How many single pairs does nh3

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Batman girl



Registration date: 04/17/2016
Posts: 8

Posted: Apr 30, 2016 13:39 Title: Differences PH3 and NH3

My question:
Good day first of all,
I found a task in an old exam that I have the impression that I cannot answer adequately:
Comparison of PH3 and NH3. The H-P-H bond angle is 93 °, the H-N-H angle is approximately 107 °
(1) Explain this difference based on the orbitals used to establish the bond
(2) In which orbitals are the non-bonding electron pairs located?
(3) What is the consequence of this for the basicity of NH3 compared to PH3?

I hope you can help me here, so far I have always been able to rely on competent and helpful answers here, which I think is really great!
regards

My ideas:
So my thoughts on this so far are the following:
(1) Since phosphorus is in the third period, unlike nitrogen, one can assume that there is no sp3 hybridization here as in the NH3 molecule. Is that correct so far?
In addition, phosphorus is a larger atom than nitrogen, which is why overlapping orbitals is generally less effective.
But then my attempts to explain begin to get thin, because otherwise the molecules of the geometry and the valence electrons are really very similar. Was that the whole answer? With the best will in the world, I can't imagine that ...
(2) In the NH3 molecule, the lone pair of electrons should then be in the spz (?) Orbital located upwards. The 4 sp3 orbitals are arranged in a tetrahedron.
In PH3, assuming that there is no hybridization, possibly even in the s orbital (?) Because all p orbitals are used for an overlap with the H atoms. Or is the pz orbital not used because hydrogen only has one s orbital and this cannot overlap with an upright p orbital? Then the lone pair of electrons would be in this pz orbital?
(3) This is where I am most at a loss. I thought that it probably had something to do with the strength of the bond with the lone pair of electrons to which a proton should possibly bind, but I haven't been able to develop much more ideas about it
magician4
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Registration date: 05.10.2009
Posts: 11677
Place of residence: Hamburg
Posted: May 01, 2016 12:32 AM Subject:

to 1)
the difference may seem small, but in fact "almost 90 °" (and thus "practically unhybridized" all 3 X-H bonds are served by practically orthogonal p-orbital), hence PH3 , and 107 ° (which almost corresponds to the tetrahedron angle) a huge difference, into which everything possible for the inevitably connected hybridization is interpreted (like bein ammonia: sp3 - hybridized)

to (2)
here is the classic answer that with the PH3 there the lonepair sits in the s-orbital, while with ammonia it is in nem sp3-hybrid should sit
that's how people will want to read it from you

... although I don't really want to go along with the ammonia: the molecule is so special (nitrogen oscillating through the plane of the three hydrogens, sometimes tunneling through it) that I don't like to treat it as a "rigid" geometrically fixed object

to 3)
a free electron pair, which is directed unidirectionally and points out into the free space ....
... is much better protonable than sun shielded spherical object inside
that's how people will want to hear it from you
... what I think is a very short-winded explanation: to reduce the various effects to be considered to only / majorizing this aspect, a way takes a short, m.a.n.


greeting

Ingo
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Batman girl



Registration date: 04/17/2016
Posts: 8

Posted: May 01, 2016 10:55 AM Subject:

First of all, thank you for your answer!
I have now understood points (2) and (3) so far. However, it has not become quite clear to me whether you agree with (1) my assumptions or consider an addition to be appropriate?

And I think that the sometimes "vague" requested answers stem from the fact that these exams are not designed for actual chemists, but biologists, etc. Which of course shouldn't really be a reason, but I think that the university simply doesn't require that much detailed knowledge here.
magician4
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Registration date: 05.10.2009
Posts: 11677
Place of residence: Hamburg
Posted: May 01, 2016 6:20 PM Subject:

I tried to answer (1) the original question as concentrated as possible: which conclusions can be drawn from the measured bond angles with regard to the AO's / their hybridization required for these angles.

a general view on phosphorus ./. Nitrogen (can hybridize, can't hybridize ...), on the other hand, I thought neither was required nor really helpful in this rough generalization (or: how do you argue for the dihedral angle Br-P-Br in PBr from 101 °)?


greeting


Ingo
_________________
a month in the laboratory saves you a quarter of an hour in the library!