H3PO4/H3PO3 question

[Crash]

I was wondering with all the properties ive learnt about acids, why is it that H3PO3 is more acidic than H3PO4 when its got less electronegative atoms bonded to the central atom plus only two H's are bonded to the oxygens as opposed to H3PO4, can anybody explain this property to me?

[Bluenoise]

Well the conjugate base is more stable I guess.

why?

Well more of a guess then anything but I'd say that the resonance form of the conjugate base finds additional stability from the pair of electrons that form the P-H bond.

Or maybe it's stabler because of less steric hinderence with it's resonance form..

[budullewraagh]

there is no P-H bond...it may be related to the bond strengths of P-O (which differ the O-H bond energies) in each compound

[Bluenoise]

there is no P-H bond

 

Sure there is

 

http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=407

[budullewraagh]

i must disagree. the phosphorus in phosphorous acid is trivalent with the oxygen. this satisfies the octet rule, leaves a pair of electrons, so it's all good. in the structure shown in your link, phosphorus makes 4 bonds. this almost never happens without formation of ions. also, the hydrogen would definitely bond to the doubly bonded oxygen rather than the phosphorus, due to electronegativity differences and the exposed lone pairs on the oxygen.

[Bluenoise]

i must disagree. the phosphorus in phosphorous acid is trivalent with the oxygen... ....in the structure shown in your link, phosphorus makes 4 bonds.

 

No it's pentavalent. Common states for phosphorus are -3, +3, +5

 

and

 

No acutally if you'd like to count again you'd notice that it makes 5 bonds; 1 with the hydrogen and 4 with the oxygen's (2 single bound, 1 double bonded).

5 bond is very common for phosphorus.

It's the same with Phosphoric acid except one of the oxygens is missing and the hydrogen is directly bonded to the Phosphorus

 

Anyways I KNOW I am correct. Why don't you just go look up the structure from a reputible source and find out for yourself.

The source I supplied (PubChem) is a very good one, very reliable. It's actually a new offshoot directory from NCBI.

I didn't draw that myself...

 

Here's another http://www.chemicalland21.com/arokorhi/industrialchem/inorganic/PHOSPHOROUS%20ACID.htm

 

But please go prove it for yourself if you can't take my word.

[budullewraagh]

i can count. whoops.

 

i know that +5 is common for phosphorus, but please consider the fact that phosphorus is commonly found at +3. consider the halides of phosphorus; it is comparatively much more difficult to bring phosphorus to +5 than +3.

 

http://www.chemistry.emory.edu/undergrad/students/molecules/gifs/82_00.gif

 

i have seen my share of images of the structure you present as well as the structure i proposed.

[Bluenoise]

I understand what you're saying but I really have no doubt with my answer. +5 isn't rare, Phosphoric acid is +5 the Phophates in ATP are +5.

Considering the halides doesn't prove anything. Both PCl3 and PCl5 are common.

 

I can't really think of any other way to explain it. If you can't take my word there isn't much more I can say.

 

Maybe someone else can clear up this stalemate...

[Skye]

As far as I know it's as bluenoise said, and this explains why phosphorous acid is diprotic compared to the triprotic phosphoric, the hydrogen bound to the P in phosphorous is only weakly acidic.

[Bluenoise]

Right but this doesn't really explain too well why the first hydrogen dissacociates easier in Phosphorus acid then Phosphoric.

[Crash]

Ive tried looking everywhere and i still cant find an awnser. i emailed a couple of professors at oxford and cornell to try to find some awnsers but im still waiting for the email back. maybe jdurg can come up with some awnsers

[Bluenoise]

I still think it has to do with higher stability of the conjugate base,

due to less hindered overlap of the p orbitals in forming the resonance conditions. Those Oxigens need to be planar when fully dissacociated to resonate and 3 would fit better then 4...

Or due to the closer proximity of the H on the H-P bond helps stabalize the negative charge on the oxygen....

[jdurg]

Hmmm. I do know that phosphorus acid is not very stable at all and will readily decompose into phosphoric acid. Phosphorus acid is also a diprotic acid while phosphoric acid is technically triprotic. (The hydrogen bound to the phosphorus in phosphorus acid is not an acidic hydrogen). Generally speaking, the lower the number of acidic hydrogens a compound has, the stronger the acid it is. (While there are definitely exceptions to this, it's a trend that you commonly see). This leads you to believe that the more acidic hydrogens a compound has, the more it needs those hydrogens to stabilize itself. Phosphoric acid has three acidic hydrogens, so you would expect it to be more energetically stable if it held onto those hydrogens as opposed to the naked phosphate ion. Also, the phosphite ion will readily be converted to the hydrogenphosphate ion, so the 'nakedness' of the conjugate base of phosphorus acid is stablized by further reaction.

 

So I would have to say that phosphorus acid is more acidic than phosphoric acid due to its lower number of acidic hydrogens, and the stability of the conjugate base upon coversion to hydrogenphosphate.

[chemi]

I found a powerpoint site on acids & bases. Slides 6-12 explained the factors that affect the strength and acidity of acids and bases, I think it'll help with this.

 

Just go to google and search (acidity of H3PO4, H3PO3, it's the first one on the page) or use the html version(not very clear, missing images)

 

http://64.233.167.104/search?q=cache:jh-SwLZfPB8J:web.uccs.edu/bgaddis/chem106/ch15.ppt+acidity+of+H3PO4,+H3PO3&hl=en