## 8.57

Sabrina Dunbar 1I
Posts: 57
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### 8.57

How would we know to use oxygen in the proposed mechanisms for the hydrogenation of ethyne to ethane? I understand the process of Hess' Law used in the solutions manual but am confused on how the book knew to use oxygens in the separate steps.

Justin Yu 3H
Posts: 36
Joined: Fri Sep 29, 2017 7:07 am

### Re: 8.57

I'm not too sure what oxygens you are referring to. If you just use the enthalpy of combustion to calculate the reaction enthalpy, you get
-1300 - 2(-286) - (-1300) = -312 kJ mol ^-1

I looked up some stuff though; the standard enthalpy of combustion is defined as the change in enthalpy when one mole of the substance is burned completely in oxygen in standard conditions, so that might be the oxygen the problem is referring to? I don't really see how that would affect how you solve the problem though.

Sabrina Dunbar 1I
Posts: 57
Joined: Fri Sep 29, 2017 7:07 am
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### Re: 8.57

Thanks Justin. I solved it the same way you did initially but upon checking the solutions manual it proposed a three step process and used Hess's Law to solve instead of ∆H values. Your explanation was very helpful, thank you!

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