ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

Lattice energies are often reported as positive values because they represent the amount of energy required to break apart an ionic solid into its gaseous ions, which is an endothermic process. This energy requirement indicates that energy must be supplied to overcome the attractive forces holding the ions together in the solid lattice. By convention, when discussing lattice energy in terms of breaking apart the solid, it is expressed as a positive value to emphasize that energy is being absorbed from the surroundings.

In contrast, when lattice energy is considered in the context of formation of the solid from its gaseous ions—a process that releases energy—it would be expressed as a negative value in that situation. However, since the question specifically pertains to why these energies are often reported positively, the correct interpretation hinges on the endothermic nature of the lattice energy when ions are separated. This measurement aligns with the conventions of how energy changes are typically reported in thermodynamics.