Bond Dissociation Energy Problem

[Not a Wrench]

Homework Statement

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.)

Homework Equations

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
ΔH_rxn = ΣD(reactants) - ΣD(products)
http://imgur.com/a/G6e1A

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?

 

[TeethWhitener]

How many CO₂ molecules are in your balanced equation? How many did you include in your bond energy calculation?

 

[Not a Wrench]

How many CO₂ molecules are in your balanced equation? How many did you include in your bond energy calculation?

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.