How Is the Enthalpy of Formation for Lithium Fluoride Calculated?

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In summary, the enthalpy of formation for lithium fluoride (LiF) can be calculated by taking into account the following values: lithium sublimation (129 kJ/mol), lithium first ionization (520 kJ/mol), bond-dissociation energy of fluorine (159 kJ/mol F2), electron affinity for fluorine (-328 kJ/mol), and lattice energy of LiF (-1047 kJ/mol). When these values are added up, the resulting enthalpy of formation should be -617 kJ/mol. It is important to note that when calculating bond-dissociation energy and electron affinity, the values should be halved since only half of the F2 and F atoms are needed for the
  • #1
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?
 
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  • #2
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? :rolleyes:
 
  • #3
I like Serena said:
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? :rolleyes:
I'm confused why there sublimation would cost half though ;(
 
  • #4
Lori said:
I'm confused why there sublimation would cost half though ;(
Aren't we talking about the bond-dissociation energy of F2?
 
  • #5
I like Serena said:
Aren't we talking about the bond-dissociation energy of F2?
Oops I meant the bond dissociation . Yeah
 
  • #6
Lori said:
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.
 
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  • #7
I like Serena said:
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
 
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FAQ: How Is the Enthalpy of Formation for Lithium Fluoride Calculated?

1. What is the Born Haber cycle?

The Born Haber cycle is a thermodynamic model used to calculate the lattice energy of an ionic compound. It involves a series of steps that describe the formation of the compound from its constituent elements.

2. Why is the Born Haber cycle important?

The Born Haber cycle allows scientists to determine the lattice energy, which is a crucial factor in determining the stability and physical properties of ionic compounds. It also provides insight into the energetics of chemical reactions and helps to understand the bonding in ionic compounds.

3. What are the steps involved in the Born Haber cycle?

The steps of the Born Haber cycle include the formation of gas phase atoms, the formation of ions, the formation of the solid compound, the sublimation of the solid compound, and the formation of the gas phase compound. These steps are connected by Hess's Law, which states that the overall energy change of a reaction is independent of the pathway taken.

4. How is the Born Haber cycle used in practice?

The Born Haber cycle is often used in laboratory experiments to determine the lattice energy of ionic compounds. It is also used in theoretical calculations to predict the lattice energy of compounds and understand the factors that influence it, such as ionic size and charge.

5. Are there any limitations to the Born Haber cycle?

Yes, there are limitations to the Born Haber cycle. It assumes ideal conditions, such as perfect crystalline structures and no thermal or electronic defects, which may not always be the case in real compounds. It also does not take into account factors such as solvation energy or covalent interactions, which can affect the lattice energy of certain compounds.

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