Are lattice energies always positive or negative?

Question

apparently, when I use the Born-Haber cycle to calculate it it's negative, but in the textbooks all the lattice energies are listed as positive. Why?

Answer 1

Lattice energy is always positive. It's the energy change for the process of the ionic compound dissociating into the separated ions in the gas phase. The dissociated ions are always higher in energy than the ionic solid itself.

If you think you have a negative lattice energy, it's probably because you're thinking about the energy change for the reverse process, going from the dissociated ions to the ionic solid. (You'd have the right number but opposite sign if this is the case.) It's also possible you might have just forgotten a term in your Born-Haber cycle, and haven't gotten to the correct answer yet.

Good luck!

Answer 2

if you use enthalpy values to calculate the energy, it is always negative, because it represents the energy released in the process of binding ions into a crystal, it's always a good idea to specify what your value is , some books consider lattice energy to be the energy necesary to break the crystal bonds and form ions, other books consider lattice enrergy to tbe the energy released when bonding the ions into a crystal, it's thr same value, just with an opposite sign.

Answer 3

most of the time it is positive but i think your case is exception negative( the magnitude must be quite small, if too large try to check your born haber cycle again...probably somteing go wrong). it must be due to factor like entropy factor that the ionic compound to be able to be formed in the first place.