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Homework Help: Calculating heat of combustion

  1. Apr 24, 2005 #1


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    I am trying to solve this question and have been stumped for quite a while. Can some tell me the steps I have to take in order to solve this.
    Calculate the heat of combustion for C2H6 from the following information:
    C2H4+ 3O2----> 2 CO2 + 2 H2O -1409.5kJ
    C2H4 + H2 ----> C2H2 -136.7 KJ
    H2 + 1/2 O2 -----> H2O -285.5 kJ

    Thanks for the help
  2. jcsd
  3. Apr 24, 2005 #2
    C2H4 + H2 ----> C2H2 -136.7 KJ

    theres clearly a typo here, since the rxn isn't balanced. Is the product supposed to be C2H6?
  4. Apr 24, 2005 #3
    if so flip the 2nd equation around so you have C2H6--->C2H4+H2
    Now add that to the first one to get


    Balance the reactions and cancel out the C2H4's
    Now add in the 3rd eq H2+O2---->H20

    Balance and cancel out the H2's. You should be able to figure out the heat of combustion following this. I of course did this real sloppily. You will have to make sure you use the correct balanced equations when you do it.
  5. Apr 28, 2005 #4
    Look up Hess's Law.
    Solve for reaction: ' C2H6 -> ? '
  6. May 14, 2007 #5
    C2H6 --> C2H4 + H2 +136.7kJ
    + C2H4 + 302 --> 2CO2 + 2H20 -1409.5kJ
    = C2H6 + 3O2 --> 2CO2 + 2H2O + H2 -1272.8kJ
    + H2 + 1/2 O2 --> H2O -285.5Kj
    = C2H6 + 3.5O2 --> 2CO2 + 3H2O - 1558.3kJ

    So 1558.3kJ is the final answer?
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