# Explain why the lattice enthalpy of magnesium oxide (MgO) is much greater in magnitude than the lattice enthalpy of sodium chloride (NaCl), using Coulomb's law to support your answer.

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

> Lattice enthalpy is a key quantity used in Born–Haber cycles to assess the stability of ionic compounds. The lattice enthalpy of MgO is approximately −3791 kJ mol⁻¹, whilst that of NaCl is approximately −787 kJ mol⁻¹.

## Mark scheme (4 marks)

1. Lattice enthalpy depends on the charges of the ions and the distance between them (Coulomb's law / electrostatic attraction).
2. Mg²⁺ and O²⁻ each carry a charge of 2+ and 2− respectively, giving a charge product of 4, compared with Na⁺ (1+) and Cl⁻ (1−) with a charge product of 1.
3. Mg²⁺ and O²⁻ are smaller ions than Na⁺ and Cl⁻ respectively, so the interionic distance in MgO is shorter.
4. Both the higher charge and the shorter interionic distance result in a much stronger electrostatic attraction in MgO, giving it a significantly larger (more negative) lattice enthalpy than NaCl.

## Key terms

- [lattice enthalpy](https://www.gradenine.co.uk/glossary/lattice-enthalpy)
- [Coulomb's law](https://www.gradenine.co.uk/glossary/coulomb-s-law)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-lattice-enthalpy-of-ffd364a7) · Published by Druglandscape Ltd.