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For an ionic compound the lattice enthalpy is the heat energy released when one mole of solid in its standard stateis formed from its ions in the gaseous state.
This value cannot be determined directly and so we make use of changes for which data are available and link them together with an enthalpy cycle.
This enthalpy cycle is based on the formation of the compound from its elements in their standard states.
From John Slater:
BORN-HABER CYCLEFOR
SODIUM CHLORIDE
Atomisation of sodium
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
kJ
-400
-300
-200
-100
H= +107kJmol-1
Atomisation of chlorine
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
H = +121kJmol-1
+
First Ionisation of sodium
e-e-e-e-
e-e-
e-e-e-e-e-
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na+(g) + Cl(g)
H = +502kJmol-1
First electron affinity of chlorine
e--
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na+(g) + Cl(g)
Na+(g) + Cl-(g)
H = -355kJmol-1
- -
-
--
+
++
+
Formation of sodium chloride
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na+(g) + Cl(g)
Na+(g) + Cl-(g)
NaCl(s)
H = -411kJmol-1
- -
-
--
+
++
+
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na(s) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + Cl(g)
0
+100
+200
+300
+400
+500
+600
+700
+800
-400
-300
-200
-100
kJ
Na+(g) + Cl(g)
Na+(g) + Cl-(g)
NaCl(s)
Lattice enthalpy for sodium chloride
H = -786 kJmol-1
