# Do I need to include lattice energy when calculating the energy required to vaporize NaCl?

I understand that the lattice energy is the energy released when a crystal forms and that's the reason it takes the negative sign. But do we need to add this energy again when we are trying to vaporize an ionic compound ?

For example, let's assume we want to completely vaporize a mole of NaCl starting at room temperature, how much energy do we need ?

I don't know neither the specific heat nor the latent heat of vaporization of NaCl, but I understand we would need:

1- Energy to raise the temperature of mole of NaCl from 298 °K to its boiling point.

2- Energy equal to the latent heat of vaporization of a mole of NaCl.

I don't know whether NaCl would go through the liquid state or not, but if it does then we would need to add the latent heat of fusion too.

Now, according to wikipedia, NaCl has a lattice energy of −756 kJ/mol. So my question here is: do we need to add this lattice energy to the calculated steps above to account for all the energy required to vaporize a mole of NaCl ?

• Well you left out steps...(1) Energy to raise NaCl to melting point (2) Energy to melt solid NaCl to liquid NaCl at its melting point (3) Energy to raise liquid NaCl from melting point to boiling point (4) Energy to vaporize NaCl at its boiling point. // So the lattice energy is (2) – MaxW Nov 18 '15 at 23:49
• @MaxW, I wasn't sure if NaCl would turn to liquid first. But anyway, is't (2) the latent heat of fusion ? Also, once an ionic compound is melted, this means that there are no more lattices to break, is that correct ? – Abanob Ebrahim Nov 18 '15 at 23:54
• Sorry to have mislead you. The lattice energy is defined as the energy liberated when gaseous ions are united to form the solid lattice. This would be at a constant temperature. So the whole heating it to some other temperature doesn't apply. – MaxW Nov 19 '15 at 0:12
• I'm sure that measuring lattice energy from ions is impractical so that the Born-Haber cycle is used. en.wikipedia.org/wiki/Born%E2%80%93Haber_cycle // I upvoted you question in hope that someone who has studied thermodynamics more recently than I can answer your question. I'm a bit confused about the temperature dependence of the lattice energy. – MaxW Nov 19 '15 at 0:42
• @MaxW, its also important to find out what we will end up with once we vaporize the NaCl mole. Will we get Na+ and Cl- or just molecules of NaCl in gas phase ? – Abanob Ebrahim Nov 19 '15 at 2:47