Why is #Na^+# a WEAKER Lewis acid than #Fe^(3+)#?

1 Answer
Jul 14, 2016

Answer:

This is because the sodium ion in #[Na(OH_2)_6]^+# is not particularly polarizing.

Explanation:

We would normally write #[Na(OH_2)_6]^+# as #Na^+(aq)#. Nevertheless, here the sodium ion is an aquated ion, and not particularly polarizing with respect to electron density. Compare this to #[Fe(OH_2)_6]^(3+)# or #Fe^(3+)(aq)#. Here, the valence electrons poorly shield the nuclear charge, and electron density is shunted towards the iron centre. The result is that the #O-H# bond is weakened, and the iron complex is weakly acidic.

I do not know offhand what the #pK_a# values of the iron complex is, but I suspect it would be a reasonable Bronsted acid. If you find quantitative data, please post them here.