Lattice enthalpy measures the strength of the electrostatic attractions within an ionic lattice. Its magnitude increases when the ions have higher charges and smaller ionic radii.
In the IB ionic model, oppositely charged ions form a three-dimensional lattice held together by electrostatic attraction. Lattice enthalpy of formation is the enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions:
This process is exothermic, so the lattice enthalpy of formation is negative. Some sources instead define lattice enthalpy as the energy required to separate the solid into gaseous ions; this has the same magnitude but a positive sign. Always check the definition given in the question.
The attraction follows the principle expressed by Coulomb's law: its strength is approximately proportional to
where and are the ionic charges and represents the distance between the ion centres.
| Factor | Effect on lattice enthalpy magnitude | Reason |
|---|---|---|
| Higher ionic charges | Increases | A larger charge product produces stronger electrostatic attraction. |
| Smaller ionic radii | Increases | The ion centres are closer together, strengthening attraction. |
| Larger ionic radii | Decreases | Greater separation weakens electrostatic attraction. |
For example, has a much greater lattice enthalpy magnitude than . The charge product is for , compared with for , and the ions in are relatively small.
A common misconception is that a “stronger” lattice has a more positive formation value. In fact, stronger attraction gives a more negative lattice enthalpy of formation, but a larger absolute magnitude.
For an IB explain question, compare both ionic charge and ionic radius, then link them explicitly to electrostatic attraction. Lattice enthalpy calculations using Born Haber cycles are AHL content, while the underlying ionic bonding model belongs to Structure 2.1.
