A chemical reaction which the hydration energy is greater than the lattice energy is referred to as

When an ionic solid dissolves in water, two energy changes compete:

• Breaking the ionic lattice (lattice enthalpy) requires energy — it is endothermic.
• Hydrating the separated ions (hydration enthalpy) releases energy as water molecules surround the ions — it is exothermic.

The overall enthalpy of solution is:

\(\Delta H_{\text{sol}} = \Delta H_{\text{latt}} + \Delta H_{\text{hyd}}\)

If the magnitude of the hydration enthalpy exceeds the lattice enthalpy (i.e., \(|\Delta H_{\text{hyd}}| > \Delta H_{\text{latt}}|\)), then \(\Delta H_{\text{sol}} < 0\). That means the dissolution releases heat and is therefore exothermic.

Note: Exothermic does not automatically mean spontaneous; spontaneity depends on \(\Delta G = \Delta H - T\Delta S\).