The Effect of Ionic Charge & Radius on Enthalpy (OCR A Level Chemistry)

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The Effect of Ionic Charge & Radius on Enthalpy

Factors affecting lattice enthalpy

  • The two key factors which affect lattice energy, ΔHlatt, are the charge and radius of the ions that make up the crystalline lattice

Ionic radius

  • The lattice energy becomes less exothermic as the ionic radius of the ions increases
  • This is because the charge on the ions is more spread out over the ion when the ions are larger
  • The ions are also further apart from each other in the lattice
    • The attraction between ions is between the centres of the ions involved, so the bigger the ions the bigger the distance between the centre of the ions

  • Therefore, the electrostatic forces of attraction between the oppositely charged ions in the lattice are weaker
  • For example, the lattice energy of caesium fluoride (CsF) is less exothermic than the lattice energy of potassium fluoride (KF)
    • Since both compounds contain a fluoride (F-) ion, the difference in lattice energy must be due to the caesium (Cs+) ion in CsF and potassium (K+) ion in KF
    • Potassium is a Group 1 and Period 4 element
    • Caesium is a Group 1 and Period 6 element
    • This means that the Cs+ ion is larger than the K+ ion
    • There are weaker electrostatic forces of attraction between the Cs+ and F- ions compared to K+ and F- ions
    • As a result, the lattice energy of CsF is less exothermic than that of KF

Chemical Energetics - Lattice Energies against Ionic Radius, downloadable AS & A Level Chemistry revision notes

The lattice energies get less exothermic as the ionic radius of the ions increases

Ionic charge

  • The lattice energy gets more exothermic as the ionic charge of the ions increases
  • The greater the ionic charge, the higher the charge density
  • This results in stronger electrostatic attraction between the oppositely charged ions in the lattice
  • As a result, the lattice energy is more exothermic
  • For example, the lattice energy of calcium oxide (CaO) is more exothermic than the lattice energy of potassium chloride (KCl)
    • Calcium oxide is an ionic compound which consists of calcium (Ca2+) and oxide (O2-) ions
    • Potassium chloride is formed from potassium (K+) and chloride (Cl-) ions
    • The ions in calcium oxide have a greater ionic charge than the ions in potassium chloride
    • This means that the electrostatic forces of attraction are stronger between the Ca2+ and O2-compared to the forces between K+ and Cl-
    • Therefore, the lattice energy of calcium oxide is more exothermic, as more energy is released upon its formation from its gaseous ions
    • Ca2+ and O2- are also smaller ions than K+ and Cl-, so this also adds to the value for the lattice energy being more exothermic

Factors affecting enthalpy of hydration

  • 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

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.