Chemistry
posted by Garcia .
Consider an ionic compound, MX2, composed of generic metal M and generic halogen X.
The enthalphy of formation of MX2= 891.
The enthalphy of sublimation of M= 141.
The first and second ionization energies of M are 605 and 1392.
The electron affinity of X= 339.
The bond energy of X2= 177.
I used the BornHaber cycle and came up with:
Lattice energy of MX2=891(141+88.5+605678) to come up with the answer that the lattice energy of MX2 is 1047.5 but i'm getting marked wrong. Where did I go wrong?????

I just took a brief look; where is the 1392 for second ionization potential? I don't see that in your calculation.

Where would I add that in to my calculation? Like would it look like this:
Lattice energy of MX2=891(141+88.5+605+1392678) 
Yes, that should do it.
Also, I wonder about taking 1/2 x 177. If you do Cl2 bond dissociation for say NaCl, then you take 1/2 Bond energy because you're using only 1/2 of it to make Cl^. But in this case you're using both parts of the X2 bond energy to make 2X^. You multiplied 2 x electron affinity (and that is proper). 
where is the 678 coming from?

678 is electron affinity * 2

Shouldn't it be +678 because the formula is E= deltaHf(H sub + IE + HBE/2) HEA?