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Hess's Law?
by Maggie16 Leaving Cert Chemistry — 11/06/16 5

Coming up with the equations is grand, and finding out the heat of reaction. But I'm always confused about the signs? How do you know whether the heat of formation is a plus or a minus. Eg. when they give you the heats of formation of carbon dioxide, water, butane: -393.5, -285.8, -125.7. Why are carbon dioxide and water values negative but the butane sign is changed to +125.7. Thanks in advance.


5 Comments
taengoogle.09 — 06/06/16
I don't know if you do the same method as us but this should be the solution. Total Heat of Products - Total heat of reactants is the heat of combustion. For example in this one it'd be CO2 = -393.5 , h20 - -285.8 and butane = -125.7. in the balanced equation there is 4 co2 and 5 water. so the equation would be like this -> (4(-393.5) + 5(-285.8)) - (-125.7) = -2877.3 kJ the reason itt was changed to 125.7 was because (-)(-) =(+)
taengoogle.09 — 06/06/16
I don't know if you do the same method as us but this should be the solution. Total Heat of Products - Total heat of reactants is the heat of combustion. For example in this one it'd be CO2 = -393.5 , h20 - -285.8 and butane = -125.7. in the balanced equation there is 4 co2 and 5 water. so the equation would be like this -> (4(-393.5) + 5(-285.8)) - (-125.7) = -2877.3 kJ the reason it was changed to 125.7 was because (-)(-) =(+)
Maggie16 — 06/06/16
I use the equation method. I come up with the equations first, and then I add up/take away the Heat of formations to get the answer. But it's hard to do that if you don't know the signs if you get me xD 4C + 4O2 → 4CO2 ΔH = – 1574 kJ 5H2 + 2½O2 → 5H2O ΔH = – 1429 kJ C4H10 → 4C + 5H2 ΔH = 125.7 kJ (see here, the heat of formation is a plus?) _____________________________________________________________ C4H10 + 6½O2 → 4CO2 + 5H2O ΔH = – 2877.3 kJ mol–1
taengoogle.09 — 07/06/16
It looks like it is because it is being taken away. As you know, you're cancelling the other because they are equivalent (+x)-(+x) = 0 In this case, since it is minus and being taken away, it becomes a plus; (+x)-(-x)= x+x / (-x)-(-x)= -x+x
aichayabre — 11/06/16
you change the sign because the C4H10 is located on a different side in the Equation which your trying to find the Heat of combustion/formation. so if they ask you to find the heat of combustion for C4H10. Given that the heats of formation of carbon dioxide, water, butane: -393.5, -285.8, -125.7. The equation for the heat of combustion of C4H10 would be C4H10 + 6.5O2 → 4CO2 + 5H2O And equation for heat of formation of C4H10 would be 4C + 5H2 →C4H10 So you see the way the C4H10s are on different sides? when you change the sign, it's like you are bringing the C4H10 to the same sides. Don't know if you can understand. Hoped it helped
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