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The key word here is formation. The formation of bonds requires energy. When finding enthalpies of formation for an entire reaction using the change in enthalpy at standard state of molecules, you can rely on enthalpy's status as a state function and subtract the change in enthalpy of the reactants from the change in enthalpy of the products. For bond enthalpies, you have to know that forming bonds requires energy and breaking bonds releases energy. Therefore the bond enthalpy values of breaking a bond is a positive number, while the bond enthalpy values of forming a bond is a negative number. In order to find the total change in H of the reaction, you would be adding the mean bond enthalpies of the reactants (whose values are positive, since they are breaking) and the mean bond enthalpies of the products (whose values are negative, since they are forming).
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