# Explain why the lattice enthalpy of magnesium oxide is significantly greater in magnitude than that of sodium chloride, using the Born–Haber cycle concept and relevant periodic trends.

> IB DP Chemistry Higher Level (2023 syllabus) — R1.2 Energy cycles in reactions · Explain · 4 marks

> Magnesium oxide (MgO) and sodium chloride (NaCl) both adopt the rock salt crystal structure, yet their standard lattice enthalpies differ considerably: MgO has a lattice enthalpy of approximately −3850 kJ mol⁻¹ compared to approximately −787 kJ mol⁻¹ for NaCl.

## Mark scheme (4 marks)

1. MgO involves 2+ and 2− ions whereas NaCl involves 1+ and 1− ions, so the ionic charges (charge product) are greater in MgO.
2. Greater ionic charges lead to stronger electrostatic (Coulombic) attraction between oppositely charged ions in the lattice.
3. The ionic radius of Mg²⁺ is smaller than that of Na⁺, and the ionic radius of O²⁻ is smaller than that of Cl⁻, so the interionic distance (r⁺ + r⁻) in MgO is shorter.
4. A shorter interionic distance further increases the electrostatic attraction (force ∝ 1/r²), so both the higher charge and the smaller ionic radii act together to make the lattice enthalpy of MgO much more exothermic than that of NaCl.

## Key terms

- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)
- [Born–Haber cycle](https://www.gradenine.co.uk/glossary/born-haber-cycle)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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