Aamc 3 #50

[Havik21]

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It basically gives a structure of HNO3 and labels all of the bonds between Nitrogen and Oxygen as well as the one bond between Oxygen and Hydrogen. and it then asked to which of the bonds was the longest.

I've always thought that the weakest bond is often the longest bond so I chose the option which was the bond between the O and H seeing that HNO3 is a strong acid and it dissociates completely and thus this would be the longest (weakest) bond however the answer was the O-N bond on the on the protic oxygen (H-O-N)

I still don't understand why the HO-N bond is longer than the O-H bond.

 

[V5RED]

It basically gives a structure of HNO3 and labels all of the bonds between Nitrogen and Oxygen as well as the one bond between Oxygen and Hydrogen. and it then asked to which of the bonds was the longest.

I've always thought that the weakest bond is often the longest bond so I chose the option which was the bond between the O and H seeing that HNO3 is a strong acid and it dissociates completely and thus this would be the longest (weakest) bond however the answer was the O-N bond on the on the protic oxygen (H-O-N)

I still don't understand why the HO-N bond is longer than the O-H bond.

Look at the hybridization of the atoms.

Hydrogen is unhybridized so it has an S-orbital (short)

The protic oxygen is sp3 hybridized, so its bonding orbitals are sp3(long)

The nitrogen is sp2 hybridized(medium)

The non-protic oxygens are between sp2 hybridized and sp3 hybridized due to resonance.(medium-long)

The longest bond is the bond that combines the longest orbitals which is the protic oxygen to the nitrogen.

 

[LoLCareerGoals]

Nice explanation V5RED. I didn't know all that about hybr. orbitals.
I would've picked one of the N-O bonds but only because N and O are closest in electro-negativity and thus the bond is not as strong and hence long? Is this a valid trend to use or did I just get lucky this time?
Then I would know that double (or partially double) bonds are shorter and narrow it down to the only single N-O bond.

Havik just because the acid dissociates completely does not make O-H bond the weakest. This question is asked in a vacuum. There is not solvent to pull H+s away, just a molecule by itself. Look at bond enthalpies and you will see.

 

[shaboobly]

look at the bond orders:

3<2<1.5<1 this tells you the length.

 

[V5RED]

look at the bond orders:

3<2<1.5<1 this tells you the length.

This method would eliminate the bonds between the 2 oxygens that are in resonance with the nitrogen, but it would still leave you guessing between the O-H bond and the O-N bond since they both have a bond order of one.

 

[V5RED]

I would've picked one of the N-O bonds but only because N and O are closest in electro-negativity and thus the bond is not as strong and hence long? Is this a valid trend to use or did I just get lucky this time?

Looking at some tables of bond length, electronegativity, and bond energy, this trend seems useful when you are talking about comparing bonds where one of the elements is the same between the bonds and the bonds are all equally hybridized and the atoms are similar in size.(ie C-O vs C-N where they are bonds between sp3 hybridized orbitals whose valence shell is in the second energy level) (edit: C-F can also sort of be looked at this way. Technically C-F is an sp3 carbon bonding to the p-orbital of a fluorine, but p orbitals are pretty close to sp3 orbitals in size and fluorine's large effective nuclear force does make its p orbitals a bit smaller than those of something like carbon, so here electronegativity seems like a good option for differentiation and explaining why its bond length is even shorter than C-O or C-N)

It is probably a useful tool to use if you have boiled it down to two choices that have equal hybridization and nearly equal atomic radius(usually atoms in the same valence energy level and not many protons apart have similar radii) and the only difference is electronegativity.