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Basically you compare the molecule(s) before and after (it would thus be very helpful to draw them out) and identify (based off of your drawing) which bonds are present before that are not present after AND which bonds are present after that were not there before.
Try drawing out the Lewis structures for the molecules that are the reactants. From there, cross out bonds that need to be broken and draw in bonds needed to get your Lewis structures to look like the Lewis structures of the products. Hope this helped!
You can list all bonds on the reactant's side and the product's side, and then see if there are bonds on the product side that are not featured in the reactant side (the bonds formed) and then see if there are bonds that appeared in the reactant side but no longer appear in the product side (the bonds broken).
i think when comparing structures before and after, its easiest to use colors and determine which bonds changed by either being added or formed. you should ask yourself, which bonds were there before but aren't there now? or which bonds look different?
Bond enthalpy questions usually give you the reaction (A + B --> C) and then you just look at each molecule that is in the reaction and look up their bond enthalpy values. Bonds broken are usually correlated to those of the reactants and bonds formed correlate to those of the products.
If it's a confusing problem with bonds, you can count up all of the bonds on either side and write them out. For example, 3 carbon-carbon bonds, 2 oxygen-carbon bonds. Do this for the reactants and the products. Then you can easily figure out which bonds are present on both sides and ignore them, since they did not contribute to the enthalpy change. From there, simply subtract the bonds that were formed from the bonds that were broken.
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