23.2.4—Explain, in qualitative terms, the effect

Hydration enthalpy measures attraction between an ion and polar water molecules. Higher charge density gives stronger attraction and a more negative hydration enthalpy.

Compare charge first, then radius among ions of the same charge. Smaller ions place charge closer to water dipoles, but hydration also depends on the ion’s coordination environment.

Mg²⁺ has a more negative hydration enthalpy than Na⁺ because of its higher charge density; Na⁺ is more strongly hydrated than K⁺ because it is smaller.

Do not claim that every larger ion has a more negative value; for a fixed charge, the trend is usually toward less exothermic hydration.