Using Bond Enthalpies to Calculate Total Enthalpy Change [ENDORSED]

[Michelle_Nguyen_3F]

When we use bond enthalpies to calculate the total enthalpy change, how can we tell which bonds are broken and which bonds are formed? Is this skill more intuitive or is there a specific way to tell which bonds are broken/formed i.e looking at Lewis structures or the chemical reaction? Thank you!

[Manpreet Singh 1N]

Hello,

I feel like in order to know what bonds are broken and formed there is some extent of knowledge necessary. Adding on, the wording of the question and the chemical formula should tell you which bonds are broken and which are formed.

Hope that helps :)

[Chem_Mod]

You are correct. You must have or must draw out the Lewis structures of both the reactants and products. If you are unsure which bonds are being broken/formed, you can always just calculate the enthalpy for all the bonds in the reactants and then calculate the enthalpy for all the bonds in the products. For the reactants, since we are breaking bonds the enthalpy will be positive (absorbing energy). But for the products, since we are forming bonds the enthalpy will be negative (releasing energy). You just add their two enthalpies to get the total enthalpy change for the overall reaction. Yes it's a little more work to calculate the enthalpy for all reactants and products, rather than just for the individual bonds being broken and formed. But if you are confused as to which bonds are the ones involved in the reaction, this method will always work.