what is hydration enthalpies ?
f 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) + mH2O -------> Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution. The approximate hydration energies of some typical ions are listed here. The table illustrates the point that as the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration.