Test 3 bond length question

[Emely Reyna 1F]

There was a question on Test 3 that asked which of the two bonds was longer and whether or not it was the stronger bond of the two? This question confused me because isn't the longer the bond mean that it is the weaker the bond? Am I thinking about this incorrectly?

[yazminedesan1F]

the bond that is shortest is stronger and the bonds that are longer are weakest. It goes, in order of increasing bond strength and in order of decreasing bond length: single bond<double bond<triple bond

[Anthony Mercado 1K]

Additionally, part of the question was a comparison of two double bonds, each with two lone pairs. In this scenario, how would you discover which of the two bond lengths was longer? * Also, is it correct in thinking the lone pairs stretch out the bond length?

[Alicia Beebe]

Additionally, part of the question was a comparison of two double bonds, each with two lone pairs. In this scenario, how would you discover which of the two bond lengths was longer? * Also, is it correct in thinking the lone pairs stretch out the bond length?

I struggled thinking through this question as well. I know it has to do with electrons repelling one another, so I also guessed that the lone pairs stretched out the bond length, but I am not 100% sure. Does anyone remember all the different factors that affect bond length? I know that triple bonds are shorter and single bonds are longer, but get confused when other factors come in to play.

[BriannaParrington-1B]

Additionally, part of the question was a comparison of two double bonds, each with two lone pairs. In this scenario, how would you discover which of the two bond lengths was longer? * Also, is it correct in thinking the lone pairs stretch out the bond length?

I struggled thinking through this question as well. I know it has to do with electrons repelling one another, so I also guessed that the lone pairs stretched out the bond length, but I am not 100% sure. Does anyone remember all the different factors that affect bond length? I know that triple bonds are shorter and single bonds are longer, but get confused when other factors come in to play.

I agree, the two double bonds threw me off as well. I was unable to tell the difference between the two even though both had the same type of bond.

[octaviahuang1f]

Additionally, part of the question was a comparison of two double bonds, each with two lone pairs. In this scenario, how would you discover which of the two bond lengths was longer? * Also, is it correct in thinking the lone pairs stretch out the bond length?

It basically still depends on the bond multiplicity of the molecule. Though they both have a double bond, PO4 3- has more resonance structures of the double bond, so it has more double bond characteristics than the other. I'm not sure if lone pair stretch out the bond length, but it definitely repels other bonds and decrease the bond angles. Hope this helps!!

[Chem_Mod]

The wording of the question may have been somewhat confusing. We were first asking you to circle the bond that is shorter, or longer depending on your test day. Then you were asked to say if the one you circled is the stronger of the two (or weaker depending on the day). So no matter which one you circled for a and b your answer to the yes or no question should have been the same for both a and b. For example Wednesday's test asked which bond was longer and then if that bond was stronger. So for both a and b whichever one you circled your answer should have no, since the longer bond will be weaker since shorter bonds are stronger. For all of the tests part b was more straight forward, usually asking between a single and a double or triple bond. However part a usually gave you two seemingly identical bonds which only differed in their resonance. Whichever structure had more resonance options possible would have longer bonds, this is because the double bond (all tests had a double bond in them for part a) will be spread out over more atoms, meaning it will not be purely a double bond. For Wednesday's test part a was H₂PO₃^1- vs PO₄^3-, the first option only had 2 possible resonance structures while the second option had 4. This means that the second option will have longer bonds since the double bond is going to be shared by 4 atoms making it more of a 1 and 1/4 bond instead of a double. Since the second option is longer it will be weaker meaning that you should have said no to the second part of the question for Wednesday when asked if the one you circled is stronger. This case is similar over all days where part a had two options, and the one with more resonance is weaker and longer overall.

[Jennifer 1G]

The wording of the question may have been somewhat confusing. We were first asking you to circle the bond that is shorter, or longer depending on your test day. Then you were asked to say if the one you circled is the stronger of the two (or weaker depending on the day). So no matter which one you circled for a and b your answer to the yes or no question should have been the same for both a and b. For example Wednesday's test asked which bond was longer and then if that bond was stronger. So for both a and b whichever one you circled your answer should have no, since the longer bond will be weaker since shorter bonds are stronger. For all of the tests part b was more straight forward, usually asking between a single and a double or triple bond. However part a usually gave you two seemingly identical bonds which only differed in their resonance. Whichever structure had more resonance options possible would have longer bonds, this is because the double bond (all tests had a double bond in them for part a) will be spread out over more atoms, meaning it will not be purely a double bond. For Wednesday's test part a was H₂PO₃^1- vs PO₄^3-, the first option only had 2 possible resonance structures while the second option had 4. This means that the second option will have longer bonds since the double bond is going to be shared by 4 atoms making it more of a 1 and 1/4 bond instead of a double. Since the second option is longer it will be weaker meaning that you should have said no to the second part of the question for Wednesday when asked if the one you circled is stronger. This case is similar over all days where part a had two options, and the one with more resonance is weaker and longer overall.

This question confused me too. I got full credit but I wasn't sure how to work it out, especially since the structures in part a had double bonds and in part b had single bonds. Now I know that the structure with the most possible resonance structures has the longest bond, thank you for the post. Will there be a scenario when they have the same number of resonance structures? If so, how should we determine the answer. Thank you!

[yuetao4k]

Longer bonds mean that they are weaker.