Hydration Energy
Definition and meaning of Hydration Energy in chemistry.
Hydration energy is the energy released when one mole of gaseous ions is surrounded by water molecules to form hydrated (aqueous) ions in solution.
In more detail
The process is exothermic because ion-dipole attractions form between the ion and the partially charged oxygen or hydrogen atoms of surrounding water molecules, releasing more energy than is needed to disrupt the water's existing hydrogen-bond network. Hydration energy becomes more negative (larger in magnitude) as ionic charge increases and ionic radius decreases, since both raise the ion's charge density and strengthen ion-dipole attraction. Together with lattice energy, it determines the overall enthalpy of solution for an ionic compound, helping explain why some salts dissolve readily while others do not.
Key facts
| Also called | Ionic hydration enthalpy |
|---|---|
| Sign | Always negative (exothermic process) |
| Main trend | Increases in magnitude with higher charge and smaller ionic radius |
| Field | Physical Chemistry |
The hydration energy of Mg2+ is about -1920 kJ/mol, far more negative than that of Na+ (about -406 kJ/mol), reflecting Mg2+'s smaller radius and higher charge (+2 vs +1).
Frequently asked questions
How is hydration energy related to lattice energy?
Lattice energy is the energy required to separate an ionic solid into gaseous ions, while hydration energy is released when those gaseous ions become surrounded by water. The balance between the two (lattice energy input versus hydration energy released) gives the enthalpy of solution, which determines whether dissolving is exothermic or endothermic.
Why does Mg2+ have a much larger hydration energy than Na+?
Mg2+ carries twice the charge of Na+ and has a smaller ionic radius, giving it a much higher charge density. This attracts surrounding water molecules more strongly, releasing more energy upon hydration.