College Chemistry 100 Level Questions

Question

By convention, is Lattice Energy a positive or negative value?

I wrote Negative on my test, but the professor says the answer is positive. Is she right?

Answer 1

Depends on the definition as both signs can be valid. There is no standard convention. You can read more of theory here

https:(//)chem.libretexts.org(/)Courses(/)City_College_of_San_Francisco(/)CCSF_Chemistry_Resources(/)03%3A_CHE_202_-_General_Chemistry_II(/)3.06%3A_Intermolecular_Forces_in_Action(/)3.6.02%3A_Lattice_Energies

Answer 2

Lattice energy is conventionally negative because it represents the energy released when gaseous ions come together to form an ionic solid. The process of forming the lattice is exothermic. However, if lattice energy is being discussed in the context of breaking the lattice (separating the ions into the gaseous state), it is expressed as a positive value because energy is required to overcome the ionic attractions.

If you’re calculating it using the Born-Haber cycle, the lattice energy is typically written as a positive value because you're calculating the energy required to break the lattice.

So really it would depend on how the professor asked the question, might be worth discussing it with them though.