How Is the Enthalpy of Formation for Lithium Fluoride Calculated?

[Lori]

Homework Statement

calculate enthalpy of formation for lithium fluoride... LiF

Homework 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

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?

 

[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?

 

[Lori]

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?

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

 

[I like Serena]

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

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

 

[Lori]

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

Oops I meant the bond dissociation . Yeah

 

[I like Serena]

Oops I meant the bond dissociation . Yeah

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.

 

[Lori]

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.

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