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Homework Help: Thermochemistry homework

  1. Oct 19, 2007 #1
    1. The problem is to calculate the delta H.

    (1/2)A====> B 150 KJ/mol

    3B====> 2C + D -125 KJ/mol

    E + A====> D 350 KJ/mol

    We have to find the delta H for: B + D====> E + 2C

    2. The equation to use is just Hess's Law II

    3. Now I have tried to combine these three reactions everyway I can think of but I can't get it the reaction B + D====> E + 2C

    Is it even possible to get it there because my D's cancel out. The closest I get is to is this B====> 2C + E

    What am I doing wrong? Any help would be appreciated. :smile:
  2. jcsd
  3. Oct 19, 2007 #2

    edit: Did I not state the problem clear enough?
    edit: Never had zero responses. Oh well :confused:
    Last edited: Oct 19, 2007
  4. Oct 19, 2007 #3
    hi again,

    I'm with you on this one, I can't see a solution either, sure the original equations are correct? would be helpful if it were E + A --> 2D?
  5. Oct 19, 2007 #4


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    There is no solution from the given data. It's fairly easy to show that the final equation can not be written as a linear combination of the first 3 equations, with non-zero coefficients.

    Probably a typo!

    PS: E + A --> 2D will fix it, as would 3B --> 2C
    Last edited: Oct 19, 2007
  6. Oct 19, 2007 #5
    Thank you I've been going mad. It was on one of my midterm's and my professor said that it was possible. I knew it couldn't be because the D's cancel out. Thank you again

    Edit: The wierdest thing is that he told me the answer is -175 KJ/mol. How does he come up with an answer with no reaction. :biggrin:
    Last edited: Oct 19, 2007
  7. Oct 19, 2007 #6
    maybe use one of these, that might give his answer of -175kJ, then show him the typo!
  8. Oct 20, 2007 #7


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    Also, in general reaction enthalpies can not be specified in kJ/mol, simply in kJ. That's at least one more typo!
  9. Oct 20, 2007 #8
    Yea, we told him that part. I guess I will try the E + A --> 2D and see if I get that answer.

    Edit: Yea it looks like it was a typo. It should have be E + A --> 2D. But he didn't say their was a typo and he better not mark my answer as wrong then. Anyway thanks :wink:
    Last edited: Oct 20, 2007
  10. Oct 20, 2007 #9
    Can mark this as solved. I would do it myself but it's not letting me for some reason
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