Calculating heat of combustion

RPN

Hello
I am trying to solve this question and have been stumped for quite a while. Can some tell me the steps I have to take in order to solve this.
Question
Calculate the heat of combustion for C2H6 from the following information:
C2H4+ 3O2----> 2 CO2 + 2 H2O -1409.5kJ
C2H4 + H2 ----> C2H2 -136.7 KJ
H2 + 1/2 O2 -----> H2O -285.5 kJ

Thanks for the help

gravenewworld

C2H4 + H2 ----> C2H2 -136.7 KJ

theres clearly a typo here, since the rxn isn't balanced. Is the product supposed to be C2H6?

gravenewworld

if so flip the 2nd equation around so you have C2H6--->C2H4+H2
Now add that to the first one to get

C2H6+C2H4+O2----->CO2+H20+C2H4+H2

Balance the reactions and cancel out the C2H4's
Now add in the 3rd eq H2+O2---->H20
C2H6+H2+O2----->CO2+H20+H2

Balance and cancel out the H2's. You should be able to figure out the heat of combustion following this. I of course did this real sloppily. You will have to make sure you use the correct balanced equations when you do it.

r3dxP

Look up Hess's Law.
Solve for reaction: ' C2H6 -> ? '

ND3G

C2H6 --> C2H4 + H2 +136.7kJ
+ C2H4 + 302 --> 2CO2 + 2H20 -1409.5kJ
= C2H6 + 3O2 --> 2CO2 + 2H2O + H2 -1272.8kJ
+ H2 + 1/2 O2 --> H2O -285.5Kj
= C2H6 + 3.5O2 --> 2CO2 + 3H2O - 1558.3kJ

So 1558.3kJ is the final answer?