Chemical Bonding and Molecular Structure
2.0 Lattice Energy
2.0 Lattice Energy
When one mole of an ionic solid is formed from its constituent gaseous ions, the energy released is called the lattice energy.
Energetics of Formation of Ionic Substances: The energy included in the formation of an ionic compound from its constituent elements may be considered as shown by the Born-Haber Cycle for the formation of one mole of sodium chloride from sodium and chlorine.
where
$S$ = heat of sublimation of sodium metal
$I$ = ionization energy of sodium
$D$ = heat of dissociation of molecular chlorine
$E_a$ = electron affinity of chlorine, and
$U$ = lattice energy of sodium chloride
The amount of heat liberated in the overall reaction is the heat of formation of sodium chloride. From the above $$\Delta {H_f} = S + I + \frac{1}{2}D - {E_a} - U$$
The most important of these energy terms are $I$, $E_a$ and $U$, since these are considerably greater than the remaining terms $S$ and $D$.
More the negative value of the heat of formation, greater would be the stability of the ionic compound produced. Thus on the basis of the above equation, formation of an ionic compound is favoured by
- low ionization energy $(I)$ of the metal.
- high electron affinity $(E_a)$ of the other element.
- higher lattice energy $(U)$ of the resulting compound.
