Factors Affecting Lattice Energy (CIE A Level Chemistry)

• The two key factors which affect lattice energy, ΔH_latt^ꝋ, 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

Trends in the lattice energy of different metal halides

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 (O^2–) 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 O^2– 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 O^2– are also smaller ions than K⁺ and Cl^–, so this also adds to the value for the lattice energy being more exothermic