Final answer:
The enthalpy of hydration (ΔHh) is the enthalpy change when one mole of gaseous ions become hydrated by water molecules, typically resulting in energy being released as the ions are solvated by water.
Step-by-step explanation:
The enthalpy of hydration (ΔHh) is the change in enthalpy when one mole of gaseous ions becomes hydrated; that is, when they are surrounded by water molecules. This process happens when an ionic substance dissolves in water, and the ions are attracted to the polar water molecules. The enthalpy of hydration can be thought of as the sum of the ion-dipole attractions between the ions and water molecules, where the energy required to separate the ions is considered as positive and the energy released as they are solvated is negative. The overall ΔHh is typically exothermic because more energy is released in solvation than is used to separate the ions.
The value of ΔHh is crucial because it helps to determine the solubility of compounds and the resulting energy changes when they dissolve. For instance, when considering the ion-dipole interaction, factors like the charge density of the ion and the magnitude of the dipole moment of water come into play. A high charge density, as seen with small cations or the bare proton of a hydrogen ion (H+), results in a stronger attraction to the water molecules, leading to a more exothermic hydration process.