Chapter 14, Problem 75QRT

Interpretation Introduction

Interpretation:

The significant difference between the ionization constant values of ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{3+}$ and ${\left[Fe{\left(H_{2}O\right)}_6\right]}^{2+}$ has to be explained.

Concept introduction:

Hydrated transition metal ions are weak acids.  There are $M-O-H$ bonds in the hydrated metal ions.  Transition metal ions have large charges and small sizes to attract the shared pair of the $M-O$ bonds to the metal ion itself.

This attraction weakens the $O-H$ bond making the hydrogen of the $M-O-H$ bond more acidic than it would be in the $O-H$ bond of a water molecule that is not bonded to a metal ion.  Thus, hydrated transition metal ions can donate proton and the solution becomes acidic.