Hydration enthalpy is the energy released when 1 mol of a substance is dissolved into a solution.
It is always negative, as bonds (e.g. dative, ion-dipole) are being formed. When the solvent is other than water the process is referred to as solvation..
The enthalpy change of a solution is equal to the sum of three different energy transfer processes
first is the lattice energy, the second is enthalpy change of hydration of cation and the third is the
enthalpy change of hydration of anion.
By definition, the enthalpy change of hydration Delta H hyd is the enthalpy change that takes place when 1 mole of gaseous ion is dissolved in excess water to make one mole aqueous ion.
Significance of hydration enthalpy
It gives an indication of the strength of the interaction of water molecules with the ions.
Enthalpy change of hydration is always exothermic since it involves bond making
Lattice energy contribution is always endothermic since as it involves bond breaking
of positive and negative ions in the molecule. The lattice energy and sum of two
enthalpy changes of hydration have roughly same magnitude but are of oposte sign
So overall, the enthalpy change of solution is very small.
Example : Hydration of CuSO4
As explained above, the process can be explained as written below:
delta ( hydration ) H CuSO4 = ( delta molar enthalpy of CuSO4 solution - delta molar enthalpy of CuSO4.5H2O)
delta molar enthalpy of CuSO4 solution = - 66.2 kj/mol
delta molar enthalpy of CuSO4.5H2O = + 11.5 kj / mol
delta ( hydration ) H CuSO4 , would therefore be equal to [ - 66.2 - ( - 11.5 ) ] = - 77.5 kj / mol