Phosphorus (1959)

[Music] the element phosphorous is an interesting example of an element that forms two allotropic forms the two varieties which commonly occur are known as white phosphorus and red phosphorus white phosphorus is in the tube on the right it's the form that is prepared by the condensation of phosphorous vapor and when freshly prepared is white however this form of the element is metastable and slowly turns to the red form even at room temperature for this reason the piece of phosphorus in the tube has a yellowish appearance due to a small surface coating of the red variety red phosphorous occurs in the tube on the left it's the stable form at room temperature these two uh forms of phosphorus also exhibit several remarkably different kinds of behavior white form as we'll soon see bursts into flame and air whereas the red form is quite stable and less heated because of this tendency of the white form to burst into Flame it must be stored underwater and that is the reason for the liquid in the tube on the right we can compare the activity of red phosphorus and white phosphorus by exposing samples of them to the air on the watch glass on the left I've placed a small sample of red sosus the watch class on the right has been warmed very slightly and on that watch class I'll now place a sample of white phospherous you can see that the white phosphorus almost immediately bursts into flame and burns with the production of large clouds of phosphorus pentoxide where under virtually identical conditions the red phosphorus does not react with the oxygen in the air at any appreciable rate since phosphorus is a non-metal its oxide formed during the combustion should react with water to produce an acid some of the phosphorus pentoxide will have remained on the watch glass and I'll now add a few drops of water to the residue on the glass now I'll add a few drops of methal red indicator you'll recall that this indicator assumes its red color in acid solution the appearance of the bright red color uh confirms terms our prediction the phosphorus pent oxide will react with water to form an acid solution red phosphorus like white will burn in air if it's raised to a somewhat higher temperature on the watch glass we have a few small pieces of red phosphorus and I will now light these with the flame of the bunson burner you can see that the phosphorus ignites easily and that the red phosphorus Burns producing the same clouds of white smoke we observed with the white phosphorus and the product of the reaction is the same phosphorus pentoxide we'll permit the red phosphorus uh to burn until virtually all of it has been used up and then test the residue with water and methyl red as we did before the red phosphorus has now burned itself out and the watch glass has been permitted to cool we'll add a few drops of water and some methyl red indicator as before and again the bright red color of the indicator tells us that the solution is acid it is acid because the p205 phosphorous pentoxide form on the red phosphorus burned has reacted with the water to form phosphoric acid plight phosphorus reacts rather rapidly with most of the halogens we can illustrate this reaction by putting a small piece of white phosphorus on the watch glass and to it adding some crystals of iodine the heat of the reaction causes the phosphorus to burst into Flame the product of the reaction is pH triiodide and the balance of the phosphorus is now simply burning forming phosphorus pentoxide we observed that when a piece of white phosphorus was placed on the watch glass it almost immediately burst in flame and produced large clouds of white smoke the reaction involved was between phosphorus and oxygen yielding the phosphorus pentoxide a white powder which caused the smoke we then added some water to the residue on the watch glass and tested with an indicator when the oxides of phosphorus like p25 react with water the initial product is metaphosphoric acid and this then may react with an additional molecule of water to yield orthophosphoric acid and both of these acids ionize yielding hydrogen ions which we detected with the indicator the reactions we observed with red phosphorus were entirely similar the phosphorus in this equation then may be either white or red the equation is the same producing p25 and then of course the subsequent equations are the same also when we treated a small piece of phosphorus with crystals of iodine we found that a rapid reaction took place with the production of phosphorus triodine when phosphorus reacts with an active metal such as calcium a compound called calcium phosphide is formed this material also results from the reduction of calcium phosphate calcium phosphide ca2 P3 reacts with slightly acidified water to produce mostly phosphine ph3 and Trace impurities of another hydride of phosphorus P2 H4 when these compounds come in contact with the air the P2 H4 ignite spontaneously setting off the phosphine so that little puffs of flame result I'll now Place several lumps of calcium phosphide in the water in the beaker so that you may observe this reaction down here the calcium phosphide is reacting with the water forming bubbles of phosphine these rise to the surface and ignite here giving puff