23.2.1—Hydration and solution enthalpies

Hydration enthalpy is the enthalpy change when one mole of gaseous ions becomes aqueous. Solution enthalpy is the enthalpy change when one mole of an ionic solid dissolves to form aqueous ions.

Hydration involves ion–water attractions; solution combines lattice separation and hydration. State the ion charge, state symbols and direction before comparing values.

Dissolving NaCl(s) gives Na⁺(aq) and Cl⁻(aq); the solution enthalpy equals lattice dissociation plus the two hydration enthalpies.

Hydration is not the same as dissolving a molecular solute, and solution enthalpy is not automatically exothermic.