Chemical elements
  Phosphorus
    Isotopes
    Energy
    Preparation
    Applications
    Physical Properties
    Chemical Properties
      Alkali Phosphides
      Alkaline Earth Phosphides
      Copper Silver and Gold Phosphides
      Zinc Group Phosphides
      Aluminium Phosphide
      Titanium Group Phosphides
      Tin Phosphides
      Lead Phosphides
      Arsenic Phosphides
      Antimony Phosphides
      Bismuth Phosphides
      Chromium Phosphides
      Molybdenum and Tungsten Phosphides
      Manganese Phosphides
      Iron Phosphides
      Cobalt Phosphides
      Phosphonium Chloride
      Phosphonium Bromide
      Phosphonium Iodide
      Hydrogen Phosphides
      Alkylphosphines
      Phosphorus Trifluoride
      Phosphorus Pentafluoride
      Phosphorus Trifluorodichloride
      Phosphorus Trifluorodibromide
      Fluophosphoric Acid
      Phosphorus Dichloride
      Phosphorus Trichloride
      Phosphorus Pentachloride
      Phosphorus Chlorobromides
      Phosphorus Chloroiodides
      Phosphorus Tribromide
      Phosphorus Pentabromide
      Phosphorus Diiodide
      Phosphorus Triiodide
      Phosphorus Oxytrifluoride
      Phosphorus Oxychloride
      Pyrophosphoryl Chloride
      Metaphosphoryl Chloride
      Phosphoryl Monochloride
      Phosphoryl Dichlorobromide
      Phosphoryl Chlorodibromide
      Phosphoryl Tribromide
      Metaphosphoryl Bromide
      Phosphoryl Oxyiodides
      Phosphorus Thiotrifluoride
      Phosphorus Thiotrichloride
      Phosphorus Thiotribromide
      Mixed Phosphorus Thiotrihalides
      Phosphorus Suboxides
      Phosphorus Trioxide
      Phosphorus Dioxide
      Phosphorus Pentoxide
      Hypophosphorous Acid
      Phosphorous Acid
      Meta- and Pyro-phosphorous Acids
      Hypophosphoric Acid
      Tetraphosphorus Trisulphide
      Diphosphorus Trisulphide
      Tetraphosphorus Heptasulphide
      Phosphorus Pentasulphide
      Phosphorus Oxysulphides
      Phosphorus Thiophosphites
      Phosphorus Thiophosphates
      Phosphorus Selenophosphates
      Phosphorus Sulphoselenides
      Diamidophosphorous Acid
      Phosphorus Triamide
      Monamidophosphoric Acid
      Diamidophosphoric Acid
      Triamidophosphoric Acid
      Dimetaphosphimic Acid ≡P=
      Trimetaphosphimic Acid
      Tetrametaphosphimic Acid
      Penta- and Hexametaphosphimic Acid
      Monamidodiphosphoric Acid
      Diamidodiphosphoric Acid
      Triamidodiphosphoric Acid
      Nitrilotrimetaphosphoric acid
      Monothioamidophosphoric Acids
      Thiophosphoryl Nitride
      Di- Tri-imido- and -amido-thiophosphates
      Imidotrithiophosphoric Acid =
      Phosphorus Chloronitrides
      Triphosphonitrilic Chloride
      Tetraphosphonitrilic Chloride
      Pentaphosphonitrilic Chloride
      Hexaphosphonitrilic Chloride
      Heptaphosphonitrilic Chloride
      Triphosphonitrilic Bromide
      Phosphorus Halonitrides
      Phosphorus Nitride
      Phosphine
      Pyrophosphoric Acid
      Phosphoric acids
    Slow Oxidation
    Phosphatic Fertilisers

Phosphorus Pentoxide, P2O5






Vigorous combustion of phosphorus in air produces a voluminous white powder which is very deliquescent and hisses when dropped into water, evolving much heat and giving a liquid of acid reaction. These salient facts were observed by the early workers on phosphorus, e.g. by Robert Boyle in 1681. The compound was analysed by many of the leading chemists at the beginning of the nineteenth century, and the empirical formula P2O5 was established.

This oxide is formed by combustion, in a full supply of air or oxygen, of white phosphorus (ignition temperature about 60° c.), of phosphorous oxide (50°-70° c.), of red phosphorus (about 260° c.), and of the phosphines and other combustible compounds. The white powder prepared in the laboratory or technically by these methods is always impure, containing a little phosphorous oxide, metaphosphoric acid, etc., while a part of the phosphorus usually escapes combustion and remains as red phosphorus. The preparation of a pure product requires further treatment. Thus the pentoxide may be thrown into a red-hot porcelain basin and stirred in a current of oxygen. The crude oxide may also be resublimed in a current of oxygen and passed over platinised asbestos or platinum sponge. The product used by Baker in experiments on intensive drying was also distilled, without the aid of a catalyst, in a current of oxygen at 180° to 210° c. The yield was about 10 per cent. A convenient apparatus for this preparation was described by Finch and Peto. The ordinary pentoxide was pushed continually through a glass tube down the vertical limb of a heated iron T-piece which was traversed by a current of oxygen. The product, which was collected in a wide-mouthed glass bottle, was partly crystalline and partly amorphous.

The purified oxide should be devoid of any alliaceous odour or odour of phosphine, and should not reduce a boiling solution of mercuric chloride. When dissolved in water and the solution neutralised with alkali (to methyl orange) it should give a white precipitate with silver nitrate which should not darken after boiling for five minutes.


