Why adding NH4OH to Al3+ , forms Al(OH)3 and not complex ion Al(NH3)3 ?
thanks so much
Al(OH)3 is more stable than the Al(NH3)3 complex ion.
When NH4OH, also known as ammonia solution, is added to Al3+ ions, the reaction that occurs is as follows:
Al3+ + 3NH4OH → Al(OH)3 + 3NH4+
In this reaction, Al3+ ions are combined with hydroxide ions (OH-) from NH4OH to form Al(OH)3, which is aluminum hydroxide. On the other hand, NH4+ ions remain as NH4+ ions.
Now, let's dive into the reasons why Al(OH)3 is formed instead of the formation of a complex ion like Al(NH3)3.
1. Strength of Ligands:
In this case, NH4OH acts as a ligand. Ligands are molecules or ions that have the ability to donate a pair of electrons to a metal ion (Al3+ in this case) to form a coordinate bond. The strength of the ligand determines the likelihood of forming a complex.
Ammonia (NH3) is a relatively weak ligand compared to hydroxide (OH-) ion. Hydroxide is a stronger base and has a higher affinity to donate electrons compared to ammonia. Therefore, hydroxide ions are preferred over ammonia as ligands when reacting with Al3+ ions.
2. Stability Constant:
Stability constants play a crucial role in determining whether a complex ion will form or not. Complex ions will only form if the stability constant is sufficiently high.
In this case, the stability constant of Al(OH)3 is higher compared to that of Al(NH3)3. When the stability constant of a particular complex is higher, it means that the complex ion formed is more stable. Consequently, the reaction is more likely to favor the formation of the more stable complex.
3. pH of the Solution:
The pH of the solution also affects the formation of complexes. When NH4OH dissolves in water, it acts as a weak base and increases the concentration of hydroxide ions (OH-) in the solution. The increased concentration of hydroxide ions favors the formation of Al(OH)3 over Al(NH3)3.
In summary, the preference for the formation of Al(OH)3 instead of Al(NH3)3 when NH4OH is added to Al3+ ions is due to the strength of ligands, stability constants, and pH of the solution.