[عاطف خليفة]

Transition-Metal Ions as Brønsted Acids

It is easy to understand why aqueous solutions of HCl or CH3CO2H are acidic. The following data for the pH of 0.1 M solutions of transition-metal ions are a bit harder to explain.

FeCl3: pH = 2.0AlCl3: pH = 3.0Cu(NO3)2: pH = 4.0

We can't attribute the acidity of these solutions to the Cl- or NO3- ions because these ions are weak bases. The acidity of these solutions must result from the behavior of the Fe3+, Al3+, and Cu2+ ions.
The Fe3+, Al3+, and Cu2+ ions can't be Brnsted acids by themselves. They can only act as proton donors by influencing the ability of the neighboring water molecules to give up H+ ions. They do this by first forming covalent bonds to six water molecules to form a complex ion.
Water molecules covalently bound to one of these metal ions are more acidic than normal. Thus, reactions such as the following occur.

Al(H2O)63+(aq) + H2O(l) Al(H2O)5(OH)2+(aq) + H3O+(aq)

Fe(H2O)63+(aq) + H2O(l) Fe(H2O)5(OH)2+(aq) + H3O+(aq)
These reactions give rise to a net increase in the H3O+ ion concentration in these solutions, thereby making the solutions acidic.