Properties

The pentoxide exists in crystalline and vitreous forms, the transformation temperature of which has been given as 440° c. Sublimation proceeds with moderate speed between 180° and 250° c. and the vapour pressure may reach 760 mm. at 360° c. When it is sublimed at 360° c. in a current of oxygen the oxide forms brilliant crystals together with some of the amorphous material which is considered to be a product of polymerisation. The crystalline form, by x-ray examination, is that of the rhombohedral system with 12P2O5 in a unit cell; the lengths of the axes are a = 11.12, b = 1.12 Å.

The melting-point was found to vary between 560° and 570° C. according to the time of heating.

The vapour density indicated a molecular weight of 336 at a red heat and 300 at 1400° C. At the higher temperature, therefore, the molecule approximates to P4O10 (M = 284).

The inconsistent behaviour on sublimation suggests that phosphorus pentoxide, like sulphur trioxide and phosphorus itself (q.v.), contains at least two crystalline forms, a metastable form with a higher and a stable form with a lower vapour pressure. These are present as a mixture below 300° C., at which temperature the vapour pressure of the metastable form becomes appreciable and then increases rapidly, reaching 3.5 atm. at 400° C. From this temperature upwards the pressure of the stable form becomes appreciable and equilibrium conditions are more easily obtained. The more volatile form may be sublimed away; consequently an abrupt fall of pressure above 400° C. has been observed. A stable vapour pressure curve has been obtained up to and beyond the triple point. Pure dry oxygen was passed over English P2O5 which was heated to 270° C. The sublimate was received directly in the tensimeter, which was contained in an electric furnace wound with nichrome wire which gave temperatures above 400° C., which were controlled within ±2° C. The pressures were measured by a glass manometer with quartz thread indicator. The following typical values are taken from the tabulated results:—

Vapour pressures of phosphorus pentoxide

t° CSolid.Liquid.
449522.5557.5596.5613.5656 700
p cm. mercury2.312.632.768.783.9135.6216


The triple point is found at 55.5 cm. of mercury and 580° C.

The affinity of formation of P2O5 evidently is very great, since no dissociation was observed at the highest temperatures mentioned above. When heated in the oxyhydrogen flame the oxide gives a continuous spectrum. In respect of this great stability P2O5 differs markedly from its congeners N2O5 and As2O5.

The heat of formation is also by far the highest in the Group and is higher than the heat of oxidation per atom of any other non-metal. The value per mol of solid P2O5 from solid white phosphorus and gaseous oxygen is given as 369.9 Cals., 369.4 Cals. The heats of formation from red phosphorus are rather lower.

Phosphoric oxide has also an exceptionally high affinity of hydration, on which account it is universally used, where its chemical properties permit, as the most powerful drying agent for neutral or acid gases and liquids and also in desiccators. The heat of hydration of crystalline P2O5 is given as 44.6 Cals., 40.8 Cals. The amorphous variety when dissolved in much water evolved 33.8 Cals. and the vitreous variety 29.1 Cals. Hence heat is evolved when the crystalline variety is transformed into the amorphous variety.

Structure

Many possible structures may be assigned to the molecule. Whichever of these is adopted the phosphorus must be quinquevalent according to the old representation, or quadrivalent with a semipolar bond according to the "octet " theory. Thus

or

Chemical Properties

Phosphorus pentoxide was reduced by hydrogen at a red heat in the presence of nickel, and by carbon at high temperatures, but not by silicon.

The alkali and alkaline earth metals react with great energy when heated with the oxide, giving oxides and phosphides.

When heated with halides of phosphorus the two molecules give oxyhalides, the change being represented by the equation

P4O10 + 6PCl5 = 10POCl3

or P4O10 + 6PBr5 = 10POBr3

The pentoxide also gives the oxychloride when heated with the halides of other non-metals, although the other non-metal does not necessarily give oxyhalide also. Thus with boron trichloride

P2O5 + 2BCl3 = POCl3.BCl3 + BPO4

and with carbon tetrachloride in a sealed tube between 200° and 300°

P2O5 + 2CCl4 = COCl2 + CO2 + 2POCl3

With an excess of P2O5

2P2O5 + 3CCl4 = 3CO2 + 4POCl3

Oxyhalides of phosphorus are also produced when the pentoxide is heated with fluorides or chlorides of the alkali and alkaline earth metals; thus POF3 is produced with calcium fluoride and POCl3 with sodium chloride.

When brought into contact with water under various conditions phosphorus pentoxide forms one or more of differently hydrated acids. Metaphosphoric acid (q.v.) probably is the first product.

The pentoxide has a powerful dehydrating effect upon oxyacids and is therefore used in preparing anhydrides from these. It also removes halogen from halogen hydracids under some conditions, giving oxyhalides, e.g. POF3 from HF. Hydrogen chloride is completely absorbed by the oxide and gives a liquid from which POCl3 can be distilled.

Phosphorus pentoxide cannot be oxidised further by ordinary oxidising agents, except to compounds of the " peracid " type. The oxygen is displaced by metathesis as just shown, but not by anhydrous halogens, except fluorine, which at a dull red heat gives PF5 and POF3.

Dry ammonia gives amido- or imido-phosphoric acids (q.v.), and also salts such as diammonium amidopyrophosphate.
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