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.
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.
1
Born-Haber Cycles
magnesium chloride
Mg2+(g) + 2e- + 2Cl
enthalpy H
H
bond energy of chlorine
Mg2+
H
(g)
+
2e-
+ Cl2
(g)
(g)
2 x H
Mg2+
first electron affinity
(g)
+ 2Cl-
(g)
second ionisation energy
Mg+(g) + e- + Cl2
(g)
H
first ionisation energy
Mg
(g)
+ Cl2
(g)
H atomisation
Mg (s) + Cl2
H
formation
H lattice
association
(g)
MgCl2
(s)
2
Born-Haber Cycles
magnesium chloride
Mg2+(g) + 2e- + 2Cl
enthalpy H
H
bond energy of chlorine
Mg2+
H
(g)
+
2e-
+ Cl2
(g)
(g)
2 x H
Mg2+
first electron affinity
(g)
+ 2Cl-
(g)
second ionisation energy
Mg+(g) + e- + Cl2
(g)
H
first ionisation energy
Mg
(g)
+ Cl2
(g)
H atomisation
Mg (s) + Cl2
H
formation
H lattice
association
(g)
MgCl2
(s)
3
Apply Hess’s Law to calculate ∆HLattice Energy for MgCl2
setting out the values in a methodical manner:
1
=
2
+
3
+
4
+ 5 +
6
+
7
-641 = +148 + 738 + 1451 + 242 + (2x-364) + ∆HLE
-641 = + 1851 + ∆HLE
∆HLE
= - 1851 - 641
= - 2492 kJ mol-1
4