Review A-Level Edexcel Chemistry lattice-energy comparison through Born-Haber values, theoretical assumptions and covalent-character evidence.
This question concerns calcium chloride, CaCl2.
The theoretical lattice energy of formation of CaCl2 is −2223 kJ mol−1. Explain why the experimental lattice energy of CaCl2 is more exothermic than the theoretical value.
An explanation that makes reference to the following points:
Either:
- bonding is partially covalent / not purely ionic / has covalent character (1)
- Ca^2+ ion is small and highly charged / has a high charge density (1)
- leading to polarisation / distortion in the electron cloud of the chloride / Cl^- ion / anion (1)
Or:
- bonding is almost purely ionic / slightly covalent (1)
- Ca^2+ is not sufficiently small and highly charged / has too small a charge density (1)
- to polarise the chloride / Cl^- ion very much / to distort its electron cloud (1)
Penalise use of calcium and/or chlorine atom once only. Ignore descriptions of possible macroscopic features of covalent character.