# Born Haber Cycle (help please)

1. Dec 1, 2017

### Lori

1. The problem statement, all variables and given/known data
calculate enthalpy of formation for lithium fluoride... LiF

2. Relevant equations
Given from the worksheet is:

Lithium sublimation = 129 kj/mol
Lithium first ionization = 520 kj/mol
bond-dissociation energy of fluorine is 159 kj/mol F2
electron affinity for fluorine is -328 kj/mol
lattice energy of LiF = -1047 kj/mol

3. The attempt at a solution
The chemical equations i wrote/figured out are:

Li(s) -> Li(g) 159 kj
Li(g) -> Li+(g) + e- 520 kj
1/2F2(g) - > F(g) -159 kj
F(g) + e- -> F-(g) -328 kj
Li+(g) + F-(g) -> LiF(s) -1047

When i added it all up to find enthalpy formation , i get -537 kj but the answer is -617. what did i do wrong?

Last edited by a moderator: Dec 1, 2017
2. Dec 1, 2017

### I like Serena

Hey Lori!

Shouldn't we have F2(g) - > 2F(g) -159 kJ, so that it only costs half?
And shouldn't Li+(g) + F-(g) -> LiF(s) have a positive energy that is released when we bring oppositely charged particles together?
And shouldn't F(g) + e- -> F-(g) have a positive energy as well, since the electron bonds naturally?
We should be consistent with the minus signs, shouldn't we?

3. Dec 1, 2017

### Lori

I'm confused why there sublimation would cost half though ;(

4. Dec 1, 2017

### I like Serena

Aren't we talking about the bond-dissociation energy of F2?

5. Dec 1, 2017

### Lori

Oops I meant the bond dissociation . Yeah

6. Dec 1, 2017

### I like Serena

Isn't bond dissociation for F2, leaving us with 2 F atoms?
We can make 2 LiF with that.
So the cost for 1 LiF is half of that.

7. Dec 1, 2017

### Lori

Oh it makes sense now. It's cause we only needed 1 so I took one half of the F so I should also take one half of the cost