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-   -   Heat of combustion (http://www.physicsforums.com/showthread.php?t=75750)

 123456 May15-05 06:35 PM

heat of combustion

Calculate the heat of combustion for C2H6 from the following information:
C2H4 + 3 CO2 --> 2 CO2 + 2 H2O /\H f -1409.5 kJ
C2H4 + H2 --> C2H2 /\H f - 136.7 kJ
H2 + 1/2 O2 --> H2O /\H f - 285.5 kJ

i saw someone else ask this question and they were referred to look at Hess's law.. i have done that but i am still unable to solve this i am so stuck.
could someone walk through the steps please?

 gravenewworld May15-05 07:03 PM

C2H4 + H2 --> C2H2 /\H f - 136.7 kJ
C2H4 + 3 CO2 --> 2 CO2 + 2 H2O /\H f -1409.5 kJ

these makes no sense(they are not balanced eqs)....did you mean for the product to be C2H6 in the 2nd eq.????????

 123456 May15-05 07:06 PM

hmm

no, what i wrote was exactly how its written in the book... is it not able to be solved, possibly they made a typo in the book???

 Astronuc May15-05 07:38 PM

This C2H4 + H2 --> C2H2 does not make sense.

Try C2H4 + H2 --> C2H6 perhaps. If this is in the book, there is a probably a typo.

 123456 May15-05 09:05 PM

I see, must be a typo...

Ohhh I see what you mean now, when you said it doesnt balance. It must be a typo then. Is there a formula or anything you use to calculate heat of combustion.. or is it just knowledge...???

 gravenewworld May16-05 02:39 PM

play around w/ the equations like a puzzle and treat them like math equations. if two of the same things appear on the left and right side of the arrow you can cancel them out. use those equations to come up w/ the equation for C6H6+O2-->CO2 +O2 (i'm too lazy to balance). Remember, if you flip an equation around you change teh sign of the Enthalpy of the rxn...

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