The Effect of Ionic Charge & Radius on Enthalpy
Factors affecting lattice enthalpy
- The two key factors which affect lattice energy, ΔHlattꝋ, 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
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 (ΔHhydꝋ) 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
- ΔHhydꝋ 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 ΔHhydꝋ becomes more exothermic
- For example, the ΔHhydꝋ of magnesium sulfate (MgSO4) is more exothermic than the ΔHhydꝋ of barium sulfate (BaSO4)
- Since both compounds contain a sulfate (SO42-) ion, the difference in ΔHhydꝋ 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
- ΔHhydꝋ 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 ΔHhydꝋbecomes more exothermic
- For example, the ΔHhydꝋ of calcium oxide (CaO) is more exothermic than the ΔHhydꝋof 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 ΔHhydꝋ 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