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Homework Help: Do enthropy and enthalpy changes depend on pressure?

  1. Oct 28, 2013 #1
    1. The problem statement, all variables and given/known data
    I have a homework which include this question: Al+1/2 N2-> AlN in this reaction find enthropy and enthalpy changes at 298 K and 1 atm. and repeat these at 298 K and 10 atm.

    entropy changes at 298 K
    aluminum 6.77 cal/mol-K
    Nitrogen 45.77
    Aluminum nitride 4.82

    enthalpy change of formation of AlN is -76,470 cal/mol

    3. The attempt at a solution

    i found them at 1 atm but i could not find at 10 atm. are the answers same or do they depend on pressure, if they depend, how can i calculate them?
  2. jcsd
  3. Oct 28, 2013 #2


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    Staff: Mentor

    How did you get your values for 1 atm? Did you use the pressure there?
  4. Oct 28, 2013 #3
    The entropy and the enthalpy of the N2 and the AlN (if it is a gas) are functions of pressure. The changes for a solid with pressure can usually be neglected. You can get the change in free energy with respect to pressure of a gas pretty easily if you know the P-V-T behavior of the gas, and you can also do the same for the change in enthalpy. The change in entropy then follows from these. Is AlN a solid or a gas at 1 atm and 298? Of course, if you are willing to assume ideal gas behavior, it's much simpler, because the change in enthalpy is zero, and the change in entropy is R times the natural log of the pressure ratio.
    Last edited: Oct 28, 2013
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