Factors Affecting Hydration Enthalpies

Factors Affecting Hydration Enthalpies: Hydration enthalpies affected by ionic size and ionic
charge in same way as lattice enthalpy.
 Effect of Ionic Size: Ionic size increase down a group.
– MN+ is has a smaller ionic radius than Y+.
– Attraction between ion and water molecules decreases.
– Hydration energy less exothermic.
 Effect of Ionic Charge:
– M+ has greater charge than Y+.
– Attraction between ion and water molecules increases.
– Hydration energy more exothermic.
 If have to compare a number of elements, just write in words something like Na > Mg > Al.
 Predicting Solubility: If hydration enthalpy larger than lattice enthalpy, overall enthalpy charge
exothermic and compound should dissolve. But many compounds with endothermic enthalpy
changes are soluble. Therefore solubility also depends on temperature and entropy.
 You’d expect the enthalpy change of solution to be endothermic as bonds are being broken
however some are exothermic as polar water molecules attract the positive metal ions to form
complex ions.
 Enthalpy Change of Hydration exothermic because the gaseous ions attract the polar water
molecules.
 Explain the difference in enthalpy changes of solution between KF and RbF: K+ has a smaller
ionic radius than Rb+. K+ has stronger electrostatic attraction to F- more than Rb+. ∆H lattice
enthalpy value of K+ more exothermic than Rb+. ∆H Solution affected more by lattice enthalpy
than by hydration enthalpy