CIE A Level Chemistry (9701) exams from 2022

Revision Notes

5.1.9 Factors Affecting Enthalpy of Hydration

Enthalpy of Hydration: Ionic Charge & Radius

  • The standard enthalpy change of hydration (ΔHhyd) 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

  • ΔHhyd 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 ΔHhydbecomes more exothermic
  • For example, the ΔHhyd of magnesium sulfate (MgSO4) is more exothermic than the ΔHhyd of barium sulfate (BaSO4)
    • Since both compounds contain a sulfate (SO42-) 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

  • ΔHhyd 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 ΔHhydbecomes more exothermic
  • For example, the ΔHhydof calcium oxide (CaO) is more exothermic than the ΔHhydof potassium chloride (KCl)
    • Calcium oxide is an ionic compound that consists of calcium (Ca2+) and oxide (O2-) 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 O2- 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

Chemical Energetics - Trends Enthalpy of Hydration, downloadable AS & A Level Chemistry revision notes

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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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