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Lattice Enthalpy and Ionic Attraction

Define lattice formation and dissociation, and explain charge and radius trends.

  • GCE A-Level H2 Chemistry 9476-2027

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An ionic solid contains oppositely charged ions held together by electrostatic attraction. Separating them into gaseous ions requires energy; bringing the gaseous ions together releases it. Lattice enthalpy measures this energy change for one mole of solid.

Define the process before deciding the sign

For lattice enthalpy of formation, the starting particles are gaseous ions:

Na⁺(g) + Cl⁻(g) → NaCl(s)

Energy is released as the lattice forms, so the formation enthalpy is negative. The reverse process is lattice dissociation:

NaCl(s) → Na⁺(g) + Cl⁻(g)

It has the same magnitude and the opposite sign. If formation is -787 kJ mol⁻¹, dissociation is + 787 kJ mol⁻¹. Both refer to one mole of solid, not one mole of each ion when the formula contains several ions.

Check: Write the lattice formation equation for magnesium chloride.

Mg²⁺(g) + 2Cl⁻(g) → MgCl₂(s). The coefficient 2 is needed to conserve atoms and charge. The process forms one mole of MgCl₂.

Compare charge, then separation

For comparable ionic structures, higher ionic charges give stronger electrostatic attraction and a larger lattice-enthalpy magnitude. With the same charges, smaller ions allow a shorter distance between their centres, strengthening the attraction.

Always distinguish a signed value from its magnitude: -900 is more negative than -700, but its magnitude is larger. Do not say that a more negative formation value means a weaker lattice.

Lattice enthalpy magnitudes for two ionic solidsThe supplied example values give MgO a much larger magnitude than NaCl. Charge, ion size and lattice structure affect the values; this comparison does not isolate charge alone.Lattice enthalpy magnitudes for two ionic solidsIonic solidMagnitude of lattice formation enthalpy (kJ mol⁻¹)
The supplied example values give MgO a much larger magnitude than NaCl. Charge, ion size and lattice structure affect the values; this comparison does not isolate charge alone.
Data table
Ionic solidMagnitude
NaCl (1+/1−)787
MgO (2+/2−)3891

Modelled example 1

Compare lattice-enthalpy magnitude

Core

Problem

Which has the more exothermic lattice enthalpy of formation, MgO or NaCl? Explain.
Study the worked solution

  1. Compare ionic charges

    Method

    Identify Mg²⁺ and O²⁻ against Na⁺ and Cl⁻.

    Reason

    The charge product is much larger for the doubly charged ion pair.

    Working

    |(+2)(-2)| = 4 for MgO; |(+1)(-1)| = 1 for NaCl.

  2. Link charge to energy

    Method

    Choose magnesium oxide and state the signed comparison.

    Reason

    Stronger electrostatic attraction releases more energy when the gaseous ions form the lattice.

    Working

    MgO has the more negative lattice enthalpy and the larger magnitude.

Try a comparison with equal charges

Which should have the more negative lattice formation enthalpy, NaF or NaCl? Assume comparable lattice structures and use the fact that F⁻ is smaller than Cl⁻.

Show the reasoning

The ionic charges are the same. The smaller fluoride ion gives a shorter distance between oppositely charged ions, so attraction is stronger. NaF therefore has the more negative formation value and larger magnitude.

Explain a sign mistake

A learner says: “MgO has a more negative lattice enthalpy than NaCl, so less energy is needed to separate its ions.” What is wrong?

Check your explanation

The learner has confused formation with dissociation. A more negative formation value means more energy is released when the lattice forms. Reversing that process requires a larger positive energy input.

Use the model to check a calculation

The charge-and-radius model predicts a trend, not an exact numerical value. To calculate a lattice enthalpy from formation, atomisation and electronic data, continue to Born–Haber cycles. For topic practice, use the Energetics and Thermodynamics check.

Syllabus and review details

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