textbook 4E. 5

[Christine Nguyen 3D]

Hi, I was wondering if anyone has any tips when solving problems that require you to use bond enthalpies when finding the enthalpy of the overall reaction? I tend to get confused on the number of bonds when drawing out the structure.

The example I had in mind was 3 C2H2 (g) --> C6H6

How would you know which bonds are broken or forming? The answer key said that there were 6 of the C-C (<--with a dashed line above) bonds that were formed. But I'm confused as to what the dashed line is for. Is it just saying that there's a resonance structure (another way of placing the bonds)?

Thanks!

[Sana Nagori 2H]

Yes this structure has resonance (I think it is benzene). So instead of forming 3 double and 3 single bonds you're actually forming 6 mixed bonds. This is assuming you're breaking all the bonds first and then reforming everything again (which is the method I prefer)

[Bella Wachter 1A]

If you're ever completely stuck, and can't figure out which bonds are broken and formed, you can always just break all the bonds in the reactants and form all the bonds in the products. It will still give you the same answer (but it's just a longer process).