Lattice energy

The lattice energy of a crystalline solid is a measure of the energy released when ions are combined to make a compound. It is a measure of the cohesive forcess that bind ions. Lattice energy is relevant to many practical properties including solubility, hardness, and volatility. The lattice energy is usually deduced from the Born–Haber cycle. The lattice energy of a crystalline solid is a measure of the energy released when ions are combined to make a compound. It is a measure of the cohesive forcess that bind ions. Lattice energy is relevant to many practical properties including solubility, hardness, and volatility. The lattice energy is usually deduced from the Born–Haber cycle. The lattice energy is exothermic, i.e., the value of ΔHlattice is negative because it corresponds to the coalescing of infinitely separated gaseous ions in vacuum to form the ionic lattice. The lattice enthalpy is reported as a positive value. 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 textbooks and the commonly used CRC Handbook of Chemistry and Physics 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 Δ G U {displaystyle Delta _{G}U} is the molar lattice energy, Δ G H {displaystyle Delta _{G}H} the molar lattice enthalpy and Δ V m {displaystyle Delta V_{m}} the change of the volume per mole. Therefore, the lattice enthalpy further takes into account that work has to be performed against an outer pressure p {displaystyle p} . The lattice energy of an ionic compound depends upon charges of the ions that comprise the solid. More subtly, the relative and absolute sizes of the ions influence ΔHlattice. In 1918 Born and Landé proposed that the lattice energy could be derived from the electric potential of the ionic lattice and a repulsive potential energy term.