Hydration energy


Hydration energy is the amount of energy released when one mole of ions undergo hydration. Hydration energy is one component in the quantitative analysis of solvation. It is a particular special case of water. The value of hydration energies is one of the most challenging aspects of structural prediction. Upon dissolving a salt in water, the cations and anions interact with the positive and negative dipoles of the water. The trade-off of these interactions vs those within the crystalline solid comprises the hydration energy.
Hydration energy is correlated with ionic radius of cations and anions, as done by Wendell Latimer.

Examples

If the hydration energy is greater than the lattice energy, then the enthalpy of solution is negative, otherwise it is positive.
The hydration energy should not be confused with solvation energy, which is the change in Gibb's free energy as solute in the gaseous state is dissolved. If the solvation energy is positive, then the solvation process is endergonic; otherwise, it is exergonic.
For instance, water warms when treated with CaCl2 as a consequence of the large heat of hydration. However, the hexahydrate, CaCl2·6H2O cools the water upon dissolution. The latter happens because the hydration energy does not completely overcome the lattice energy, and the remainder has to be taken from the water in order to compensate the energy loss.
The hydration energies of the gaseous Li+, Na+, and Cs+ are respectively 520, 405, and 265 kJ/mol.