The Born–Haber cycle is a theoretical construct used to analyse and predict the formation of ionic compounds. While the steps in the Born–Haber cycle do not directly represent the actual processes that occur during the formation of an ionic solid from its elements, they provide a useful model for understanding and calculating the energetics involved. In reality, ionic compounds form in a more complex manner, often involving gaseous ions coming together to form a solid. Despite this complexity, the Born–Haber cycle accurately reflects the overall change in enthalpy and helps chemists understand and quantify the forces at play in the formation of ionic compounds.

Ionisation energy and electron affinity are crucial components of the Born–Haber cycle because they account for the energy changes associated with forming ions from atoms. Ionisation energy represents the energy required to remove an electron from an atom, turning it into a cation. Electron affinity, on the other hand, reflects the energy change when an electron is added to an atom, forming an anion. Since ionic compounds consist of cations and anions, these energy changes are essential for accurately calculating the lattice enthalpy and understanding the overall energetics of ionic compound formation.

The structure of an ionic compound, including the arrangement of ions and their coordination numbers, plays a significant role in determining its lattice enthalpy. In a closely packed structure, where ions are efficiently arranged and have high coordination numbers, the interactions between ions are maximised, leading to a higher lattice enthalpy. Conversely, in a less efficiently packed structure with lower coordination numbers, the interactions between ions are reduced, resulting in a lower lattice enthalpy. The specific geometry and arrangement of ions in the crystal lattice are thus key factors influencing the strength of the ionic bonds and the stability of the ionic compound.

Lattice enthalpy cannot be measured directly because it involves the hypothetical process of forming an ionic compound from its gaseous ions, which is not a practical experimental setup. Instead, the Born–Haber cycle is employed to calculate lattice enthalpy indirectly. This cycle breaks down the formation of an ionic compound into a series of steps, each of which corresponds to a physical process with an associated enthalpy change that can be measured or calculated. By using these known values and applying Hess's law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in, the lattice enthalpy can be deduced.

The lattice enthalpy of an ionic compound provides invaluable insight into its physical properties, such as melting and boiling points, solubility, and hardness. A compound with a high lattice enthalpy typically has high melting and boiling points because more energy is required to overcome the strong ionic bonds. Such a compound also tends to be less soluble in water and is usually harder. Conversely, a compound with a lower lattice enthalpy will have lower melting and boiling points, and it is likely to be more soluble in water and softer. Understanding lattice enthalpy is crucial for predicting how an ionic compound will behave under different conditions, aiding in the synthesis and application of these compounds in various fields.