Lattice EnergyLattice energy indicates the strength of the ionic bonds in an ionic lattice. The enthalphy change delta H latt is the enthalpy change that accompanies the formation of one mole of an ionic compound from its constituent gaseous ions. This is an exothermic process.X+ (g) + Y+(g) --> XY(s) delta H latt [XY(s)]Lattice enthalpies cannot be determines directly and so they must be calculated indirectly using Hess's Law.The energy cycle used to calculate a lattice enthalpy is the Born-Haber cycle.
Standard enthalpy change of formation of sodium chloride , measured using a bomb calorimeter .Standard enthalpy change of atomisation of sodium, calculated from enthalpy changes of fusion and vaporisation and the specific heat capacity of sodium.1st ionisation energy of sodium, found by spectroscopic measurements.1st electron affinity of chlorine, found by methods similar to those used to measure ionisation energies.Given the measured quantities of all these terms the lattice energy of NaCl can be calculated :delta H latt [NaCl(s)] = -411-109-494-121-(-364) KJ mol-1 = 802 KJ mol-1
Values for lattice energy have the same order of magnitude as the bond enthalpies which compare ionic bond strengths to covalent bond strengths.The difference in ionic lattices is that each ion is held by attractive forces to a number of other ions in the lattice. As a result of this the energy needed to break them is much higher than needed to break an individual covalent bond. This explains the high melting and points of ionic substances.