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Hess's law? How do I know which part is endothermic and exothermic?

So we have the two cycles, one for enthalpy formation and one for enthalpy combustion.

In the combustion the arrows go down and in the formation they go up.

My question... How do I know whether the sign is positive or negative?!

I am doing AS by the way.
Original post by roshni_khanna
So we have the two cycles, one for enthalpy formation and one for enthalpy combustion.

In the combustion the arrows go down and in the formation they go up.

My question... How do I know whether the sign is positive or negative?!

I am doing AS by the way.


combustion is always exothermic, while formation may be either ...

check out this interactive run-through of Hess' law
(edited 9 years ago)
Don't you just figure it out when you add up all the numbers together (or take away, depending on the direction of the arrows)
yeah I just realised I was being stupid haha, thanks guys

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