Standard enthalpy change of lattice energy

The standard enthalpy change of lattice energy, ΔHlatto, is the change in enthalpy to break one mole of a solid ionic compound and separate its gaseous ions to an infinite distance under standard conditions.

Since it requires a large amount of energy to carry out the process, the standard enthalpy change of lattice energy is always positive (endothermic), e.g.

KCl(s)\rightarrow K^+(g)+Cl^-(g)\; \; \; \; \; \; \; \Delta H_{latt}^{\: o}=717\: kJmol^{-1}

However, it may be defined as the change in enthalpy when one mole of an ionic solid is formed from its gaseous ions that are initially infinitely apart under standard conditions. If so, it will have values that are always negative (exothermic).

The magnitude of the standard enthalpy change of lattice energy increases for ions with higher charge densities, leading to stronger electrostatic forces of attraction between them in the ionic lattice. Hence,

\left | \Delta H_{latt}^{\: o}[MgO] \right |>\left | \Delta H_{latt}^{\: o}[LiF] \right |>\left | \Delta H_{latt}^{\: o}[LiCl] \right |>\left | \Delta H_{latt}^{\: o}[NaCl] \right |>\left | \Delta H_{latt}^{\: o}[NaBr] \right |>\left | \Delta H_{latt}^{\: o}[KBr] \right |

The standard enthalpy change of lattice energy of an ionic compound is determined theoretically using a Born-Haber cycle, as it is very hard to measure it precisely through experiments.


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