calculating H through bond enthalpies

[Agustina Santa Cruz 2F]

Why is there an added negative sign on the side of the products when using bond enthalpies to calculate total reaction enthalpy?

[Gwendolyn Hill 2F]

Hi!

The negative sign described is due to the fact that the product bonds are being formed, and the reactant bonds are being broken. So when calculation total reaction enthalpy, you subtract the amount of energy for the products!

[Mahnoor_Wani_1I]

Reactants: Breaking bonds is often an endothermic processs the value of the enthalpy will be positive because it intakes energy to break the bond. Products: forming bonds is often an exothermic reaction that releases energy aka the value of the enthalpy will be negative if it is forming bonds.

[Ken_Cheng_2A]

You can also think of calculating enthalpy using bond enthalpies as being the opposite of the other methods in that when using bond enthalpies, it's reactants minus products instead of products minus reactants, but the above people have already explained why exactly that is the case.

[Pratika Nagpal]

Is there any case where breaking bonds is not endothermic process, ie, the sign of the enthalpy change is not positive and therefore negative.