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Bond Dissociation Energy Problem

  1. Jan 29, 2017 #1
    1. The problem statement, all variables and given/known data
    Calculate an approximate heat of combustion for propane (C3H8) in kilojoules per mole by using the bond dissociation energies in the table. (The strength of the O=O bond is 498 kJ/mol, and that of a C=O bond in CO2 is 804 kJ/mol.)

    2. Relevant equations
    C3H8 + 5O2 → 3CO2 + 4H2O
    ΔHrxn = ΣD(reactants) - ΣD(products)
    http://imgur.com/a/G6e1A
    3. The attempt at a solution
    Looked at propane model found 8 C-H bonds and 2 C-C bonds.
    5 O-O (double bonds) found in oxygen
    2 C-O bonds found in carbon dioxide
    2 H-O bonds found in water

    (8(410)+2(350)+5(490)) - (2(804)+4(2*460))= 1182 kJ/mol (obviously not right. Combustion is exothermic) Could anyone help?
     
  2. jcsd
  3. Jan 29, 2017 #2

    TeethWhitener

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    Gold Member

    How many CO2 molecules are in your balanced equation? How many did you include in your bond energy calculation?
     
  4. Jan 29, 2017 #3
    There are 3CO2 molecules equaling 3(2*350) = 3(700). I reworked it again. Getting [8(410)+2(350)+5(498)] - [3(804)+4(2*460)] = -1672 kJ/mol. But that is also wrong and I only have one more chance to answer it. I'm positive I'm counting the number of bonds of one molecule incorrectly.

    Edit: Nevermind. I wrote everything down this time and got it right. I was getting lost in my head keeping track of which molecules were on the reactants side and which were on the products. I arrived at -2034 kJ/mol which is the correct answer.
     
    Last edited: Jan 29, 2017
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