LATTICE ENERGY_1

THE ENTHALPY OF FORMATION OF IONIC PAIR IN GAS PHASE

The formation of ionic compound from its atoms in gas phase occurs in three steps are:
1. the formation of cation. This is an endoterm process
2. the formation of anion. This may endoterm or exotherm process
3. the formation of ionic pair. It is an exotherm process
The energy released in the formation of ionic pair in gas phase from its ions could be estimated from Ionization Energy, IE, the electronic affinity, EA, and from its Bond Dissosiation Energy, BDE. The thermodynamic cycle can be use in the Ionic pair enthalpy calculation. Figure 1 describes the thermodynamic cycle of MgO formation.

Figure 1. The thermodynamic cycle of MgO formation
(Effendi, 2008)

The data of each process are listed below:
Mg(g) --> Mg+(g) + e                                          IE1    =  737.7 kJ/mol
Mg+(g) --> Mg2+(g) + e                                      IE2    = 1450.7 kJ/mol
O(g)  +  e --> O-(g)                                            EA1  = -140.984 kJ/mol
O-(g) + e --> O2-(g)                                            EA2  =   744  kJ/mol
Mg(g) + O(g) --> MgO(g)                                    -BDE = -377 kJ/mol

Based on Hess's Law, it can be formulated as,
-BDE = IE1 +IE2 + EA1 +EA2 +  (delta) Hip

Therefore,
(delta)Hip = -BDE - (IE1+IE2 +EA1 + EA2)
                  = -3168.4 kJ/mol


THE LATTICE ENERGY
The lattice energy of ionic solid could be defined as the enthalpy of formation of the ionic compound from gaseous ions. 
It may also defined as the energy required to completedly separate one mole of a solid ionic compound into gaseous ionis constituents. 
The Crystal of ionic compound contains of cations and anions which are regularly, alternately and repeatedly arranged.
The electrostatic force contribute dominantly in the lattice energy of ionic crystal. Other force including repulsive interaction between electrons and short-range repulsive forces which are important when atoms or ions so close and their  electron clouds begin to overlap.
The energy curve that describes the relation between ions distance with the energy is depicted in Figure 2.

Figure 2. The energy curve of ionic pair