Enthalpy change of hydration is the energy change when one mole of gaseous ions dissolves in water to form hydrated ions. This process involves water molecules surrounding the ion, and is typically exothermic, meaning energy is released.
The magnitude of hydration enthalpy depends on two main factors. First, the charge on the ion - higher charged ions have more negative hydration enthalpies. Second, the size of the ion - smaller ions have more negative hydration enthalpies due to stronger ion-dipole interactions with water molecules.
Hydration enthalpy plays a crucial role in Born-Haber cycles. These cycles help us calculate lattice enthalpies and enthalpies of solution. The enthalpy of solution equals the sum of lattice enthalpy and hydration enthalpy, showing how these thermodynamic quantities are interconnected.
The graph shows clear trends in hydration enthalpy. For Group 1 metals, hydration enthalpy becomes less negative as we go down the group due to increasing ionic size. Group 2 metals have much more negative hydration enthalpies due to their higher charge, following the same size trend. This demonstrates how charge density controls the strength of ion-water interactions.
In summary, hydration enthalpy is a fundamental thermodynamic property that describes the energy change when gaseous ions dissolve in water. It's always exothermic and depends on the charge density of ions. This concept is essential for understanding solubility, predicting ionic behavior, and performing thermodynamic calculations in chemistry and related fields.