Lattice enthalpy: the two definitions and the sign convention
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
Lattice enthalpy of formation is the enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions. For example, Na+(g) + Cl-(g) becomes NaCl(s).
Lattice enthalpy of dissociation is the reverse: one mole of an ionic solid breaks up into its gaseous ions.
Words that must appear
- One mole of an ionic solid
- From its gaseous ions
- Standard conditions
The sign
Formation is exothermic, so negative. Dissociation is endothermic, so positive. They have the same size and opposite signs, so always say which form you mean.
Do not confuse with
Its reverse: formation and dissociation have the same size and opposite signs.
Common questions
What is lattice enthalpy?
It depends which form is meant. Formation is the enthalpy change when one mole of an ionic solid forms from its gaseous ions under standard conditions. Dissociation is the reverse.
Is lattice enthalpy positive or negative?
Formation is negative. Dissociation is positive.
What is the difference between lattice enthalpy of formation and of dissociation?
They are opposites: same size, opposite sign.
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