Energy is produced by the formation of 1 mol of ion pairs from gaseous ions which approach each othe — Physical Chemistry — Thermodynamics Chemistry Question
Born-Haber cycle
Energy is produced by the formation of 1 mol of ion pairs from gaseous ions which approach each other from infinity. This evolved energy at 0 K under normal atmospheric pressure is defined as the lattice energy. The lattice energy of an ionic crystal is indirectly obtained by applying Hess’s law to its enthalpy of formation.
llustrate a Born-Haber cycle of potassium chloride (KCl) and calculate its lattice energy by use of the following thermodynamic data.
Enthalpy of formation of KCl(s): – 437 kJ mol–1
Enthalpy of sublimation of K(s): 89 kJ mol–1
Ionization energy of K(g): 419 kJ mol–1
Enthalpy of dissociation of Cl2(g): 242 kJ mol–1
Electron affinity of Cl(g) : – 349 kJ mol–1
The marks of “g” and “s” represent “gas” and “solid” state, respectively.
Model Answer
13.1 B + C + D – A + E = 89 + 419 + 121 – (– 437) + (– 349) = 717 kJ mol–1
A: Enthalpy of formation of KCl(s), – 437 kJ mol–1
B: Enthalpy of formation of KCl(s), 89 kJ mol–1
C: Ionization energy of K (g), 419 kJ mol–1
D: Enthalpy of dissociation of Cl2 (g), 121 kJ mol–1 (242/2) E: Electron affinity of Cl(g)
–349 kJ mol–1
F: Lattice energy of KCl(s), 717 kJ mol–1
[VISUAL]
KCl(s)
K+(g) + e + Cl(g)
K+(g) + Cl–(g)
K(s) + ½ Cl2(g)
K(g) + ½ Cl2(g)
K+(g) + e + ½ Cl2(g)