1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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)
    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


    User Avatar
    Science Advisor
    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted