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bond length such as the midterm question

Posted: Sun Nov 11, 2018 11:00 pm
by Ruiting Jia 4D
Hi,

When determining bond lengths with two differently valued lengths, should we write it as a range or a specific value? I know that on the midterm, any value between was accepted, but would writing a range be considered wrong?

Re: bond length such as the midterm question

Posted: Mon Nov 12, 2018 2:40 am
by Linh Vo 2J
Hello, my TA told me that because the question was asking the bond length average, it would have been best to write the hybrid "average" of the bond lengths, which in that case 120+140/2=130. However, a range would have also been accepted because we are unable to pinpoint an exact number or value of the bond length, so a range of [120-140] would not technically have been wrong either.

Re: bond length such as the midterm question

Posted: Mon Nov 12, 2018 6:46 pm
by Alan Chang
In the midterm it was asking for the resonance bond length, so in this case we can take the average of the single and double bond to make an estimate of the bond length.

Re: bond length such as the midterm question

Posted: Mon Nov 12, 2018 9:14 pm
by Raj_Bains_2C
I believe that a specific value is preferred but a range should be accepted as a correct answer.

Re: bond length such as the midterm question

Posted: Mon Nov 12, 2018 9:22 pm
by Danielle_Gallandt3I
An additional element to consider here is how many double bonds there are and how many single bonds. If having more double bonds lowers the energy of the molecule, then you can assume that structures with more double bonds are contributing more to the resonance structure. In that case, the resonance bond length is probably going to be closer to the double bond length.

Re: bond length such as the midterm question

Posted: Tue Nov 13, 2018 9:24 pm
by Cameron_Greenberg_3C
Yes, the range of 120-140 would be correct. Most likely, any number between would be counted correct also.