# Homework Help: Bond Dissociation Energy Problem

1. Jan 29, 2017

### Not a Wrench

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. Jan 29, 2017

### TeethWhitener

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

3. Jan 29, 2017

### Not a Wrench

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