Using Hess' method [ENDORSED]

[paulina_higashi_1G]

When using method 1 to calculate delta H, is there a specific rule we have to follow if we use the reverse reaction of one of the reactions to obtain the net reaction value?

[Chem_Mod]

If you flip a reaction (go from R -> P to P -> R), we flip the sign of enthalpy. This is because we're reversing the reaction, so something that was once endothermic would now be exothermic. And something that was exothermic would now be endothermic.

If you're multiplying a reaction by a coefficient (like going from R -> P to 2R -> 2P) then we multiply the enthalpy of reaction by the coefficient we multiplied the reaction by (2 in this case). This is because we're increasing the number of moles of the reaction. If we burn a toothpick, the energy released will be small. But if you burn a forest down, the energy released from the fire will be super big, because we had more moles to burn!

Not saying we should burn forests down.
-Vivek

[405995505]

If we use the reverse reaction, we simply take the inverse of the given enthalpy. Meaning, that we put 1/given enthalpy.