Answer
Lattice energy is usually highly endothermic.
Work Step by Step
By definition, lattice energy is the energy required to completely separate one mole of a solid ionic compound into its gaseous ions. Lattice energy is designed to estimate the stability of ionic bonding in ionic compounds.
First, we intuitively see that ionic compounds are very stable, so to break down an ionic compound into gaseous ions require a very large energy.
The reason for this stability is due to the strong attraction between oppositely-charge ions, which draws the ions together and form a solid array.
To break down a very stable compound into gaseous ions obviously requires much energy. Therefore, lattice energy is usually highly endothermic.
