Anhydrate lithium carbonate, Li₂CO₃(s), consists of Li⁺ ions and CO₃²⁻ ions. The Li⁺ ionic radius is 76 pm, and the planar trigonal CO₃²⁻ ion can be approximated as having an effective ionic radius of 178 pm for nearest Li⁺–CO₃²⁻ interactions.
The experimental lattice energy of Li₂CO₃ is reported as −6.58×103kJ mol−1.
(a) Using the ionic radii given and treating the cation–anion separation distance as the sum of the radii, calculate the electrostatic potential energy, in joules, for a single Li⁺ – CO₃²⁻ ion pair.
(b) Convert the value from part (a) to kJ mol⁻¹.
(c) Compare your calculated energy with the experimental lattice energy reported above. Quantify the percent difference.
(d) Provide one chemically sound reason why the simple electrostatic model used in parts (a) and (b) underestimates or overestimates the experimental lattice energy.