I need help please...?

Question

Write the balanced equation for the combustion of heptane and use the average bond energies to estimate the reaction energy (heat of combustion). Give your answer in kJ/mol of heptane . (Be sure to indicate the sign)

I keep getting -92, but that's not correct.

Answer 1

OK...Your balanced equation should be
C5H12 + 8 O2 --> 5 CO2 + 6 H2O

So the bonds broken would be 12 X 413 + 4 X 348 + 8 X 495
That's because you're breaking 12 C-H bonds, 4 C-C bonds and 8 O=O bonds.

Then, you're forming 10 C=O bonds and 12 H-O bonds so that term would be 10 X -799+ 12 X -463.

I get something like -3238 kJ for this reaction. Like that better? (You'd better check my values with the table in your text just to make sure the values I have are the same as yours.)