Hi, In my book it says that it is difficult to determine the lattice energy of NaCl so they use the Haber cycle which applies Hess' law. Lattice is the energy change when a solid ionic substance separates into ions in gas phase. We could simply increase tempetrature untill NaCl breaks down into ions & then calculate the heat that has been absorbed. So why do they say that it is difficult to determine the lattice? Is it because we must achieve very high temperatures to melt salt? Thank you for helping me.