It is the enthalpy change when 1 mole of gaseous ions are added with excess of water to form aqueous ions.It is always negative as hydration phenomena is exothermic
Hydration is defined as the sticking of water molecules (polar) around its ions.In general term is solvation,when solvent happen to be other than water
Factor affecting hydration energy:
a)Charge on ions:- magnitude of charge increases it mean magnitude of hydration energy increases
b) Size of ions:- Size of ions increase it mean amgnitude os hydration energy increases
c) Greater is the polarity of solvent or greater is the di-electric constant,magnitude of solvation energy will be increase Trends:- Li+>Na+>k+>Rb+>Cs+ depend on size factor due to decrease hydration energy
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