Why does phosphorus have two melting points


The element phosphorus occurs in several modifications: White phosphorus is a white mass at room temperature with a slightly pungent odor of garlic. In the course of time, the white phosphorus partially converts to other phosphorus modifications under certain storage conditions. With longer storage in water, the white phosphorus often appears yellow or even orange as a result of these conversions. The stored white phosphorus is therefore often referred to as “yellow” phosphorus. When it is cold, the pure, white phosphorus is crystal clear and very brittle, at room temperature it becomes wax-soft. White phosphorus does not conduct electricity, it melts at around 44 ° C to form liquid phosphorus. If the liquid is heated, it boils at 280.5 ° C and gaseous phosphorus is obtained. The white phosphorus is only sparingly soluble in water, so it can be kept in water for a long time. The modification crystallizes tetrahedral, in the P4Molecule, four phosphorus atoms are arranged in a tetrahedral manner.


Self-heating of the white phosphor
 

 
White phosphorus reacts with the oxygen in the air and forms a white smoke.
In the process, it heats up and melts. Auto-ignition can now occur at any time!


White phosphorus oxidizes spontaneously in the air, producing a white phosphorus pentoxide smoke. The exothermic reaction melts the phosphorus. Above 50 ° C it can ignite by itself and then burns with a very hot, yellowish-white and relatively bright flame to form phosphorus pentoxide:
  
P4 + 5 O2  P4O10      ΔHR. = −2995 kJ / mol
  
Therefore, white phosphorus must be kept underwater. Fires with white phosphorus are difficult to extinguish as they re-ignite after the water has evaporated. White phosphorus dissolves well in carbon disulfide CS2, Disulfur dichloride S.2Cl2 and in phosphorus trichloride PCl3, a little less good in benzene and ether. In the dark, under certain conditions, chemiluminescence occurs in extremely fine distribution with white phosphorus, as the P formed on the surface is2O3 into the more stable P2O5 converts. During this oxidation, energy is given off in the form of heat and light.

When white phosphorus is heated in an alkaline environment, the highly toxic hydrogen phosphide PH is generated3 and phosphinic acid H3PO2. If you heat white phosphorus to over 450 ° C for a long time, you get it, depending on the conditions red phosphorus and at the same time also purple phosphorus. The red, amorphous phosphorus sublimes about 30 ° C lower than the purple, crystalline phosphorus. Red phosphorus is chemically more stable than white phosphorus, it only ignites above 300 ° C and burns to phosphorus pentoxide with the formation of white smoke.
    

Red phosphorus burns
    

 
Red phosphorus burns in the air, producing a white smoke.


If red phosphorus is heated in the absence of air, for example in a test tube with a cotton ball, white phosphorus is obtained again, which is deposited on the test tube wall after the vapors have cooled down. In contrast to white phosphorus, red phosphorus is not soluble in the solvents mentioned. With strong oxidizing agents such as potassium chlorate, it forms explosive and friction-sensitive mixtures. The purple phosphorus(also known as “Hittorf's phosphorus”) can be produced by heating white phosphorus at around 550 ° C for several weeks. The product has a metallic sheen and appears purple on the edges, it is easily split and, like black phosphorus, does not dissolve in carbon disulfide. Purple phosphorus is a crystalline modification, the phosphorus atoms are arranged in pentagonal tubes, these form lattice-like networks.
     

Heat red phosphorus in the absence of air
 

 
White or yellow phosphorus forms in the absence of air.


If you expose the white phosphorus to high temperatures and high pressure of up to 12,000 bar, you get black phosphorus. Another manufacturing option would be the addition of finely divided mercury and heating to 380 ° C. If you then add black phosphorus seed crystals to the melt, the black phosphorus is created even without the high pressure. This modification forms gray-black glossy, rhombic crystals that have good thermal and electrical conductivity. It is also suitable as a semiconductor material. In the case of black phosphorus, the atoms are called P.6- Rings arranged in a chair shape, with each phosphorus atom having three neighboring atoms. In addition to the modifications mentioned, one also knows the fibrous phosphorus, which is formed when gaseous phosphorus cools slowly at 500 to 600 ° C. The thin fibers are often fused with platelets of purple phosphorus.