Solutions/Answers to Exercises
Exercise 1. Calculate the net energy change in kJ/mol when solid MgF2 is formed from its elements: Mg (s) + F2 (g) → MgF2 (s). Sketch a Born-Haber cycle for the process.
First write the equations and energy for each step. Add the equations and the energies to get the net equation and net energy. Multiply the electron affinity by 2 since there are 2 F atoms.
Mg (s) → Mg (g) +147.7 kJ
F2 (g) → 2 F (g) +158 kJ
Mg (g) → Mg+ (g) + e– +737.7 kJ
Mg+ (g) → Mg2+ (g) + e– 1450.7 kJ
2 F (g) + 2 e– → 2 F– (g) -656 kJ
Mg2+ (g) + 2 F– → MgF2 (s) -2957 kJ
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Mg (s) + F2 (g) → MgF2 (s) -1120 kJ
Born Haber Cycle for MgF2
Back to Formation of Ionic Compounds: The
Born-Haber Cycle
Exercise 2. Calculate the net energy change in kJ/mol when solid Al2O3 is formed from its elements: 2 Al (s) + 3/2 O2 (g) → Al2O3 (s).
There are 2 aluminum ions and 3 oxide ions in Al2O3 (s).
2 Al (s) → 2 Al (g) 2 mol x 660 kJ/mol
3/2 O2 (g) → 3 O (g) 3/2 mol x 498 kJ/mol
2 Al (g) → 2 Al+ (g) + 2 e– 2 mol x 578 kJ/mol
2 Al+(g) → 2 Al2+ (g) + 2 e– 2 mol x 1830 kJ/mol
2 Al2+ (g) → 2 Al3+ (g) + 2 e– 2 mol x 2750 kJ/mol
3 O (g) → 3 O– (g) + 3 e– 3 mol x -142 kJ/mol
3 O (g) → 3 O– (g) + 3 e– 3 mol x 844 kJ/mol
2 Al3+ (g) + 3 O2- (g) → Al2O3 (s) 1 mol x -15,916 kJ/mol
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2 Al (s) + 3/2 O2 → Al2O3 (s) -1427 kJ
Back to Formation of Ionic Compounds: The
Born-Haber Cycle
Exercise 3. Below is a figure of a Born-Haber Cycle where A is the metal and X2 is a nonmetal. Answer the following questions.
1451 kJ
b) What is Eea?
-697 kJ
c) What is the net energy change for the formation of the solid?
-642 kJ
d) What is the sublimation energy?
148 kJ
e) What is the energy for the formation of the solid from its ions?
-2524 kJ
