Explain why the lattice enthalpy of magnesium oxide is much more negative than the lattice enthalpy of sodium chloride.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Lattice enthalpy is a measure of the strength of ionic bonding in a giant ionic lattice. The lattice enthalpy of magnesium oxide is −3791 kJ mol⁻¹ and the lattice enthalpy of sodium chloride is −787 kJ mol⁻¹.
Model answer (5 marks)
Lattice enthalpy is proportional to the product of the charges of the ions and inversely proportional to the distance between them. Magnesium ions are 2+ and oxide ions are 2−, giving a charge product of 4, whereas Na⁺ and Cl⁻ give a product of 1. The larger charge product means a stronger Coulombic attraction. In addition, Mg²⁺ and O²⁻ are smaller than Na⁺ and Cl⁻, so the ions are closer together in the crystal lattice. The combination of higher charges and shorter inter‑ionic distances results in a much larger (more negative) lattice enthalpy for MgO compared with NaCl.
Examiner tips
- Use the formula ΔH_latt ∝ z⁺z⁻/r to justify the points
- Mention both charge and size effects explicitly
- Keep the answer concise – 5 marks only
- Use correct UK spelling and chemical symbols
Common mistakes
- Confusing lattice enthalpy sign (more negative = stronger bond)
- Forgetting to mention ionic radii
- Using the wrong charge product (e.g. 2×1 instead of 2×2)
Mark scheme (5 marks)
- Lattice enthalpy depends on the charges of the ions involved
- Magnesium ions carry a 2+ charge and oxide ions carry a 2− charge, giving a greater charge product than Na⁺ and Cl⁻
- Greater ionic charges produce stronger electrostatic attraction between oppositely charged ions in the lattice
- Magnesium and oxide ions are also smaller than sodium and chloride ions, so the ions are closer together in the lattice
- Both the higher charge and smaller ionic radius together mean the electrostatic attraction (and therefore the magnitude of the lattice enthalpy) is much greater for magnesium oxide
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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