Acid Strength Of Hydrated Cations

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Acid Strength of Hydrated Cations

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(H2O6]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+

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