Lattice Energies for Some Ionic Compounds TABLE 8.1 Lattice Energy (kJ/mol) Compound Lattice Energy (kJ/mol) Compound 1030 LiF MgCl2 2526 LİCI 834 SrCl2 2127 Lil 730 NaF 910 MgO 3795 NaCl 3414 788 Cao NaBr 732 Sro 3217 Nal 682 KF 808 ScN 7547 КСІ 701 Kвг 671 CSCI 657 600 CsI TABLE 7.2 Successive Values of Ionization Energies, I, for the Elements Sodium through Argon (kJ/mol) Is 13 14 I5 Element 12 4562 496 738 578 786 1012 Na 1451 1817 (inner-shell electrons) Mg 7733 11,577 4356 4964 4556 5159 5771 2745 Al 1577 16,091 Si 3232 1907 2252 2298 2914 21,267 6274 27,107 11,018 11,995 3357 1000 8496 7004 6542 3822 1251 9362 CI 1521 7238 8781 Ar 3931 2666
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
Construct a Born–Haber cycle for the formation of the hypothetical
compound NaCl2, where the sodium ion has a 2+
charge (the second ionization energy for sodium is given in
Table 7.2). (a) How large would the lattice energy need to be
for the formation of NaCl2 to be exothermic? (b) If we were
to estimate the lattice energy of NaCl2 to be roughly equal
to that of MgCl2 (2326 kJ/mol from Table 8.1), what value
would you obtain for the standard enthalpy of formation,
ΔHf°, of NaCl2?
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