Lattice energy and energy required to vaporize MgO ?

[Hurricane939]

I understand that the lattice energy is the energy released when an solid ionic compound forms or it is the energy required to separate completely a mole of a solid ionic compound into its gaseous ions. So is second definition the same thing as vaporizing an ionic compound and so we can directly use the lattice energy instead of the calculations we usually do ?

For example, I tried to calculate the energy required to vaporize one mole of Magnesium oxide (MgO) using the ordinary method. I got the melting and boiling points, latent heat of fusion and vaporization from here: http://www.microkat.gr/msdspd90-99/Magnesium%20oxide.htm. I then calulated the energy to raise the temperature to melting point + the heat of fusion + the energy to raise the temperature to boiling point + and the heat of vaporization. The result was 560 kJ/mole of MgO.

By trying to do the same using the lattice energy approach, I found that the lattice energy of MgO is 3800 kJ/mole, which is about 7 times the calculated energy above.

So my questions here are:
1- Is the second definition of lattice energy the same as the energy needed to vaporize an ionic compound and so they can be used interchangeably ?
2- If the answer to question (1) is yes, then why don't the two approaches give the same result ?

 

[DrDu]

Look for Born-Haber cycle.

 

[Hurricane939]

Look for Born-Haber cycle.

I did. But the cycle starts (or ends) with Mg++ and O-- ions, and I am not sure what the products of vaporizing a mole of MgO will be. In other words, if I vaporize MgO, will I get MgO gaseous molecules, or Mg++ and O-- gaseous ions ?

 

[DrDu]

From what I remember, it is the second statement which is right, i.e the energy needed to make up an ionic crystal from infitely separated ions.
Upon vaporization, you get almost exclusively neutral atoms, so you will have to include ionization energies in your calculation.

 

[Hurricane939]

I think I found the answer here: https://books.google.com.eg/books?i...attice energy and vaporization energy&f=false
When an ionic compound like NaCl (or MgO) vaporizes, it becomes gaseous molecules rather than ions. The difference in energy between what I calculated and the lattice energy is the energy needed to turn the molecules into ions.

 

[DrDu]

I fear reality is even more complicated than this. In gas phase you will have a mixture of atoms, diatomic molecules and small clusters and also some ions, their relative proportion depending on temperature and pressure.

 

[Hurricane939]

I am talking about the least temperature that an ionic compound can vaporize at. As mentioned in the link above, the heat of sublimation is about 1/3 the value of lattice energy for NaCl. Remember that lattice energy is the energy needed to vaporize an ionic compound into its gaseous ions, so if the heat of sublimation is 1/3 of this lattice energy, then this definitely means that this ionic compound will vaporize into ion pairs first, and if you supply it with the 2/3 remaining energy, it will become gaseous ions.

What I mean here is that ion pairs of NaCl or MgO will be the most abundant once the gas vaporizes. One other way to think about this is to think about plasma since plasma by definition is a gas of ions. And that's why I think there will be very very few ions once an ionic compound vaporizes, because you can't make plasma by just vaporizing some table salt, can you ?

 

[DrDu]

http://scitation.aip.org/content/aip/journal/jcp/104/17/10.1063/1.471389

 

[Hurricane939]

There is still nothing that says ions can exist in an ionic compound vapor that has been vaporized with temperature just above the boiling point. You will have to supply much more energy to start getting separate ions.

 

[PietKuip]

The O2- ion does not even exist in vacuum.
In the solid, two charges per ion is not realistic. The compound is quite covalent.

 

[Hurricane939]

The O2- ion does not even exist in vacuum.
In the solid, two charges per ion is not realistic. The compound is quite covalent.

I am sorry I don't get what you mean. Do you mean that MgO is not an ionic compound ?!

 

[PietKuip]

I just mean that there is no such thing as a free O2- ion. The single-charge negative oxygen ion will repel an extra electron.

The purely ionic model works reasonably well for alkali halides, but also those have some degree of covalency. In the case of oxides and sulfides, the ionic model overestimates cohesive energies.