The enthalpy of lattice dissociation is the enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions. This is always endothermic because energy must be supplied to overcome the ionic bonding
e.g. NaCl(s)-> Cl-(g) + Na+(g)
The enthalpy of lattice formation is the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions. This exothermic s energy is released when the ionic bonds form.
Cl-(g) + Na+(g)-> NaCl(s)