Enthalpies of solution and hydration — FAQ

Both processes involve the formation of attractive forces, which releases energy and is therefore exothermic. Lattice enthalpy involves forming strong electrostatic attractions between oppositely charged gaseous ions to make a solid lattice. Hydration enthalpy involves forming ion-dipole attractions between gaseous ions and polar water molecules.

Yes. The spontaneity of a process is determined by the Gibbs free energy change, $\Delta G = \Delta H - T\Delta S$. When an ionic solid dissolves, there is a large increase in disorder, so the entropy change ($\Delta S$) is large and positive. If this positive entropy term is large enough, it can make $\Delta G$ negative even if $\Delta H$ is positive (endothermic), allowing the salt to dissolve spontaneously.

The Born-Haber cycle is used to determine the lattice enthalpy ($ΔH_{\text{latt}}^{\ominus}$) from other experimental values (like ionisation energy and electron affinity). The enthalpy cycle for solution uses the lattice enthalpy value, which may have been found via a Born-Haber cycle, to calculate the enthalpy of solution. They are two different, but related, applications of Hess's Law.