A2-Level Enthalpy and Entropy

  • It is very difficult to find lattice enthalpies experimentally.

  • Born-Haber energy diagrams are used to find a lattice enthalpy by using Hess’ Law.

  • Hess’ Law states that the overall energy change that occurs when a product is formed from its elements is the same, regardless of the route taken to form the product.

  • Different standard reactions are used to construct a Born-Haber cycle

    • Standard enthalpy of atomisation (forming gaseous atoms)

    • Ionisation energies (removing electrons from gaseous atoms to form positive gaseous ions)

    • Electron affinities (addition of electrons to gaseous atoms to form negative gaseous ions)

    • Standard enthalpy change of formation (enthalpy change when one mole of a compound is formed from constituent elements in standard states)


Born-Haber Cycles


It is incredibly difficult and practically unrealistic to find lattice enthalpies experimentally. Separate samples of gaseous positive and negative ions would need to be kept apart, then carefully mixed and the thermal energy released measured to give an enthalpy change.


Instead, to find out lattice enthalpies, we find them indirectly using an energy cycle (utilising Hess’ Law).  The energy cycles used to find lattice enthalpies are referred to as Born-Haber cycles.


By taking the elements (that make up a compound) in their standard states, we can measure the enthalpy change that occurs when the lattice is formed (enthalpy of formation), and we can also measure the enthalpy changes that occur when we form the gaseous ions required to find the lattice enthalpy.


There are some ‘standard’ reactions involved in a Born-Haber cycle.