What Factors Affect the Strength of Lattice Enthalpy? (HL)

The magnitude of lattice enthalpy increases when the ions have higher charges and smaller ionic radii. Both factors strengthen the electrostatic attraction between oppositely charged ions in the lattice.

The lattice enthalpy of formation is the enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions. A stronger lattice has a more negative formation value. If lattice enthalpy is defined as lattice dissociation enthalpy, the value is more positive, so it is safest to compare magnitudes.

The electrostatic attraction can be modelled qualitatively using:

$$|\Delta H_{\text{latt}}| \propto \frac{|q_+q_-|}{r}$$

Here, $q_+$ and $q_-$ are the ionic charges, while $r$ is the distance between the ion centres, approximately related to the sum of their ionic radii.

This electrostatic model treats ions as spherical point charges. Real lattices may show partial covalent character because a small, highly charged cation can polarize a large anion's electron cloud. Therefore, experimental lattice enthalpy may differ from a purely ionic theoretical value, especially when polarization is strong.

For example, $\text{MgO}$ has a much greater lattice enthalpy magnitude than $\text{NaCl}$. The charge product is $|(+2)(-2)|=4$ for $\text{MgO}$, compared with for . Similarly, has a greater magnitude than because is smaller than .

A common misconception is that larger ions form stronger lattices because they contain more electrons. In fact, larger ionic radii increase the distance between charges and weaken electrostatic attraction.

In an IB Chemistry HL explain question, identify charge and radius, then connect each explicitly to electrostatic attraction. In BorntoHaber calculations, state the lattice enthalpy convention and sign clearly before substituting values.