Hydration energy (also hydration enthalpy) 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 dissolution energy, with the solvent being water. The value of hydration energies is one of the more challenging aspects of structure 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.
If the hydration energy is greater than the lattice energy, then the solvation energy is positive, otherwise it is negative.
For instance, water warms when treated with CaCl2 (anhydrous calcium chloride) 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.
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