Sciencemadness Discussion Board

Why is there no such thing as H3NO4 ?

metalresearcher - 29-9-2019 at 11:23

There is phosphoric acid H3PO4, arsenic acid H3AsO4, both from the same group as nitrogen and in oxidation state +5.

There is N2O5 and HNO3,like P2O5 and HPO3, but adding water does not make the fictional acid H3NO4.
Why ?

Sigmatropic - 29-9-2019 at 13:47

The term to look for is ortho nitrate, similar to ortho esters in naming. https://en.m.wikipedia.org/wiki/Orthonitrate

Interesting question, I'm curious for the theoretical explanation. My guess is it involves thermodynamics and the high stability of nitrate.

fusso - 29-9-2019 at 13:56

Now I wonder why is H3PO4 stable but HPO3 and its salts don't exist.

DraconicAcid - 29-9-2019 at 14:43

Quote: Originally posted by fusso  
Now I wonder why is H3PO4 stable but HPO3 and its salts don't exist.


Nitrate is stabilized by three sigma bonds and a pi bond. Pi bonds between oxygen and phosphorus are much weaker than N-O pi bonds (the orbitals don't match in size, so they don't overlap as well), so phosphorus would much rather have four sigma bonds instead.

fusso - 29-9-2019 at 15:46

Then why don't P go full sigma and prefer P(OH)5?

DraconicAcid - 29-9-2019 at 16:08

Then it would have to use its d orbitals in its hybridization, and those are pretty high energy.

Keras - 30-9-2019 at 02:04

Quote: Originally posted by metalresearcher  
There is phosphoric acid H3PO4, arsenic acid H3AsO4, both from the same group as nitrogen and in oxidation state +5.

There is N2O5 and HNO3,like P2O5 and HPO3, but adding water does not make the fictional acid H3NO4.
Why ?


In H₃PO₄, the central P has five bonds. N cannot do that, because N has no d orbitals (not even vacant ones – they physically don't exist at all), therefore it is limited to 4 bonds only, much like carbon, oxygen and boron. The ability to hybridise d orbitals comes with the third row.

DraconicAcid - 30-9-2019 at 18:27

Quote: Originally posted by Keras  

In H₃PO₄, the central P has five bonds. N cannot do that, because N has no d orbitals (not even vacant ones – they physically don't exist at all), therefore it is limited to 4 bonds only, much like carbon, oxygen and boron. The ability to hybridise d orbitals comes with the third row.


Nitrogen does have d orbitals, just not in the valence shell (all atoms have an infinite number of shells- even lowly hydrogen has higher shells, and transitions from the seventh shell can be seen in its emission spectrum).

Nitrogen could conceivably form H3NO4 without a double bond to one of the oxygens (and thus only having four bonds), in a similar bonding arrangement as in trimethylamine N-oxide (https://en.wikipedia.org/wiki/Trimethylamine_N-oxide ), but that just isn't stable.


As an aside, the conjugate base of H3NO4 has been made: https://en.wikipedia.org/wiki/Orthonitrate


[Edited on 1-10-2019 by DraconicAcid]