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Lattice Energy - Hard question - I keep getting wrong answer

  1. Dec 14, 2009 #1
    1. The problem statement, all variables and given/known data

    What is the lattice energy of KI? Given the information below.

    Heat of formation for KI = -328 kJ/mol

    Heat of sublimation for K = 89.20 kJ/mol

    Ionization energy for K = 419 kJ/mol

    Bond dissociation energy for I2 = 149 kJ/mol

    Electron affinity for I = - 295 kJ/mol

    Heat of sublimation for I2 = 48.30 kJ/mol

    I used Born-Haber's law: Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

    2. Relevant equations

    Born-Haber's law = Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

    3. The attempt at a solution

    I used Born-Haber's law: Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

    -328 = (89.20 - 48.30) + .5 (149) + 419 - 295 + Lattice Energy

    Lattice Energy = -567.4 (Which is wrong; it should be a positive answer)
     
  2. jcsd
  3. Dec 14, 2009 #2
  4. Dec 14, 2009 #3
    I plugged in +567.4 and it was wrong. What is going on? What does that variable V equal to in relation to my problem in the equation that you gave me from wiki?
     
  5. Dec 14, 2009 #4
    also, it says you should subtract the electron affinity, there for making it positive in this problem. if that's the right equation you're trying to use.
     
  6. Dec 14, 2009 #5
    Ok but what is the variable "V" in my problem in relation to the equation from wikipedia:

    Delta H = V + .5 B + IE - EA - U
     
  7. Dec 14, 2009 #6
    It explains all of the variables underneath, but..

    V is the enthalpy of vaporization for metal atoms.
     
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