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Just confirming - when using the bond enthalpies method, it's the sum of the bonds broken minus the sum of the bonds formed, right? This goes against the sum of products minus the sum of reactants in other forms of the equation, so I wanted to make sure I have it right. Thanks!
It takes more energy to break the bonds, thus breaking the bonds is an endothermic reaction. Forming is the bonds is an exothermic reaction, so the equation to find the total bond enthalpy is in the fact bonds broken - bonds formed.
You are correct, we use this convention because all bond enthalpies given are positive and when bonds are broken, energy is required into the system therefore the H is positive and when bonds are formed, energy is released therefore H is negative so a negative sign is placed in front of the sum of bonds formed. Since enthalpy is a state function, which disregards the path taken, you add the bonds broken and bonds formed together and get: (delta H) = ((delta H)[BROKEN]) + (-(delta H)[FORMED]), resulting in BONDS BROKEN - BONDS FORMED.
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