enthalpy reaction

[AMahadi]

When solving the enthalpy of a reaction using bond enthalpies, why do we perform reactants minus product while for all other enthalpy calculations we use products minus reactants?

[Elena Bell 1C]

We do reactants minus the products for bond enthalpies because it is broken minus made. The bonds broken require energy, while forming bonds releases energy.

[Kayla Maldonado 1C]

We do reactants minus the products for bond enthalpies because it is broken minus made. The bonds broken require energy, while forming bonds releases energy.

Why is it that in order to break bonds, energy is needed? And to form bonds energy is released?

[Sydney Myers 4I]

When you form a bond, you're taking atoms from a state of high energy (free floating) to a more stable state (in a bond). This is a net decrease in potential energy, and the excess is given off as heat and the reaction is exothermic.

When you break a bond, you need some form of energy to take the atoms from a state of lower potential energy back up to a state of higher potential energy, so the reaction is endothermic and requires energy.

[Max Madrzyk Dis 4G]

We subtract the enthalpy of the products from the reactants when we are calculating enthalpy using bond strengths because forming a bond in the products has a negative enthalpy while breaking one in the reactants has a positive enthalpy.