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Method 2 (Bond Enthalpies) requires us to recognize the structure of both the reactants and products to calculate the enthalpy of reaction using bond enthalpies. In reference to method 2's example regarding the reaction between C2H4 and HBr to form CH3CH2Br, how would you recognize which bonds are broken in the reactants and consequently formed in the products?
On the course reader, you would have to compare the product bonds with the reactant bonds, finding out which ones stay intact and which ones are broken and then formed again. In that problem, one could see the double bond is broken and then formed as a single bond. Since we cannot calculate the energy required to do that we would have to take into account the energy needed to break a double CC bond and the energy needed to form a single CC bond. Also the Hydrogen and the Bromine in HBr need to be broken so that they can be added to one of the carbon atoms each. Knowing this we can see that the double CC bond and the HBr bond are broken. Also the single CC bond, the CH bond, and CBr bonds are formed.
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