Explain why the enthalpy of lattice dissociation of potassium oxide is less endothermic than that of sodium oxide.

Lattice dissociation enthalpy is the enthalpy change when one mole of a gaseous ionic lattice dissociates into isolated gaseous ions. The process is endothermic because energy is required to overcome the electrostatic attraction between oppositely charged ions. Sodium and potassium ions both have the same charge (+1) but the potassium ion is larger so the electrostatic forces of attraction are weaker in potassium oxide. Hence less energy is required to separate the ions making the enthalpy of lattice dissociation less endothermic.