5.1.9 Factors Affecting Enthalpy of Hydration

• The standard enthalpy change of hydration (ΔH_hyd^ꝋ) is affected by the amount that the ions are attracted to the water molecules
• The factors which affect this attraction are the ionic charge and radius

Ionic radius

• ΔH_hyd^ꝋ becomes more exothermic with decreasing ionic radii
  • Smaller ions have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution
  • Therefore, more energy is released when they become hydrated and ΔH_hyd^ꝋ becomes more exothermic

• For example, the ΔH_hyd^ꝋ of magnesium sulfate (MgSO₄) is more exothermic than the ΔH_hyd^ꝋ of barium sulfate (BaSO₄)
  • Since both compounds contain a sulfate (SO₄^2-) ion, the difference in ΔH_hyd^ꝋ must be due to the magnesium (Mg²⁺) ion in MgSO₄ and barium (Ba²⁺) ion in BaSO₄
  • Magnesium is a Group 2 and Period 3 element
  • Barium is a Group 2 and Period 6 element
  • This means that the Mg²⁺ ion is smaller than the Ba²⁺ ion
  • The attraction is therefore much stronger for the Mg²⁺ ion
  • As a result, the standard enthalpy of hydration of MgSO₄ is more exothermic than that of BaSO₄

Ionic charge

• ΔH_hyd^ꝋ is more exothermic for ions with larger ionic charges
  • Ions with large ionic charges have a greater charge density resulting in stronger ion-dipole attractions between the water molecules and the ions in the solution
  • Therefore, more energy is released when they become hydrated and ΔH_hyd^ꝋ becomes more exothermic

• For example, the ΔH_hyd^ꝋ of calcium oxide (CaO) is more exothermic than the ΔH_hyd^ꝋ of potassium chloride (KCl)
  • Calcium oxide is an ionic compound that consists of calcium (Ca²⁺) and oxide (O^2-) ions
  • Potassium chloride is formed from potassium (K⁺) and chloride (Cl^-) ions
  • Both of the ions in calcium oxide have a greater ionic charge than the ions in potassium chloride
  • This means that the attractions are stronger between the water molecules and Ca²⁺ and O^2- ions upon hydration of CaO
  • The attractions are weaker between the water molecules and K⁺ and Cl^- ions upon hydration of KCl
  • Therefore, the ΔH_hyd^ꝋ of calcium oxide is more exothermic as more energy is released upon its hydration

The enthalpy of hydration is more exothermic for smaller ions and for ions with a greater ionic charge