What am I doing wrong? Thermochemistry

[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 that's not one of the choices. So what did I do wrong?

 

[Gokul43201]

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?

 

[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

 

[Gokul43201]

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

 

[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