Explain why the lattice enthalpy of magnesium chloride (MgCl₂) is significantly more exothermic than that of sodium chloride (NaCl).

IB DP Chemistry Higher Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

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 crystalline solid and depends on the charges and sizes of the ions present.

Model answer (4 marks)

Magnesium chloride has a 2+ ion (Mg²⁺) while sodium chloride has a 1+ ion (Na⁺). The higher charge on Mg²⁺ gives a stronger electrostatic attraction to the chloride ions. In addition, Mg²⁺ is smaller than Na⁺, so the inter‑ionic distance in MgCl₂ is shorter. The combination of a higher charge and a shorter distance increases the charge density on Mg²⁺, producing a much more exothermic lattice enthalpy for MgCl₂ than for NaCl.

Examiner tips

Use the command word ‘explain’ – give reasons, not just facts.
Mention both charge and size – both are needed for full marks.
Show the link between charge density and lattice enthalpy.
Keep the answer concise and use correct terminology (e.g., ‘electrostatic attraction’, ‘inter‑ionic distance’).

Common mistakes

Confusing lattice enthalpy with lattice energy; students may write the opposite sign.
Forgetting to mention the smaller ionic radius of Mg²⁺.
Using vague terms like ‘stronger’ without linking to charge and distance.

Mark scheme (4 marks)

Magnesium ion (Mg²⁺) carries a 2+ charge whereas sodium ion (Na⁺) carries only a 1+ charge
Greater ionic charge leads to stronger electrostatic attraction between oppositely charged ions
Mg²⁺ is smaller than Na⁺ (has a smaller ionic radius), so the interionic distance is shorter in MgCl₂
Smaller interionic distance (and higher charge) means greater charge density on Mg²⁺, resulting in a more exothermic lattice enthalpy for MgCl₂

Key terms in this question

lattice enthalpy