1. Lattice enthalpy
For an ionic compound the lattice enthalpy is the heat energy released when one mole of solid in its standard state is formed from its ions in the gaseous state.
Eg Na+(g) Cl-(g) NaCl(s)
- L.E cannot be determined directly
experimental data is used
enthalpy cycle (Hess’s Law) links the data.
- enthalpy cycle is based on the formation of the compound from its elements in their standard states.

2. BORN-HABER CYCLE
FOR
SODIUM CHLORIDE

3. kJ H= +107kJmol-1 Atomisation of sodium +800 +700 +600 +500 +400 +300
+200
Na(g) + 1/2 Cl2(g)
+100
H= +107kJmol-1
Na(s) + 1/2 Cl2(g)
-400
-300
-200
-100
Atomisation of sodium

4. kJ H = +121kJmol-1 Atomisation of chlorine +800 +700 +600 +500 +400
+300
Na(g) + Cl(g)
+200
H = +121kJmol-1
Na(g) + 1/2 Cl2(g)
+100
Na(s) + 1/2 Cl2(g)
-400
-300
-200
-100
Atomisation of chlorine

5. kJ e- e- e- e- e- e- e- e- e- e- e- + H = +502kJmol-1
+800
Na+(g) + Cl(g)
+700 e- e- e- e-
+600 e- e-
+500 e- e-
+400 e-
H = +502kJmol-1 e- e-
+300
Na(g) + Cl(g)
+200 +
Na(g) + 1/2 Cl2(g)
+100
Na(s) + 1/2 Cl2(g)
-400
-300
-200
-100
First Ionisation of sodium

6. First electron affinity of chlorinekJ kJ kJ
+800
+800
+800
Na+(g) + Cl(g)
+700
+700
+700
+600
+600
+600
H = -355kJmol-1
+500
+500
+500
+400
+400
+400
Na+(g) + Cl-(g)
+300
+300
+300
Na(g) + Cl(g)
Na(g) + Cl(g)
Na(g) + Cl(g)
+200
+200
+200 - e-
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
+100
+100
+100
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g)
-400
-300
-200
-100
-400
-300
-200
-100
-400
-300
-200
-100
First electron affinity of chlorine

7. kJ kJ kJ - + H = -411kJmol-1 +800 +800 +800 +700 +700 +700 +600 +600
Na+(g) + Cl(g)
+700
+700
+700
+600
+600
+600
+500
+500
+500
+400
+400
+400
Na+(g) + Cl-(g)
+300
+300
+300 - +
Na(g) + Cl(g)
Na(g) + Cl(g)
Na(g) + Cl(g)
+200
+200
+200
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
+100
+100
+100
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g)
-400
-300
-200
-100
-400
-300
-200
-100
-400
-300
-200
-100
H = -411kJmol-1
Formation of sodium chloride
NaCl(s)

8. kJ kJ kJ - - - - - + + + + H = -786 kJmol-1 +800 +800 +800 +700 +700
Na+(g) + Cl(g)
+700
+700
+700
+600
+600
+600
+500
+500
+500
+400
+400
+400
Na+(g) + Cl-(g)
+300
+300
+300
Na(g) + Cl(g)
Na(g) + Cl(g)
Na(g) + Cl(g)
+200
+200
+200 - -
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g)
Na(g) + 1/2 Cl2(g) +
+100
+100
+100 + - +
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g)
Na(s) + 1/2 Cl2(g) - + -
-400
-300
-200
-100
-400
-300
-200
-100
-400
-300
-200
-100
H = -786 kJmol-1
Lattice enthalpy for sodium chloride
NaCl(s)