Lattice enthalpy values can be obtained from Born–Haber cycles and by calculations based on a perfect ionic model - AQA - A-Level Chemistry - Question 33 - 2018 - Paper 3

To determine the compound that shows the greatest percentage difference between its lattice enthalpy values calculated from Born–Haber cycles and those from a perfect ionic model, we need to consider factors such as ionic charge and size.

1. Calculate Lattice Enthalpy: Use the Born–Haber cycle to understand the theoretical lattice enthalpy versus the values derived from the perfect ionic model. The difference in these values is influenced by the ionic character of the compounds.

2. Evaluate Ionic Characteristics:

   • CsF: Contains larger ions and high charge, leading to distinct lattice energies in different models.
   • CsI: Similar data trends to CsF but even larger ions may influence this further.
   • LiF: To be checked for smaller ionic sizes, typically with higher lattice energies.
   • LiI: Has a comparative lower charge distribution affecting lattice.

3. Analyze Percentage Difference: After calculating the values, compute the percentage difference using the formula:
   ext{Percentage Difference} = rac{| ext{Calculated Value} - ext{Theoretical Value}|}{ ext{Theoretical Value}} imes 100

4. Identify the Greatest Difference: After performing the calculations, it will usually turn out that LiI (Option D) has the greatest percentage difference due to significantly varying ionic interactions, leading to a higher discrepancy between models.