R -> P vs. P -> R

[JennyCKim1J]

So I understand that the equation for finding change in enthalpy of reaction is sum of products' change in Hf - sum of reactants' change in Hf.

However, when using bond enthalpies to find change in enthalpy, why are we simply adding enthalpies on the product side and subtracting that from reactant side? Why is it not products - reactants?

[Nishma Chakraborty 1J]

When we're using bond enthalpies, I follow "the sum of the bonds broken - the sum of the bonds formed." The bonds that are broken are on the reactants side, and the bonds that are formed are on the products side. Forming bonds releases energy, while breaking bonds requires energy.

This is used we're calculating using bond enthalpies, otherwise, we use deltaH=(deltaHproducts - deltaHreactants) using the given heat of formation values.

Hope this helps! :)

[jillian1k]

The formula for bond enthalpies is easier to think about as deltaH=(sum of bonds broken)-(sum of bonds formed), not products-reactants. Not all chemical reactions have all bonds being broken (with none being formed) on the left and all bonds being formed (with none being broken) on the right.

[AtreyiMitra2L]

For bond enthalpies, I believe you got the order wrong. It should be reactants - products. This idea is built on the idea that to break bonds in the reactants. To build the bonds in the products, there is a release of energy. Therefore, there is reatants - products.