1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Born Haber Cycle (help please)

  1. Dec 1, 2017 #1
    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. jcsd
  3. Dec 1, 2017 #2

    I like Serena

    User Avatar
    Homework Helper

    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:
  4. Dec 1, 2017 #3
    I'm confused why there sublimation would cost half though ;(
  5. Dec 1, 2017 #4

    I like Serena

    User Avatar
    Homework Helper

    Aren't we talking about the bond-dissociation energy of F2?
  6. Dec 1, 2017 #5
    Oops I meant the bond dissociation . Yeah
  7. Dec 1, 2017 #6

    I like Serena

    User Avatar
    Homework Helper

    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.
  8. Dec 1, 2017 #7
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted