Wed November 07, 2012 By:

Why does hydration enthalpy decrease with decrease in size.

Expert Reply
Fri November 09, 2012
Enthalpy of hydrationHhyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process,
Mz+(g) + mH2® Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution. 
As the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration. For exmple; 
Enthalpy of Hydration (Hhyd kJ/mol) of Some Typical Ions
IonHhyd      IonHhyd      IonHhyd
H+ -1130 Al3+ -4665 Fe3+ -4430
Li+ -520               Be2+ -2494            F- -505
Na+ -406               Mg2+ -1921           Cl- -363

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