Ionic Molecules And The Born Haber Cycle
ADVERTISEMENT
1
2
3
4Chapter 9 looks at forces that keep atoms together.
Ionic molecules and the Born-Haber cycle
(g) MX(s)
The formation of an ionic solid can be described by the following equation. M(s) + 1/2X
2
The energetics or enthalpy associated with this process cannot be determined directly. You have been recently
presented with the processes of ionization (loss of an electron from a gaseous atom), electron affinity (gain of
an electron by a gaseous atom) and lattice energy (change in energy when two gaseous ions form an ionic
solid). These can be used, according to Hess’s Law to calculate the energetics of formation of an ionic solid.
Note that the starting materials are in their elemental state.
(g) NaF(s)
Let’s use the formation of NaF(s) as an example.
Na(s) + ½ F
2
The following steps must occur:
Process
Energy change
Na(s) Na(g) sublimation
109 kJ/mol
Na(g) Na
+
(g) ionization
495 kJ/mol
(g) 2F(g) bond breaking
F
154 kJ/mol
2
F(g) F
-
(g) electron affinity
-328 kJ/mol
(g) NaF(s) lattice energy
+
-
Na
(g) + F
-923 kJ/mol
When these equations and the energetics are added, the net result is the formation of the ionic solid with an
enthalpy of formation H
of –570 kJ
f
Na(s) Na(g) sublimation
109 kJ/mol
Na(g) Na
+
-
(g) + e
ionization
495 kJ/mol
(g) F(g) bond breaking
½ F
½ (154 kJ/mol)
2
F
-
-
F(g) + e
(g) electron affinity
-328 kJ/mol
(g) NaF(s) lattice energy
+
-
Na
(g) + F
-923 kJ/mol
(g) NaF(s)
Na(s) + ½ F
-570 kJ/mol
2
A second example:
2+
-
What is the lattice energy associated with Ca
(g) and Cl
(g) coming together to form CaCl
?
2
(g) CaCl
2+
-
Ca
(g) + 2Cl
(s)
2
sublimation energy of calcium
+178 kJ/mol
first ionization of calcium
+590 kJ/mol
second ionization of calcium
+1150 kJ/mol
bond dissociation of Cl
(g)
+244 kJ/mol
2
-
electron affinity of Cl
-349 kJ/mol
enthalpy of reaction
-326 kJ
Ca(s) Ca(g)
178 kJ
Ca(g) Ca
+
-
(g) + e
590 kJ
+
(g) Ca
2+
-
Ca
(g) + e
1150 kJ
(g) 2Cl(g)
Cl
244 kJ
2
2Cl(g) 2Cl
-
(g)
2(-349 kJ)
(g) CaCl
2+
-
Ca
(g) + 2Cl
(s)
x
2
(g) CaCl
Ca(s) + Cl
(s)
-326 kJ
2
2
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education