A student constructs a Born–Haber cycle for NaCl and finds that the experimental lattice enthalpy is 400 kJ mol⁻¹, whereas the theoretical lattice enthalpy calculated from the cycle is 420 kJ mol⁻¹. Explain the difference between the experimental and theoretical lattice enthalpies and why the theoretical value is higher.

The experimental lattice enthalpy is the energy released when gaseous ions combine to form the solid crystal, measured directly from calorimetric data. The theoretical lattice enthalpy is derived from the Born–Haber cycle, which uses the enthalpies of formation, ionisation energies, electron affinities and sublimation energies. The theoretical value is higher because it assumes ideal point‑charge ions and neglects factors such as covalent character, lattice defects and the finite size of ions, which reduce the actual energy released. Thus the theoretical lattice enthalpy represents an upper limit, whereas the experimental value reflects the real, slightly lower energy of crystal formation.

The answer defines both experimental and theoretical lattice enthalpies, identifies the key difference (idealised assumptions vs real measurements), explains when each applies (calorimetry vs cycle calculation), and concludes why the theoretical value is higher. This tests understanding of Born–Haber cycle components and the limitations of theoretical estimates.