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Could someone explain the negative signs in this reaction? I was able to get up to the point where you have the enthalpy values to break the bonds, but from there, I don't understand why in the solutions manual it is -6*518, and +3*837. Do you add the two numbers after that?
It is -6*518 because you are forming six c-c bonds and the enthalpy of each one (with resonance) is 518. 3*837 refers to the enthalpy of c-c triple bonds broken. The enthalpy of bonds formed is subtracted from that of bonds broken.
Using bond enthalpies, we can calculate the enthalpy of the reaction. Breaking bonds requires energy (positive change) while creating bonds releases energy (negative change). Therefore, we can find the net energy change by adding all the bond enthalpies of the bonds broken, and subtracting from that value the bond enthalpies of the bonds formed. For question 8.73a, doing this method using the values in the book gives us a final answer of -597 kJ/mol.
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