe(OH)₂ (s) + 2NH₄OH (aq) → [Fe(NH₃)₆]²⁺ (aq) + 2H₂O (l) + 2OH⁻ (aq)

* The excess ammonium hydroxide reacts with the ferrous hydroxide to form a soluble complex ion, hexammineiron(II) ion ([Fe(NH₃)₆]²⁺).

* This complex ion is colorless.

Oxidation in Air:

* The hexammineiron(II) ion is unstable in air and readily oxidizes to form a reddish-brown precipitate of ferric hydroxide (Fe(OH)₃).

* 4[Fe(NH₃)₆]²⁺ (aq) + O₂ (g) + 8H₂O (l) → 4Fe(OH)₃ (s) + 8NH₄⁺ (aq) + 4OH⁻ (aq)

Overall Observations:

1. Initial Green Precipitate: The formation of the pale green ferrous hydroxide precipitate is the first visible reaction.

2. Dissolving of the Precipitate: With the addition of excess ammonium hydroxide, the precipitate dissolves due to the formation of the soluble hexammineiron(II) complex.

3. Reddish-Brown Precipitate: The solution eventually turns reddish-brown as the hexammineiron(II) complex oxidizes in air, forming ferric hydroxide.

Important Note: This reaction is a classic example of how excess reagent can lead to the formation of a different product due to complex formation and subsequent oxidation.