Explain why the standard enthalpy of atomisation of a metal is always a positive value, and describe the role that this step plays in a Born–Haber cycle for an ionic compound.

IB DP Chemistry Standard Level (2023 syllabus) — R1.2 Energy cycles in reactions · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Atomisation is the process of breaking the metallic bonds in the standard‑state solid metal to give gaseous metal atoms. Breaking a bond requires energy input, so the enthalpy change for atomisation is always positive (endothermic). In a Born–Haber cycle for an ionic compound, atomisation is the first step that produces the gaseous metal atoms from the solid metal. These atoms are then ionised in the next step. The atomisation enthalpy is included in the Hess’s‑law energy cycle; a larger positive atomisation enthalpy makes the overall formation of the ionic compound less exothermic, unless compensated by other terms such as a large lattice enthalpy.

Examiner tips

  • Use the word ‘endothermic’ to justify the positive sign; link bond breaking to energy input.
  • State that atomisation produces gaseous metal atoms, a prerequisite for ionisation.
  • Explain that the atomisation term is added to the cycle and affects the overall ΔH of formation.
  • Mention that a large atomisation enthalpy reduces the exothermicity unless offset by lattice enthalpy.

Common mistakes

  • Saying the atomisation enthalpy is negative or that it releases energy.
  • Confusing atomisation with ionisation or lattice formation.
  • Failing to explain why the step is necessary before ionisation.

Mark scheme (4 marks)

  1. Atomisation involves breaking metallic bonds / converting the metal from solid to gaseous atoms, which requires energy input.
  2. Because energy is absorbed / the process is endothermic, ΔH atomisation is always positive.
  3. In a Born–Haber cycle, atomisation is the step that produces gaseous metal atoms from the standard-state solid metal, which is necessary before ionisation (removal of electrons) can occur.
  4. The atomisation enthalpy contributes to the overall (Hess's law) energy cycle, meaning a larger atomisation enthalpy makes the formation of the ionic compound less energetically favourable (less exothermic overall), unless offset by other terms such as lattice enthalpy.

Key terms in this question

standard enthalpy of atomisation · Born–Haber cycle

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