• The standard enthalpy change of hydration (ΔH_hyd^?) 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^??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 ΔHhyd??becomes?more?exothermic
• For example, the ΔH_hyd^??of magnesium sulfate (MgSO₄) is?more exothermic?than the ΔH_hyd^??of barium sulfate (BaSO₄)
  • Since both compounds contain a sulfate (SO₄^2-) ion, the difference in ΔHhyd??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^??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 ΔHhyd??becomes?more?exothermic
• For example, the ΔH_hyd^??of calcium oxide (CaO) is?more exothermic?than the ΔHhyd??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 ΔHhyd??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