Hydration energy is the heat change accompanying the formation of 1 mole of a specific hydrate. Hydration energy of [Ba(H₂O₂)₂]²⁺, [Cu(H₂O₄]²⁺ and [AI(H₂O)₆]³⁺ are approximately 30, 75 and 320 KJ/mol(-ve, exothermic), respectively. This large variation in hydration ratio for the action (AI³⁺ > Cu²⁺ >Ba²⁺) is responsible for high hydration energy . However, closely associated to hydration is the phenomenon of hydrolysis, such as:
                                                        Hydration                         Hydrolysis
                               Fe³⁺ + 6H₂O      →        [Fe(H₂O)₆]³⁺    →          [Fe(H₂O)₅(OH)²⁺ +  H⁺

Small and highly charged cation, that is, large charge-to-size ratio, can lead to the rupture of O-H bonds in the hydrated cation to yield hydrolyzed species and the hydrated proton as shown:
                               [ M(H₂O)n ]^M+_(aq) + H₂O  <==>    [ M (H₂O)_n-1 (OH ] _(aq) ^(m-1)+ + H₃O⁺.

The larger the charge-to-size (acidity) of the cation, the more polar the O-H bond in the coordinated H₂O and the more easily a proton is released as H₃O⁺. On the basis  of charge-to-size ratio, the relative acid strength decreases as :
              [ Fe(H₂O)₆ ]³⁺ > [ V (H₂O)₆]³⁺ > [Cr(H₂O_6]³⁺ > [Sc(H₂O₆]³⁺ > [AI (H₂O)₆]³⁺ > [La (H₂O)₆]³⁺

Among divalent hydrated cations, the acid strength decrease in the order:
              [ Be(H₂O)_n ]2+ > [Cu(H₂O)_n]²⁺ > [ Mg(H₂O)_n]²⁺ > [Ca(H₂O)_n]²⁺ > [ Ba(H₂O)_n ]²⁺


For more help in  Acid Strength of Hydrated Cations click the button below to submit your homework assignment