Calculating Reaction Enthalpies using Standard Enthalpies of Combustion

[William_Hora_2H]

Why do we calculate reaction enthalpies using standard enthalpies of combustion as follows? (i.e. Why reactants minus products)?

[Kaitlyn Bateman 1L]

I don't believe that we do that. typically it is sum of delta H of products minus sum of delta H of reactants. The only reasoning I can think of is if you have to switch the sign and deal with the reverse reaction, but even then, I'm not sure how that would make it reactants minus products.

[Jason Ho 2L]

I agree with Kaitlyn. Based on Dr. Lavelle's lecture, I have it in my notes as the sum of products minus the sum of reactants as well.