Lattice energy
The lattice energy of a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions and as such is invariably positive. The precise value of the lattice energy may not be determined experimentally, because of the impossibility of preparing an adequate amount of gaseous ions or atoms and measuring the energy released during their condensation to form the solid. However, the value of the lattice energy may either be derived theoretically from electrostatics or from a thermodynamic cycling reaction, the Born–Haber cycle.
The concept of lattice energy was originally developed for rocksalt-structured and sphalerite-structured compounds like NaCl and ZnS, where the ions occupy high-symmetry crystal lattice sites. In the case of NaCl,lattice energy is the energy released by the reaction
which would amount to -786 kJ/mol.
Some older textbooks define lattice energy with the opposite sign, i.e. the energy required to convert the crystal into infinitely separated gaseous ions in vacuum, an endothermic process. Following this convention, the lattice energy of NaCl would be +786 kJ/mol. The lattice energy for ionic crystals such as sodium chloride, metals such as iron, or covalently linked materials such as diamond is considerably greater in magnitude than for solids such as sugar or iodine, whose neutral molecules interact only by weaker dipole-dipole or van der Waals forces.
The relationship between the molar lattice energy and the molar lattice enthalpy is given by the following equation:
where is the molar lattice energy, the molar lattice enthalpy and the change of the volume per mol. Therefore, the lattice enthalpy further takes into account that work has to be performed against an outer pressure . Lattice Energy of an ionic compound depends upon charge of the ion and size of the ions. Moreover, factors such as packing of ions doesn't matter efficiently.
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