Calculating for Bond Enthalpies

[Yijia_ZHOU]

I was wondering for bond enthalpy calculations, do we always need to draw out the structures to see what bonds are broken and formed? Is there a way to tell from the chemical formula? Also is it always (the bonds broken) - (the bonds formed)?

[Sanjana Tottemp 3C]

Hello,


It can be a lot easier to calculate bond enthalpies when you draw out the structure, however, it still possibly to calculate bond enthalpies without drawing out the structure by trying to think about the chemical formula of the reactants and products but this can be a little hard, since you would have to imagine the structure in your head. I believe drawing out the structure is Chem 14A material, however, in the problem they give you all the bond enthalpies for each bond, which can guide you on how the structure would look like. It is always going to be bond broken minus bond formed, because bonds being broken are an endothermic reaction therefore they require energy to be broken cause the bond enthalpy of those bonds to be positive and the bonds being made to have a bond enthalpy that is negative.

Hope that Helps!

[Emmet Asker]

If you are ever unsure of whether you calculated the reaction enthalpy correctly, I believe you can perform a calculation where all the bonds are broken and reformed to make the product. This would be a lot more calculations, but it might help to better understand how the calculations work when you solely calculate the bonds broken and formed in the reaction.

[805939635]

You do need to know which atoms are bonded to which, so if you can do that in your head (or if it's an obvious compound like H2), there is no need to draw it out. But you will need to have a total of bonds broken vs bonds formed, regardless of how you get there. I think usually the examples are molecules that are pretty easy to find the structures of, from the problems I've seen so far.

[Viviana Hsiao 1G]

It's not necessarily needed, but I think it's more helpful and a good way to double check the different bonds that are formed vs bonds that are broken. If you do not draw it out, you just need to know which bonds are formed and which are broken, which can be done most of the time by eyeballing the chemical formula.