Hi,

I think because the Fe3+ is smaller and has a more positive charge potential than Cu2+, therefore, the electrons between the O-H bond of the water ligands become polarised until an H+ from water is liberated.  This makes the Fe3+ solution more acidic than that of Cu2+.  

In fact it could be written as and equilibrium reaction:

[Fe(H2O)6]3+(aq) equilibrium  [Fe(H2O)5OH]2+(aq)  + H+(aq)

I hope this makes sense and it helps! 

Hallow 

I think, in aqueous solution the aqua complex ions will form, that is [Fe(H2O)6]+3 and [Cu(H2O)6]+2, both are classified as aqua acids. The strength of aqua acids increases with increasing positive charge on the metal and decreasing size. Since Fe+3 has the higher charge, the removal of acidic proton from coordinating water molecule as a result of inductive effect will be much easier and hence a stronger acid

Please refer to "Inorganic Chemistry" by Shriver and Atkins, foundations, acids and bases. 

It completely made sense all your contributions. thanks. I thought as much also

They are all  correct. The simple reason is bcos  Fe3+ has a higher charge density than Cu2+.

The ionic potential of iron (III) due to latice size and electron positions contributes to the observed effect.

Thanks for your generous contributions

The rule in chemistry is simple. The Acidity increases when is higher the oxidation state ( same anion). The justifications mentioned by other authors, is the correct one.

due the small size of Fe+ the electrons of O-H bond of water ligand are more attracted towards oxygen atom which create a more positive charge on Hydrogen atom of which can then easily be removed by water in solution.

Kaana Asemave

FeCl3 is a fairly strong Lewis acid, in which Fe exhibits 5 electrons in d-orbital and 0 electron in s-orbital. During the formation of transition metal complexes, the d- and p-orbitals of Fe are of particular interest. Because of the trigonal planar structure of Fe3+, the ligands interactions are strong with the orbital of Fe having components along z-axis, such as pz, dxz, dyz, or dz2. Moreover, the ligand s and p orbital are perpendicular to the metal p and d orbital in the x and y plane and therefore, having non-bonding interaction. As the Fe3+ has an empty s-orbital, the dz2 is available to form bonding molecular orbital with s-orbital of halogen. Due to symmetry the px and py of ligand will not interact and their electrons will go to anti-bonding orbitals. And the pz-electrons of the ligand will go to bonding orbital. Because of the interactions along z-axis, the electron density locates in the molecular orbitals along z-axis and the dxy- and dx2-y2-orbitals remains empty and destabilized due to higher energy. Because of their higher energy these empty orbitals (dxy and dx2-y2) cannot be filled by the electrons from Fe3+, but these can readily accept electron from a Lewis base and results in acidic solution.