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Energetics of Compunds 2

  1. Oct 10, 2004 #1
    last question

    Calculate the energy change for the formation in kJ of 0.39 mol of KI(s)
    given the following information: I2(s) --> I2(g) : 62.44 KJ/mol

    K(s) --> K(g) : 180.4 KJ/mol

    1/2 I2(g) -->I(g) : 138.05 KJ/mol

    K+(g) + e- --> K(g) : -419 KJ/mol

    I-(g) --> I(g) + e- : 295.16 KJ/mol

    Lattice energy for KI = -649 KJ/mol

    E= E(vaporization)+IE+1/2bond energy+EA+lattice energy

    how would i do this, should i just add everything up?? but then what about the .39 mol??
  2. jcsd
  3. Oct 10, 2004 #2


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    First, do the necessary redox chemistry:

    2K --> 2K+ + 2e-
    I2 + 2e- --> 2I-

    2K + I2 --> 2KI

    Just add the necessary numbers; but if you multiply any reaction with a number, don't forget to multiply the value with it, too.

    And note that the reaction I mentioned can only become in the gaseous phase, so you'll need to use their gas-phase-transition energies given already.

    About .39 moles, just multiply the value given for 1.00 moles with .39 to learn how much energy is released (or required) for this amount.
  4. Oct 10, 2004 #3
    ok thanks, but can u set the numbers up, this is highly new to me
  5. Oct 10, 2004 #4


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    1) Convert solid potassium to the gaseous one (requires E1)
    2) Convert solid iodine to gaseous iodine with noticing its mole ratio (requires E2)
    3) Use these data in the redox reaction I posted in my previous thread (requires E3; but one released and one required energies are present here)
    4) Subtract the lattice energy from the total you found from 1-3 (releases E4); it is worth noting that the 0.39 moles are to be used here.
    5) Relax, as you've solved the problem :wink:
  6. Oct 10, 2004 #5
    i still dont understand how u convert solid potassium to the gaseous state?? I havent done this yet in class, but for some reason they give us h.w. in advance. Would it be too much if u could solve the problem and explain how u did it??
  7. Oct 10, 2004 #6


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    Gasesous state.......to become their ions; how is solid K supposed to lose an electron when its bonded strongly (and thus a solid) each other?
  8. Oct 11, 2004 #7


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    Dear Parwana,

    It is better for you to try to solve the problem by yourself. Please review what you have in hand; there are some energies given which will be used to bring solid potassium to the gaseous one, and ionize it; and also some for sublime iodine and ionize. Just find them and calculate the overall energy for one mol; and as you have less then one mol, multiply the value with the one you have.
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