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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: aziza on November 21, 2007, 01:53:58 AM

Title: TAUTOMERS & BOND DISSOCIATION ENERGIES
Post by: aziza on November 21, 2007, 01:53:58 AM

I'm hoping someone can shed some light on this question for me:

Consider the molecule diazomethane (CH2N2).  Two possible Lewis Structures:

H-C-trpl bond-N-N-H         and         H2C-dbl bond-N-dbl bond-N

Determine which one is the more favourable structure using bond dissociation energies.

Now, when I work out the bond dissociation energies, the first structure has a total of 1857KJ/mol (H-C=414;C-N=891;N-N=163;N-H=389) and the second structure has a total of 1861KJ/mol (2(H-C)=828;C-N=615;N-N=418).  I would say that the first structure is the more favourable structure because it is the lower energy tautomer, but the answer in my book says the second structure is favourable.  What am I doing wrong here?????


Regards,


Michelle

Title: Re: TAUTOMERS & BOND DISSOCIATION ENERGIES
Post by: Dan on November 21, 2007, 03:23:06 AM

Bond dissociation energy is the energy you must put in to break the bonds.

The higher the bond dissociation energy, the stronger the bond, so the tautomer with the greater total bond dissociation energy will be the more stable one (and will itsself have a lower free energy).