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Calculating lattice energy on ionic compound

  1. Jan 21, 2013 #1
    1. The problem statement, all variables and given/known data

    Given the following thermodynamic data, calculate the lattice energy of CaBr2(s) caculate the lattice enegy:

    (A) Δ°Hf CaBr2(s) = -675 kJ/mol
    (B) Δ°Hf Ca(g) = 179 kJ/mol
    (C) Δ°Hf Br(g) = 112 kJ/mol
    (D) 1st ionization energy of Ca = 590 kJ/mol
    (E) 2nd ionization energy of Ca = 1145 kJ/mol
    (F) Electron affinity of Br = -325 kJ/mol


    2. Relevant equations

    Heats of Formation + ionization energies + electron affinity - lattice energy = heat of formation



    3. The attempt at a solution

    (B) + (C) + (D) + (E) + (F) + Lattice Energy = -675

    solving for lattice energy:

    [179 + 112 + 590 + 1145 + -325] + Lattice Energy = -675

    Lattice energy = -675 - 1701

    Lattice energy = -2376 kJ/mol

    I think I am going wrong with something in the calculations of the energy bromine contributes to the equation. I know bromine is diatomic but I'm not sure how to incorporate that into the equation. I don't want you to give me the answer because I really want to be able to do this on my own but anything you can point out that can get me in the right direction are appreciated.
     
  2. jcsd
  3. Jan 22, 2013 #2

    Borek

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    Staff: Mentor

    You are not following stoichiometry - you need to deal with two Br atoms all the time. 112 kJ/mol for Δ°Hf Br(g) is a dissociation enthalpy - but it produces just one Br atom, and you need two.

    It should became obvious if you will try to write a "reaction equation" - after all, Hess law is just a way of dealing with the energy conservation. Mass conservation and charge conservation are used to balance reaction equation, same thinking can be applied to the energy conservation.
     
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