Energy Changes in the Formation of Ionic Crystals

The overall change in these five steps is the same as the reaction in which NaCl(s) is formed from its elements in their standard states—that is, ArHoveraU = AfH°[NaCl(s)]. From Appendix D, we see that AfH°[NaCl(s)] = -411 kj moP so in the following setup, the lattice energy of NaCl is the only unknown. 

Na (g7 -b GHgJ NaCl(s) ArHg = lattice energy of NaCl =  

Shown here is a five-step sequence for the formation of NaCI(s) from its elements in their standard states. The sum of the five enthalpy changes gives AfH [NaCI(s)]. The equivalent one-step reaction for the formation of NaCI(s) directly from Na(s) and Cl2(g) is shown in color. (The vertical arrows representing A,/-/ values are not to scale.) 

One way to use the concept of lattice energy is in making predictions about the possibility of synthesizing ionic compounds. In Example 12-12, we calculate the enthalpy of formation of MgCl(s). Then, we predict the likelihood of obtaining this compound. 

EXAMPLE 12-12 Relating Enthalpy of Formation, Lattice Energy, and Other Energy Quantities 

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The Nature of Bonding in Solids 12-6 Crystal Structures 12-7 Energy Changes in the Formation of Ionic Crystals... 

Energy Changes in the Formation of Ionic Crystals— Lattice energies of ionic crystals can be related to certain atomic and thermodynamic properties by means of the Born-Fajans-Haber cycle (Fig. 12-51). 

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