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Cha. 8 HW Problem #57

Posted: Wed Jan 11, 2017 5:49 pm
What does the symbol "delta H subscript c" stand for in this problem? At first I thought it was heat of combustion, but the reaction does not involve oxygen:

C2H2(g) + 2H2(g) –> C2H6(g)

Thanks!

Re: Cha. 8 HW Problem #57

Posted: Wed Jan 11, 2017 6:08 pm
I think it IS supposed to be entalphy of combustion! on page 296 towards the bottom in Example 8.11, it says entalphy of combustion and then writes "delta H subscript c." Either way, the numbers for the entalphy are given to you, so what EXACTLY the c means does not matter in this case!

Hope this helps!

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 3:26 pm
The h sub c does in deed stand for enthalpy of combustion. I have attached a copy of my explanation of the problem. Hope you find it helpful!

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 6:28 pm
do you always need to go through the process of canceling out or can you stop before that?

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 6:44 pm
also, can you explain why we need to combine the 3 reactions to find the reaction we are looking for? Can't we just plug in the values (-1300, -1560, and -1560) we are given for C2H2, H2, and C2H6 to find the delta H of the reaction from the start without doing all the other work?

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 8:08 pm
Just using the values given, you get=>
C2H2 + 2H2 --> C2H6
-1300kJ + 2(-286kJ) - (-1560kJ) = enthalpy of the reaction
-312kJ would be the enthalpy of the reaction

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 8:26 pm
We have to combine the 3 reactions instead of simply plugging the numbers because they give us the the value of the combustion reactions, not the values of the standard enthalpy of formation. Also, if we were to plug in the values we would get positive 312 which is incorrect.

Re: Cha. 8 HW Problem #57

Posted: Thu Jan 12, 2017 8:37 pm
oh wow! It makes a lot more sense now! Thanks so much