Heat of combustion for C2H6 using Hess's law

[chemzz]

Homework Statement

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)

The Attempt at a Solution

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


is my final answer 1558.3 correct? can someone check this for me pleasez

 

[vili70]

It is close.
I have the following:
C2H6 + 2.5O2 --> 2CO2 + 3H2O + DH
Applying heats of formations for each compound:
-85 + 2.5*0 = 2*(-394) + 3*(-242) + DH
DH = 1429 kJ/mol

You may use the following calculator http://home.fuse.net/clymer/rq/ but I found it not being very accurate.