Homework Help: Chemistry

Posted by Anna on Saturday, April 3, 2010 at 11:58pm.

Why do we put a minus sing in front of the bond enthalpies of the bonds that are being formed?

And why are the enthalpies of formation negative in general?

I also don't understand why we subtract the enthalpy of formation of the reactants from the enthalpy of formation of the products?

Maybe someone could help me understand this.
Thanks!

Chemistry - DrBob222, Sunday, April 4, 2010 at 12:16am
The definition of bond energy is the amount of energy released when one mole of constituent gaseous atoms form. Generally it is an exothermic process and someone decided years ago that the release of energy would be counted as negative. Delta H for the reaction is the delta H products - delta H reactants. If the products are more negative than the reactants the delta H rxn is negative and that means it is spontaneous. Subtracting the other way would not give the proper sign. Said another way, the products are more stable than the reactants.

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