Hydration Energy
The energy released when one mole of gaseous ions is surrounded by water molecules to form a dilute aqueous solution.
Formula / equation
ΔH_hyd ∝ z / r
Unit
kJ/mol
In depth
Hydration energy increases with ionic charge and decreases with ionic radius, so small, highly charged ions are the most strongly hydrated. Whether a salt dissolves depends on how the sum of the hydration energies compares with the lattice energy that must be overcome.
Examples in the real world
Al³⁺ releases about −4665 kJ/mol on hydration, against Na⁺'s −406 kJ/mol.