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What am I doing wrong? Thermochemistry

  1. Oct 20, 2007 #1
    [SOLVED] What am I doing wrong? Thermochemistry

    1. Calculate the heat per mole of CO2 produced for octane.

    2. I just balanced the equation and calculated the total heat produced. Subtracted the heat of water produced from it. Then took that and divided it by the number of moles.

    2(C8H18) + 5O2====> 16CO2 + 18H20

    3. My answer was -359.5 KJ/mol. But thats not one of the choices. So what did I do wrong?
     
  2. jcsd
  3. Oct 20, 2007 #2

    Gokul43201

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    If you throw in all the enthalpies you used in the calculation, that'll make it much easier to debug any errors.

    Also, what are the choices?

    PS: Shouldn't that be 25 O2?
     
    Last edited: Oct 20, 2007
  4. Oct 20, 2007 #3
    Yes, it is 25 O2 sorry for that. The enthalpies I used:

    octane- -296 KJ
    H2O- -286 KJ
    CO2- -394 KJ

    Their are 9 choices
    all in KJ/mol

    -400
    -1000
    -740
    -1100
    -320
    -684
    -468
    -840
    -890
     
    Last edited: Oct 20, 2007
  5. Oct 20, 2007 #4

    Gokul43201

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    Looks to me like it should come out around -610kJ/mol. What data are you expected to use? Bond enthalpies, combustion enthalpies, calorific values...there are many ways to skin the bird.

    Edit: that should be -670kJ/mol (approximately) - closest option is -684kJ/mol

    Edit2: Found better data; now I get -679kJ/mol
     
    Last edited: Oct 20, 2007
  6. Oct 20, 2007 #5
    How did you do it? I did combustion enthalpies

    Edit: I just did it again and i got -690 KJ/mol I have no clue how i kept getting -359:redface:

    Edit (part 2): Thank you
     
    Last edited: Oct 20, 2007
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