Haber Cycle involves the formation of an ionic compound from the reaction of a metal with a non-metal. Born–Haber cycles are used primarily as a means of calculating lattice energies which cannot otherwise be measured directly.
The lattice enthalpy is the enthalpy change involved in formation of the ionic compound from gaseous ions. Some chemists define it as the energy to break the ionic compound into gaseous ions. The former definition is invariably exothermic and the latter is endothermic.
Enthalpy of formation of LiF:
The sum of the energies for each step of the process must equal the enthalpy of formation of the metal and non-metal, ΔHf.
- V is the enthalpy of sublimation for metal atoms (lithium)
- B is the bond energy (of F2). The coefficient 1/2 is used because the formation reaction is Li + 1/2 F2 → LiF.
- IEM is the ionization energy of the metal atom:
- EAX is the electron affinity of non-metal atom X (fluorine)
- UL is the lattice energy (defined as exothermic here)
Example of lattice formation enthalpy of NaCl:
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