Lattice Enthalpy Trends

01Lattice Enthalpy Trends

Ionic charge and radius

This section covers how ionic charge and ionic radius affect electrostatic attraction and lattice enthalpy.

Factors affecting lattice enthalpy

The magnitude of lattice enthalpy depends mainly on the charges and radii of the ions in the lattice. Both affect the strength of electrostatic attraction between oppositely charged ions.

Change	Effect on electrostatic attraction	Effect on lattice formation enthalpy
Smaller ionic radius	Ionic centres are closer, so attraction is stronger	More exothermic
Larger ionic radius	Ionic centres are further apart, so attraction is weaker	Less exothermic
Greater ionic charge	Oppositely charged ions attract more strongly	More exothermic

Ionic radius

As ionic radius increases, lattice formation enthalpy becomes less exothermic. The charge is spread over a larger ion and the centres of neighbouring oppositely charged ions are further apart.

The electrostatic attraction is therefore weaker, so less energy is released when the ionic lattice forms.

Key link: electrostatic attraction acts between the ionic centres. Increasing the distance between those centres weakens the attraction and reduces the magnitude of lattice formation enthalpy.

For example, $\ce{CsF}$ has a less exothermic lattice enthalpy than $\ce{KF}$ . Both compounds contain $\ce{F-}$ ions, but $\ce{Cs+}$ is larger than $\ce{K+}$ . The ions are therefore further apart in $\ce{CsF}$ and the electrostatic attraction is weaker.

Ionic charge

Greater ionic charge gives greater charge density and stronger electrostatic attraction between oppositely charged ions. Lattice formation therefore becomes more exothermic as ionic charge increases.

For example, $\ce{CaO}$ has a much more exothermic lattice enthalpy than $\ce{KCl}$ . Calcium oxide contains $\ce{Ca^{2+}}$ and $\ce{O^{2-}}$ , whereas potassium chloride contains $\ce{K+}$ and $\ce{Cl-}$ . The calcium and oxide ions are also smaller than the potassium and chloride ions, strengthening the attractions further.

Worked example: comparing lattice enthalpies

Question: Explain why $\ce{KF}$ has a more exothermic lattice formation enthalpy than $\ce{CsF}$ .

The ions have the same charges in both compounds, so charge does not account for the difference. $\ce{K+}$ is smaller than $\ce{Cs+}$ , giving a shorter separation between oppositely charged ionic centres. The attraction is therefore stronger in $\ce{KF}$ and more energy is released when its lattice forms.

Exam Tip: When comparing lattice enthalpies, consider both charge and radius. State how these alter electrostatic attraction and then link that change to a more or less exothermic lattice formation enthalpy.

02Lattice Enthalpy Trends

Perfect ionic model

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Theoretical lattice enthalpy

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Comparing the values

Ionic solid	Theoretical lattice enthalpy magnitude / $\mathrm{kJ\,mol^{-1}}$	Born–Haber lattice enthalpy magnitude / $\mathrm{kJ\,mol^{-1}}$	Difference
$\ce{NaCl}$	769	790	2.7%
$\ce{ZnS}$	3427	3615	5.5%

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03Lattice Enthalpy Trends

Covalent character

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Polarisation of anions

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When covalent character increases

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Evidence from lattice enthalpies

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