We have to explain why the formation of solid sodium chloride (NaCl) from solid sodium metal (Na) and chlorine gas (Cl2) an exothermic reaction, even though it takes more energy to form the Na+ ion than the amount of energy released upon formation of Cl-.
The formation of an ionic compound from its elements is explained by the Born-Haber cycle. The process for the formation of NaCl is:
Step 1: Convert Na(s) into a gas, Na(g). This corresponds to ΔHsub (heat of sublimation).
Step 2: Convert Na(g) into its ion, Na+(g). This corresponds to I.E. (Ionization energy of Na).
Step 3: Split Cl2(g) into 2 Cl(g) atoms. This corresponds to ΔHdiss (heat of dissociation)
Step 4: Convert each Cl(g) into the Cl–(g) ion. This corresponds to E.A. (electron affinity for Cl).
Step 5: Combine Na+(g) and Cl–(g) to form NaCl(s). This corresponds to U (lattice energy).
Why is the formation of solid sodium chloride from solid sodium and gaseous chlorine exothermic, even though it takes more energy to form the Na+ ion than the amount of energy released upon formation of Cl- ?