## Hw question 8.57

Chelsey_Schaum1H
Posts: 15
Joined: Fri Jul 22, 2016 3:00 am

### Hw question 8.57

Is it possible to use standard enthalpies of formation instead of Hess' Law? I did standard enthaplies of formation and got 312. The answer is -312, I can't figure out where the negative comes from.

Chem_Mod
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### Re: Hw question 8.57

Please post the entire problem, and also your work. I don't know what the question says, and without your work it is impossible to see how you got your answer.

Sue_Park_2D
Posts: 13
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Hw question 8.57

In this specific question of the HW you need to use Hess's Law, since they give you information of the standard enthalpy of combustion of C2H2, C2H6, and H2.

But I think you can use standard enthalpies of formation because I tried calculating and I got the same answer of -312 kJ/mol.

You might have gotten 312kJ/mol if you forgot that you calculate the reaction standard reaction enthalpy by calculating standard enthalpy of formation of all products and subtracting standard enthalpy of formation of all reactants from it.

You could have possibly subtracted the other way around.

Hope this helps!

Melody Li 2L
Posts: 11
Joined: Fri Jul 22, 2016 3:00 am

### Re: Hw question 8.57

In this problem specifically, I believe you need to use Hess's Law because after balancing the equations of C2H2, C2H6 and H2 using combustion, one of the equations will be flipped/reversed in order to do cancellation. This reversed equation will result in a sign change of the enthalpy. (Ex: If change in enthalpy was -1560 kJ for C2H6, it will become 1560 kJ.) Perhaps try doing it through Hess's Law and you should get the correct answer.

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