# Explain why the lattice enthalpy of magnesium oxide is much more negative than the lattice enthalpy of sodium chloride.

> Edexcel A-Level Chemistry (9CH0) — 13.1 Born-Haber cycles and entropy · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Lattice enthalpy depends on the charges of the ions involved
2. Magnesium ions carry a 2+ charge and oxide ions carry a 2− charge, giving a greater charge product than Na⁺ and Cl⁻
3. Greater ionic charges produce stronger electrostatic attraction between oppositely charged ions in the lattice
4. Magnesium and oxide ions are also smaller than sodium and chloride ions, so the ions are closer together in the lattice
5. 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

- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)

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