Lattice Energy

  • Lattice Energy refers to the energy that is needed to separate all ions in an ionic solid to gaseous ions completely. The term can be defined in two ways: enthalpy of formation of the ionic compound from gaseous ions and energy required to dissociate the ionic compound into gaseous ions.

  • In an Ionic Compound, Lattice Energy measures the strength of bonds; the stronger the bond, the higher the Lattice Energy will be.

  • Lattice energy is usually a large, exothermic value (release heat) when positive ions (cations) and negative ions (anions) combine to form an ionic crystal lattice.

  • Factors influencing Lattice Energy are the charge on the ions and the size (radius) of the ions. A greater charge and a smaller ionic radius result in stronger interaction and higher Lattice Energy.

  • The Born-Haber Cycle allows for indirect calculations of Lattice Energy. It is a thermochemical cycle involving multiple steps, providing a method to calculate otherwise difficult-to-determine enthalpy changes.

  • It’s necessary to understand that the Born-Haber Cycle also uses Hess’s Law of constant heat summation, stating that enthalpy change for any chemical reaction is independent of the route by which the reaction may occur.

  • Lattice Energy affects properties of an ionic compound, including volatility, hardness, and solubility. Higher lattice energy usually correlates with lower volatility, higher hardness, and lower solubility in water.

  • Ionic compounds with higher Lattice Energies typically have higher melting and boiling points as more energy is required to overcome the stronger forces holding the ions together.

  • Lattice Energy cannot be measured directly, but can be calculated using a Born-Haber Cycle or estimated using the Born-Lande equation or the Kapustinskii equation.

  • For compounds of similar structures, the Lattice Energy tends to increase with an increase in the charge of ion and decrease with an increase in the size of ion.

  • Keep in mind that ‘Lattice Dissociation Enthalpy’ refers to the endothermic process where ionic lattices are broken down into their constituent ions, while ‘Lattice Formation Enthalpy’ is exothermic and refers to the formation of an ionic lattice from its constituent ions.