Factors Affecting Enthalpy of Hydration (Cambridge International A-Level Chemistry)

Enthalpy of Hydration: Ionic Charge & Radius

• The standard enthalpy change of hydration (ΔH_hyd^θ) 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

• The standard enthalpy change of hydration ( ΔH_hyd^θ) becomes more exothermic (more negative) as the ionic radius decreases

• Smaller ions have a higher charge density, resulting in stronger ion–dipole attractions with water molecules

• Therefore, more energy is released when these ions are hydrated

• For example:

  • Mg²⁺ in MgSO₄ is smaller than Ba²⁺ in BaSO₄
  • As a result, ΔH_hyd^θof MgSO₄is more exothermic than that of BaSO₄

Ionic charge

• The enthalpy of hydration becomes more exothermic as the ionic charge increases

• Higher charge leads to a greater charge density, strengthening ion–dipole attractions with water molecules

• This means more energy is released during hydration

• For example:

  • Ca²⁺ and O²⁻ in CaO have higher charges than K⁺ and Cl⁻ in KCl
  • Therefore, ΔH_hyd^θof CaO is more exothermic than that of KCl

Comparing enthalpies of hydration

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