LATTICE ENERGIES

The ionic solid's lattice energy is a measure of the strength of bonds in that ionic compound. It is generally defined as the enthalpy of development of the ionic compound from gaseous ions and like is invariably exothermic. Lattice energy might also be described as the energy needed to completely separate one mole of a solid ionic compound into gaseous ionic constituents. The notion of lattice energy was initially developed for rock sphalerite-structured and  salt-structured compounds such as NaCl and ZnS, where the ions take place high-symmetry crystal lattice sites. In the example of NaCl, the lattice energy is the energy released by the reaction

Na⁺ (g) + Cl^- (g) → NaCl (s)

that would amount to -787 kJ/mol.

Some historical textbooks describe lattice energy as the energy needed to convert the ionic compound into gaseous ions which is an endothermic process and following this definition the lattice energy of NaCl would be +787 kJ/mol.

The exact value of the lattice energy may not be determined experimentally, due to the impossibility of preparing a sufficient amount of gaseous cations and ions and measuring the energy that is released during their condensation to form the solid. Though, the value of the lattice energy may either be derived hypothetically from electrostatics or from a thermodynamic cycling reaction, the known as Born-Haber cycle.