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Why is it that when you calculate delta Hrxn with enthalpy of formation you subtract sum of products from sum of reactants but with bond enthalpy its reversed and you subtract bonds of reactants from bonds of products? What is the conceptual meaning behind it, because I feel like products-reactants is usually used/makes most sense to me?
So essentially with Bond Enthalpy the concept is the change between the two values so it’s final (products) - initial (reactants). However with Enthalpy of formation we calculate the sum of the values of both sides, however when bonds are formed (in the product) the value is turned to negative. Therefor it ends up as H formation= H reactants + H products (which are usually negative).
When you calculate the overall enthalpy of a reaction using bond enthalpies, you are not subtracting the bond enthalpies of the products from the reactants. You are summing up all the bond enthalpies. The reason why the bond enthalpies on the products side are always negative is because bonds are being formed, which is an exothermic process, while on the reactants side, bonds are being broken, which require heat. Keep in mind that bond enthalpies are usually given in terms of the heat required to break a bond, so you would need to add a negative sign when bonds are being formed on the products side.
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