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When does the change in enthalpy=change in internal energy?

  1. Oct 26, 2011 #1
    Sometimes in my book, a problem justifies ΔU=ΔH for a process, such as combustion in a bomb calorimeter, by saying that since the number of moles of gas doesn't change, they are equal.

    In other questions, the number of moles doesn't change (such as an irreversible expansion of a perfect gas) but still, ΔH is different from ΔU because there is a change in temperature, so ΔH= ΔU + Δ(nRT)= ΔU + nRΔT

    When do you use the first justification? Only in a bomb calorimeter? Any time I am given a reaction and the change in molar internal energy for that reaction where the moles of gas is the same on both sides of the equation? Does this mean that the temperature of a sample in a bomb calorimeter is constant?
     
    Last edited: Oct 26, 2011
  2. jcsd
  3. Oct 27, 2011 #2

    Bill_K

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    Science Advisor

    H = U + PV, so ΔH = ΔU whenever Δ(PV) = 0.
     
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