4D.15 Sum of enthalpy in equation is positive, but answer is negative

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Hannah_1G
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Joined: Wed Sep 18, 2019 12:22 am

4D.15 Sum of enthalpy in equation is positive, but answer is negative

Postby Hannah_1G » Tue Feb 11, 2020 9:30 pm

Subtracting the enthalpy of formation of the product from the reactants generates a positive number, but the answer is negative. Can someone please explain why this is the case?

Hope Hyland 2D
Posts: 50
Joined: Wed Feb 20, 2019 12:16 am

Re: 4D.15 Sum of enthalpy in equation is positive, but answer is negative

Postby Hope Hyland 2D » Tue Feb 11, 2020 11:13 pm

I had the exact same question! The first time I did it, I got +312kJ by subtracting the enthalpies of formation of reactants (C2H2 and H2) from the enthalpy of formation of product (C2H6). But in the solution, it looks like they just added all the enthalpies of formation, but flipped the sign of the product.

But then I realized that they gave us the standard enthalpies of combustion, so they actually wanted us to use Hess's law instead of adding up the standard enthalpies of formation for each component of the reaction (because they didn't give us standard enthalpies of formation, they gave us enthalpies of combustion!). So we're supposed to then write out the combustion reaction for each reactant/product and use the same procedure we normally use for Hess's law, which is why they flipped the sign on C2H6.


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