Born-Haber Cycle
Definition and meaning of Born-Haber Cycle in chemistry.
The Born-Haber cycle is a thermochemical cycle that applies Hess's law to relate the lattice energy of an ionic compound to other measurable enthalpy changes, such as sublimation, ionization, bond dissociation, electron affinity, and enthalpy of formation.
In more detail
Lattice energy cannot be measured directly because gaseous ions never actually combine to form a solid in an isolated, observable step. The cycle instead builds an indirect path: converting elements to gaseous atoms (sublimation, bond dissociation), then to gaseous ions (ionization energy, electron affinity), and finally to the solid lattice. Because enthalpy is a state function, the sum of enthalpies around the closed cycle equals the standard enthalpy of formation, leaving lattice energy as the only unknown to solve for. Comparing this experimental lattice energy to one calculated from a purely ionic electrostatic model also reveals how much covalent character is present in the bonding.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Named after | Max Born and Fritz Haber (1919) |
| Governing principle | Hess's law (enthalpy is a state function) |
| Classic example compound | NaCl |
For NaCl: ΔHf° = ΔHsub(Na) + IE1(Na) + ½D(Cl2) + EA1(Cl) + ΔHlattice(NaCl). Since ΔHf°, ΔHsub, IE1, D, and EA1 are all independently measurable, solving this equation gives ΔHlattice(NaCl) ≈ -787 kJ/mol.
Frequently asked questions
What quantity is usually the unknown solved for in a Born-Haber cycle?
Almost always the lattice energy, since it cannot be measured directly, whereas sublimation enthalpy, ionization energy, bond dissociation energy, electron affinity, and enthalpy of formation can all be measured experimentally.
Why does the Born-Haber cycle matter beyond calculating lattice energy?
Comparing the experimental lattice energy from the cycle to a theoretical value from a purely ionic (Born-Landé or Kapustinskii) model shows how well the ionic bonding model fits, exposing covalent contributions when the two values disagree significantly.