Estimation of change in enthalpy of a gas phase reaction using bond enthalpies [closed]








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Estimate ∆H for the following gas phase reaction: 
$$\ce{C2H4 + 3 O2 ⟶ 2 CO2 + 2H2O}$$


I have been having trouble on this problem for a long time, and I was hoping someone would help me with it. I tried using the average bond enthalpies, but that always got me the wrong answer. Below is a table of average bond enthalpies in kJ/mol. 


[![enter image description here](https://i.stack.imgur.com/3eZ7Y.png)](https://i.stack.imgur.com/3eZ7Y.png)




Use [Hess' Law](https://en.wikipedia.org/wiki/Hess%27s_law): $\Delta$H = $\Sigma$ Enthalpy of Bonds Broken - $\Sigma$ Enthalpy of Bonds Formed


**Reactants** 


* Bond enthalpy of C2H4 = 602 + (413 \* 4) = 2254 kJ mol-1
* Bond enthalpy of O2 = 3 \* 498 = 1494 kJ mol-1
* $\therefore$ $\Sigma$ Enthalpy of Bonds Broken= 2254 + 1494 = 3748 kJ mol-1


**Products**


* Bond enthalpy of CO2 = (799 \* 2) \* 2 = 3196 kJ mol-1
* Bond enthalpy of H2O = (463 \* 2) \* 2 = 1852 kJ mol-1
* $\therefore \Sigma$ Enthalpy of Bonds Formed = 3196 + 1852 = 5048 kJ mol-1


**Overall**


* $\Delta$H = 3748 - 5048 = -1300 kJ mol-1




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After having a quick Google search for the enthalpy change of combustion for ethene, it came up as -1411 kJ mol-1. Our calculated answer isn't far off. I'm hoping we got the same answer.


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