Al and Zn both form hydroxides when reacted with an NH3 sloution and NaOH solution
The hydroxides are both amphoteric
When you add excess NH3 the Zn ppt dissolves, but Aluminium stays
When you add excess NaOH both ppts dissolve
why doesn't the aluminum ppt dissolve in NH3?
Do the hydroxides form complex ions when reacted with either solution and if they do what are those ions
thank you very much for your help
marysia
Al^+3 ions form Al(OH)3 when reacted with NaOH which is a ppt. The ppt dissolves in excess NaOH because of the formation of the complex ion Al(OH)4^-.
Zn ion does the same.
Zn+2 + 2OH^- ==> Zn(OH)2(s)
In excess OH^-, the ppt dissolves because of the formation of the zincate ion.
Zn(OH)2 + 2OH^- ==> Zn(OH)4^-2
The story is a little different for NH3.
For Al^+3, the NH3 forms Al(OH)3(s) but on addition of excess NH3, it does not form a complex ion; therefore, Al(OH)3, although slightly soluble in excess NH3, will not dissolve.
Zn^+2 + NH3 ==> Zn(OH)2(s)
With excess NH3, it dissolves due to the formation of the ammonia complex ion.
Zn(OH)2 + excess NH3 ==> Zn(NH3)4^+ which is soluble.
thanks very much
:)
love marysia
The reason why the aluminum hydroxide (Al(OH)3) precipitate does not dissolve in excess NH3 (ammonia) is due to the formation of a complex ion. When Al(OH)3 reacts with NH3, it forms a complex ion called [Al(H2O)6]3+. This complex ion is not readily soluble in ammonia solution, preventing the dissolution of the aluminum hydroxide precipitate.
On the other hand, the zinc hydroxide (Zn(OH)2) precipitate dissolves in excess NH3 because it forms a soluble complex ion called [Zn(NH3)4]2+. This complex ion has a greater stability and solubility in ammonia solution compared to the precipitated zinc hydroxide.
Both hydroxides, aluminum hydroxide and zinc hydroxide, also form complex ions when reacted with NaOH (sodium hydroxide) solution. The aluminum hydroxide forms the complex ion [Al(OH)4]−, while the zinc hydroxide forms the complex ion [Zn(OH)4]2−. Both of these complex ions are soluble in excess NaOH, resulting in the dissolution of the precipitates.
The difference in behavior between aluminum hydroxide (Al(OH)3) and zinc hydroxide (Zn(OH)2) when reacted with NH3 (ammonia) and NaOH (sodium hydroxide) solutions is due to their differing chemical properties.
When aluminum hydroxide reacts with NH3, it forms a complex ion called "aluminate ion" (Al(OH)4-) in solution. This complex ion is stable and does not precipitate out of the solution. On the other hand, when excess NH3 is added, the zinc hydroxide dissolves because it forms a soluble complex ion, namely "tetraamminezinc ion" (Zn(NH3)4^2+).
However, when both aluminum hydroxide and zinc hydroxide are treated with excess NaOH, both precipitates dissolve. The hydroxides form soluble "complex ions" when reacted with NaOH. For aluminum hydroxide, it forms a complex ion called "aluminate ion" (Al(OH)4-), while zinc hydroxide forms "tetrahydroxyzinc ion" (Zn(OH)4^2-).
So, to summarize:
1. When reacted with NH3:
- Aluminum hydroxide forms an aluminate ion (Al(OH)4-) which stays in solution.
- Zinc hydroxide forms a tetraamminezinc ion (Zn(NH3)4^2+) which gradually dissolves.
2. When reacted with NaOH:
- Both aluminum hydroxide and zinc hydroxide form complex ions: aluminate ion (Al(OH)4-) and tetrahydroxyzinc ion (Zn(OH)4^2-), respectively. Both precipitates dissolve.
The reason why aluminum hydroxide remains insoluble even with excess NH3 is because the formation of the aluminate ion stabilizes it, preventing it from dissolving in the solution. Zinc hydroxide, on the other hand, forms a more soluble complex ion with NH3, allowing it to dissolve.
I hope this explanation helps! If you have any more questions, feel free to ask.