lower energy vs higher energy

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Marisa Gaitan 2D
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lower energy vs higher energy

Postby Marisa Gaitan 2D » Mon Nov 09, 2020 10:03 am

I am a little confused on the concept of the differing energies of resonance structures. If I recall correctly, structures with formal charges of 0 have lower energy than those with charges? Also, how do we know which resonance structures contribute more to the overall structure? I'm stuck on this concept as well. Thanks!

Megan Sparrow 1A
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Re: lower energy vs higher energy

Postby Megan Sparrow 1A » Mon Nov 09, 2020 10:09 am

Hey! To answer your question, I believe that all resonance structures technically contribute to the overall structure, but the major resonance contributor has the lowest energy. When the formal charge is 0 or close to 0, then the structure is most stable. In general, the most stable structures (lower energy) contribute most to the resonance hybrid.

Nika Gladkov 1A
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Re: lower energy vs higher energy

Postby Nika Gladkov 1A » Mon Nov 09, 2020 10:16 am

The reason structures with a formal change of 0 are of a lower energy than structures with nonzero formal charges is because they are more stable. If all of the atoms in a structure have FC=0, it means that all of them have bonded exactly how they "like" to, and, thus, do not want to change the structure or bonding, making them of a lower energy.

I believe that, typically, all resonance structures contribute equally to the true structure of the molecule because the delocalized electrons travel throughout all of the molecule. However, in the Sapling homework, there were problems where we were asked to look at bond length to determine which structures contributed most. In this case, I think that you just need to compare the bond lengths of the lewis structures to the measured bond lengths and see which of them are the closest. The lewis structure with bond lengths most different from the measured bond lengths would contribute the least to the overall structure.

Samantha Pedersen 2K
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Re: lower energy vs higher energy

Postby Samantha Pedersen 2K » Mon Nov 09, 2020 12:52 pm

Yes, the structure that has the most atoms with a formal charge of 0 has a lower energy than structures with fewer atoms that have formal charges of 0. This is because it takes energy to separate different charges (like +1 and -1), so structures that don't have a lot of different charges don't require as much energy.

All resonance structures contribute to the true structure of the molecule, but to determine which structure contributes the most it depends on the information that you have. If you have determined the formal charges, then the structure with the most formal charges equal to 0 (the structure with the lowest energy) contributes the most to the true structure. If you're given experimentally determined bond lengths, then the structure with the bond lengths that are closest to the measured bond lengths contributes the most to the true structure. On the Sapling homework, there was also a question about using oxidation numbers to determine the structure that contributes the most to the true structure. In this case, the structure with the formal charge on an atom that is closest to the oxidation number of that atom contributes the most to the true structure.

I hope this helps!


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