What is Hydration Enthalpy?

Always Negative (Exothermic): Hydration enthalpy is always negative because when water molecules (which are polar) surround an ion, they form ion-dipole attractions, releasing energy in the process.

Factors affecting magnitude:

1. Charge of the ion: Higher charge → more attraction to water → higher (more negative) hydration enthalpy. Al³⁺ has a much higher hydration enthalpy than Na⁺.
2. Size of the ion: Smaller ions have a higher charge density, attracting water more strongly. Li⁺ has higher hydration enthalpy than Cs⁺, though both have +1 charge.

Whether an ionic compound dissolves in water depends on the competition between:

• Lattice Enthalpy (energy to BREAK the crystal lattice — always endothermic/positive)
• Hydration Enthalpy (energy RELEASED when ions are hydrated — always negative)

If |Hydration Enthalpy| > Lattice Enthalpy → Compound dissolves

This explains why NaCl dissolves readily (its hydration enthalpy is large enough to overcome its lattice enthalpy) but some ionic compounds like AgCl do not dissolve (lattice enthalpy is too large).