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Enthalpy calculations

when doing calculations I do sum of products sum of reactants but do the arrows change this rule sometimes? Like how with combustion they go down and formation they go up. Is it always P-R or can it switch up for some questions?
Hi! It changes based on the reaction pathway you're trying to find. For example, if you think back to GCSE Maths, with vector triangles, if you had a triangle like this:
enthalpy demo 2.jpg

a = b + (-c)
If you look at the blue arrows, they show the reaction pathway, it can occur by the one reaction (a) or by the two reaction enthalpies (b-c), c is minus because it is the other direction to the arrow head.
With combustion and formation, enthalpy is released so that enthalpy given is always negative, formation is always enthalpy required so is always positive. So if an enthalpy is negative, the sign might cancel out depending on the pathway you're trying to find. Hope that helps! If not, watch a couple of the worked questions on the MaChemGuy YouTube

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