Homework Help: What am I doing wrong? Thermochemistry

1. Oct 20, 2007

ace123

[SOLVED] What am I doing wrong? Thermochemistry

1. Calculate the heat per mole of CO2 produced for octane.

2. I just balanced the equation and calculated the total heat produced. Subtracted the heat of water produced from it. Then took that and divided it by the number of moles.

2(C8H18) + 5O2====> 16CO2 + 18H20

3. My answer was -359.5 KJ/mol. But thats not one of the choices. So what did I do wrong?

2. Oct 20, 2007

Gokul43201

Staff Emeritus
If you throw in all the enthalpies you used in the calculation, that'll make it much easier to debug any errors.

Also, what are the choices?

PS: Shouldn't that be 25 O2?

Last edited: Oct 20, 2007
3. Oct 20, 2007

ace123

Yes, it is 25 O2 sorry for that. The enthalpies I used:

octane- -296 KJ
H2O- -286 KJ
CO2- -394 KJ

Their are 9 choices
all in KJ/mol

-400
-1000
-740
-1100
-320
-684
-468
-840
-890

Last edited: Oct 20, 2007
4. Oct 20, 2007

Gokul43201

Staff Emeritus
Looks to me like it should come out around -610kJ/mol. What data are you expected to use? Bond enthalpies, combustion enthalpies, calorific values...there are many ways to skin the bird.

Edit: that should be -670kJ/mol (approximately) - closest option is -684kJ/mol

Edit2: Found better data; now I get -679kJ/mol

Last edited: Oct 20, 2007
5. Oct 20, 2007

ace123

How did you do it? I did combustion enthalpies

Edit: I just did it again and i got -690 KJ/mol I have no clue how i kept getting -359

Edit (part 2): Thank you

Last edited: Oct 20, 2007