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P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol - Chemistry Forum

P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 01-29-2004, 11:28 PM
G. R. L. Cowan
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol




NIST ([Only registered users see links. ]),
which I tend to believe, gives -2,214.31 kJ/mol
for P4O6 vapour's delta 'H'.

Halfway down [Only registered users see links. ], however,
is a diagram ([Only registered users see links. ]) that shows
it as -1,318 kJ/mol for the solid.

And some sources say it decomposes.
In air, sure, it would decompose to P4O10,
but I don't understand how it could decompose
all on its own.

This sort of discordancy might make sense for, um,
aluminum bismuthide but phosphorus (III) oxide
doesn't seem that obscure. Please shed some light.


--- Graham Cowan
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fireproof fuel, real-car range, no emissions
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  #2  
Old 01-30-2004, 01:30 PM
the_buckleys@optusnet.com.au
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

G. R. L. Cowan wrote:

Looks as if the NIST data is based on reacting P4 vapor, the other on
solid P4. Decomposition could be disproportionation to phosphorous and
P4O10.

Rob.
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  #3  
Old 01-30-2004, 02:02 PM
Jaak Suurpere
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

"G. R. L. Cowan" <[Only registered users see links. ]> wrote in message news:<[Only registered users see links. ]>...
Might it be capable of disproportionation to P4 and P4O10?

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  #4  
Old 01-30-2004, 04:06 PM
G. R. L. Cowan
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

Jaak Suurpere wrote:

Since posting I've thought, maybe the low-magnitude figures
actually pertain to mixtures of P4 and P4O6,
such as would be easy to get if one tried
to make P4O6 by burning P4 with limited oxygen.
P4 melts 44.1 C, boils 280 C.
P4O6 melts 23.8 C, boils 175.4 C.

If NIST is right, it won't disproportionate --

P4O6(g) ---> (2/5) P4(g) + (3/5) P4O10(s)
-2214.31 47.1 -1805.964

but if the -1,318-kJ/mol figure is right,
it should disproportionate like crazy,
and why would phosphorus combustion ever produce it.


--- Graham Cowan
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  #5  
Old 01-30-2004, 09:00 PM
G. R. L. Cowan
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[Only registered users see links. ] wrote:

So NIST's 900 more reported kJ/mol is how far P4(g)
is above P4(s)? It boils at 280 C.

As noted in my other posting,
disprop won't happen if NIST is right, otherwise maybe,
But if some work had been done on P4O6
that bore a large adulteration of P4 --
both colourless, nearly the same mp and bp --
and other, more careful work had used pure P4O6,
all this would be explained.


--- Graham Cowan
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fireproof fuel, real-car range, no emissions
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  #6  
Old 02-03-2004, 12:15 PM
the_buckleys@optusnet.com.au
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

G. R. L. Cowan wrote:

My trusty Atkins gives the following data for standard enthalpies of
formation for phosphorus species (KJ/mol):

P (wh, s) 0 (duh)
P(g) + 314.64
P2(g) + 144.3
P4(g) + 58.91
P4O10(s) -2984.0
P4O6(s) -1640.1

Note that these are enthalpies, not free energies of formation, but
disprop to P4 and P4O10 would be exothermic on that data.

5P4O6 --> 2P4 + 3P4O10 /\H = -752kJ/mol.

Maybe NIST confused data for P4O10 with P4O6? - It's in the right
ballpark for gaseous pentoxide.

A trip to the nearest Rubber Book might help.

Rob.
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  #7  
Old 02-04-2004, 03:23 PM
G. R. L. Cowan
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Default P4O6(g): fragile !? DH_f = -315 kcal/mol = -529 kcal/mol

[Only registered users see links. ] wrote:

Your rubber book has entries for P4On? Or even "P2On"?

NIST has entries for both P4O6 (gas phase only,
delta 'H' of f -2.2 MJ/mol)
and P4O10 (both gas and solid phase,
enthalpies in MJ/mol respectively -2.9, -3.0).


--- Graham Cowan
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