No, iron(II) hydroxide (Fe(OH)₂) is not soluble in ammonia. It forms a precipitate that remains insoluble even in excess aqueous ammonia.
Iron(II) hydroxide, a greenish-white solid, behaves differently from some other transition metal hydroxides when exposed to ammonia. While certain metal hydroxides like copper(II) hydroxide or zinc hydroxide dissolve in excess ammonia to form stable, soluble ammine complexes, iron(II) hydroxide does not readily form such complexes.
Why Iron(II) Hydroxide is Insoluble in Ammonia
The insolubility of iron(II) hydroxide in ammonia can be attributed to the following:
- Lack of Stable Ammine Complex Formation: Iron(II) ions do not form sufficiently stable soluble ammine complexes (e.g., [Fe(NH₃)₆]²⁺) with ammonia to overcome the low solubility product of Fe(OH)₂.
- Low Solubility Product (Ksp): Iron(II) hydroxide has a very low solubility product, meaning it is inherently difficult to dissolve in aqueous solutions, including ammonia.
The Oxidation Factor
It's also crucial to note that iron(II) compounds are easily oxidized. Iron(II) hydroxide quickly oxidizes to iron(III) hydroxide (Fe(OH)₃) in the presence of air or other oxidizing agents. Iron(III) hydroxide is a reddish-brown precipitate that is also insoluble in ammonia. This rapid oxidation often complicates observations in experiments involving Fe(OH)₂.
Solubility Comparison of Metal Hydroxides in Ammonia
Understanding how different metal hydroxides interact with ammonia is key in qualitative inorganic analysis. The table below illustrates the behavior of iron(II) hydroxide compared to other common metal hydroxides:
| Metal Hydroxide | Formula | Typical Color | Solubility in Aqueous Ammonia | Explanation/Observation |
|---|---|---|---|---|
| Iron(II) Hydroxide | Fe(OH)₂ | Greenish-white | Insoluble | Does not form a stable ammine complex. Rapidly oxidizes to Fe(OH)₃ (also insoluble) in air. |
| Copper(II) Hydroxide | Cu(OH)₂ | Blue | Soluble (forms deep blue solution) | Forms the soluble tetraamminecopper(II) complex, [Cu(NH₃)₄]²⁺. |
| Zinc Hydroxide | Zn(OH)₂ | White | Soluble (forms colorless solution) | Forms the soluble tetraamminezinc(II) complex, [Zn(NH₃)₄]²⁺, or tetrahydroxozincate(II) in excess strong base. |
| Aluminum Hydroxide | Al(OH)₃ | White | Insoluble | Does not form an ammine complex; amphoteric (soluble in strong acids and bases, not ammonia). |
| Magnesium Hydroxide | Mg(OH)₂ | White | Insoluble | Does not form an ammine complex. |
For more detailed information on solubility and complex ion formation, refer to resources on chemical solubility and coordination chemistry.
Practical Implications
In laboratory settings, the insolubility of iron(II) hydroxide in ammonia is a distinguishing characteristic. When aqueous ammonia is added to a solution containing Fe²⁺ ions, a precipitate of Fe(OH)₂ will form and will not redissolve even with excess ammonia. This behavior helps differentiate Fe²⁺ from other transition metal ions like Cu²⁺ or Zn²⁺ that form soluble ammine complexes. Due to its rapid oxidation, it's often observed as a mixture or transition from green to reddish-brown.
