Ionic crystal enthalpy diagram

Let us examine the enthalpy terms involved when an ionic crystal MX is dissolved in water. The energy diagram for a Group I halide is as shown in Figure 3.8. 

Born-Haber s cycle — Hess s law establishes that the enthalpy of a reaction is the same independently whether the reaction proceeds in one or several steps. It is a consequence of the first law of thermodynamics, which states the conservation of energy. Born and -> Haber applied Hess s law to determine the - enthalpy of formation of an ionic solid. The formation of an ionic crystal from its elements according to Born-Haber s cycle can be represented by the following diagram. 

Lattice enthalpies cannot be found directly from experiments (partly because ionic crystals form ion pairs when heated, not free gaseous ions). Therefore lattice enthalpies must be calculated indirectly from other known enthalpy changes of reaction using a Born-Haber cycle. This is an enthalpy level diagram derived from Hess s law it is used to follow the enthalpy changes which occur when an ionic compound is formed from its chemical elements and gaseous ions (Figure 15.11). 

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