Lattice Energy.pdf

Definition

LATTICE ENERGY

Lattice energy, also called lattice enthalpy, enthalpy, of an ionic solid/crystal is a measure of the strength of bonds in that ionic compound. It is usually defined as the enthalpy of formation of the ionic compound from its gaseous ions and as such is invariably exothermic; Conversely lattice lattice energy may also be d efined as the energy required to completely separate/break one mole of a solid ionic/crystalline ionic/crystalline compound into its gaseous ionic constituents (positive (positive and negative ions) at infinite distance. distance. Lattice energy is denoted by the symbol U. Lattice energy depends on two factors 01-Charges on the ions 02-The distance between the centers of neighboring positive and negative ions The greater the magnitude of ion charge, the greater is the lattice energy. Lattice energy works against the solution process, so an ionic solid with relatively large lattice energy is usually insoluble. Lattice energies cannot be measured directly, but experimental values are obtained from thermodynamic data using the BornHaber cycle. Theoretical values for lattice energy may be calculated. The ions are treated as point charges, and the electrostatic (coulombic) energy E between two ions of opposite charge is calculated: శ

Eൌ

ష

௓ ௓ ௘

మ

௥

Where, Z+ and Z- are the charges on the po sitive and negative ions e is the charge on an electron r is the inter-ionic inter-ionic distance For more than two ions, the electrostatic energy depends on the number of ions, and also on A their arrangement in space. For  one mole, the attractive energy is: ା

Eൌെ

଴

ି

2

Where,  No is the Avogadro constant constant A is the Medelung constant In the case of NaCl, the lattice energy is the energy released by the reaction  Na+ (g) + Cl− (g) → NaCl (s) which would amount to -787 kJ/mol.