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Hess's Law

  1. Jun 6, 2012 #1
    Had a recent midterm and got this question wrong: why is Hess's Law generally not obeyed? Any ideas?

  2. jcsd
  3. Jun 8, 2012 #2
    What do you understand by Hess' law?
  4. Jun 8, 2012 #3
    That enthalpy is a state function and when chemical reactions are added, their enthalpies must be added as well. Is the answer the fact that it only accounts for the energy in the chemical bonds and not the heat or work exchanged with the surroundings?
  5. Jun 9, 2012 #4
    Can you give an example where it is violated? Since enthalpy is a state function I don't see why it would not be obeyed.
  6. Jun 9, 2012 #5
    If what I said above is right, then I'm guessing the professor meant why is Hess's Law not obeyed in real world examples where conditions and paths can vary. I feel like he should have included that in the question though....oh well. If what I said isn't right then I wouldn't know under what circumstances Hess's law wouldn't be obeyed either.
  7. Jun 9, 2012 #6
    Well, as has been said, Hess's law relates to the difference of state functions so is obeyed under any conditions which these hold.

    So you need to enquire what is there about most chemical reactions that disobeys these conditons.

    I suggest the answer is not in the reactions themselves or the sequence of reactions that make up a Hess chain, but the quantities of reactants themselves.

    The ΔHs only hold good for reactions where the reagents are in the correct molar proportions.
  8. Jun 9, 2012 #7
    But even if you don't have enough of one type of reagent, it's sort of intuitive that the resulting change in enthalpy would just be a fraction of the number of moles that reacted, which wouldn't make Hess's Law wrong...right?
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