Phosphorus pentoxide

• Phosphorus ( V) oxide
• Phosphorus pentoxide
• Phosphorus pentoxide
• Tetraphosphorus decaoxide

Colorless and odorless, hygroscopic solid

Fixed

2.30 g · cm -3

340-360 ° C

With water decomposition to phosphoric acid

Risk

2 mg · m-3 (Recommendation, not the law )

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Phosphorus pentoxide, phosphorus pentoxide in more detail, is an oxide of the element phosphorus. It is a colorless, odorless, highly hygroscopic powder, which is reacted with water in a highly exothermic reaction to phosphoric acid.

Although long known that a molecule is described accurately by the empirical formula P4O10, the historical name (di ) phosphorus pentoxide has been preserved.

Production

Phosphorus pentoxide results from the combustion of white phosphorus in a dry air stream under strong heat development:

Lack of oxygen and phosphorus trioxide P4O6, wherein the four " outer " ( doubly bonded in the above formula to a phosphorus atom ) in the absence of oxygen atoms occurs.

Properties

Phosphorus pentoxide has to take a big effort to water and form metaphosphoric:

The balance here is almost entirely on the right side, so that in a phosphorus -containing, closed space sets a water vapor pressure of less than 1.3 × 10-4 Pa.

More water intake via polyphosphoric

To orthophosphoric acid

The overall reaction

Is highly exothermic. Here, a molar heat of reaction of -377 kJ · mol -1 is realized.

Use

Phosphorus is due to the equilibrium shown above, an extremely efficient drying agent (see desiccator ): water molecules from the surrounding air, which impinge on phosphorus pentoxide, are very strongly bound. Commercial desiccant containing 75 % phosphorus and 25 % inert inorganic carrier material, thereby maintaining a free-flowing structure of the drying agent. By the addition of water to the level of depletion of the indicator means is displayed. When absorbing water, the originally colorless indicator changes color to green, blue-green to blue (water content about 33 %).

In gases, liquids and solids contained water has a higher vapor pressure, making the water molecules move gradually to an almost complete phosphorus pentoxide. This also affects chemically bound water (see water of crystallization) and even bound separated hydrogen and hydroxyl groups in organic molecules or biological material: these have a certain water vapor pressure, ie, an OH group forms a H atom adjacent appended occasionally a water molecule which evaporates during extreme dry air. In the steam-containing air surrounding us this balance does not matter, but a sugar cube is black as the forgotten in the oven cake in a desiccator ( went to the lost water in the same way by the heat ).

Fertilizers containing phosphates such as potassium dihydrogen phosphate or diammonium hydrogen phosphate KH2PO4, (NH4 ) 2HPO4, wherein the phosphorus content is often indicated as converted to phosphorus pentoxide ( P2O5).

Although phosphorus is used in practice in organic chemistry just as the drying agent, it was experimentally, however, to demonstrate that the cation of phosphorus pentoxide in the gas phase is so reactive that it is even the most stable of all the carbon -hydrogen bonds, namely methane, effectively can activate at room temperature.

Safety

Phosphorus causes severe burns. Therefore, gloves, respiratory protection, and appropriate laboratory clothing must be worn during handling.