flame and producing fumes of phosphorus pentoxide some water is formed also we saw that when we reacted small lumps of calcium phosphide C3 P2 with water reaction took place rapidly with the production of calcium hydroxide and phosphate a gas which bubbled from the reaction mixture a phosphine may contain traces of other halides of phosphorus which ignite spontaneously and then set off the phosphine but in any case at the surface of the water in the reaction vessel when the phosphine encounters the oxygen in the air it does burn with the production of phosphorus pentoxide and water as shown by this equation we turn now to a consideration of some reactions of the phosphate ion in each of the two test tubes I've placed several milliliters of a solution of sodium phosphate and to each tube I'll add a few drops of silver nitrate solution the yellow precipitate is silver phosphate now we've seen before that we can dissolve a precipitate if we can cause the product of the concentrations of its ions raised to the appropriate power to fall below the solubility product constant for this precipitate in the case of silver phosphate we can lower the concentration of either the phosphate ion or of the silver to the tube on the left I'll add some dilute nitric acid you can see that this precipitate rapidly dissolves now the dilute nitric acid reacts with the phosphate ion converting it to phosphoric acid and so reducing the concentration of the phosphate ion that the solubility product conditions are not meant to the tube on the right I'll add some concentrated ammonium hydroxide and once again the precipitate dissolves the ammonium hydroxide reacts with a silver ion forming the silver ammonia complex ion and reducing the concentration of the silver ion so in this case by reducing the concentration of the phosphate ion we've caused the precipitate to dissolve and in this case by reducing the concentration of the silver ion we've caused the precipitate to dissolve when we mix Solutions of silver nitrate and sodium phosphate we obtain a precipitate of silver phosphate this precipitate must have been obtained because the product of the concentration of the silver ion cubed times the phosphate ion concentration exceeded the solubility product constant for silver phosphate we then divided this precipitate into two sections and treated one section with acid we observed that the precipitate dissolved nitric acid was used as a source of the hydrogen ion now when the precipitate dissolved it must have done so because the silver ion concentration Cube times the phosphate ion became less than the solubility product constant now in the case of the acidification of silver phosphate precipitates we were operating on the small quantity of phosphate ion in equilibrium with the precipitate when we added acid this phosphate ion was converted to the hydrogen phosphate ion and in the presence of additional acid would go to the dihydrogen phosphate ion and eventually to phosphoric acid in any case however the phosphate ion concentration is greatly reduced this term becomes very small and this product this quantity times this quantity with this very small is less than the solubility product constant and the precipitate dissolved now when we added ammonia to Solutions or suspensions of silver phosphate we're operating on a silver ion in equilibrium with this precipitate the silver ion reacts with two molecules of ammonia to produce the silver ammonia complex ion this lowers the concentration of the silver ion making this term very small and once again the product of these two terms is smaller than the solubility product constant so the precipitate dissolve a common test for the phosphate ion involves its reaction with Solutions of ammonium mdate in the presence of nitric acid in the test tube we have a few milliliters of a solution of sodium phosphate to this solution I'll add a small quantity of ammonium mdate solution and we'll then make the mixture acid with nitric acid the yellow precipitate which forms in the tube is ammonium phosph mdate the formation of this precipitate with its characteristic yellow color serves as a good test for the presence of the phosphate ion we'll now utilize the test we've just developed to study the reaction between red phosphorus and nitric acid I have a small quantity of red phosphorus in this uh brown bottle I'll place a gram or so of this material in the beaker we'll now add some dilute nitric acid to the beaker also we'll then place the buns and burner under the setup and boil the mixture for several minutes after several minutes of boiling the reaction mixture has been permitted to cool somewhat and we will now filter to remove excess phosphorus to the filtrate from the reaction between the red phosphorus and the nitric acid We'll add a few milliliters of ammonium mdate and then some additional concentrated nitric acid the appearance of the yellow precipitate of ammonium phosph mdate confirms our suspicion that the red phosphorus has been oxidized to the phosphate ion or to phosphoric acid by the action of the nitric acid we developed a test for the presence of the phosphate ion by treating a solution containing the phosphate ion with an excess of nitric acid and a solution of ammonium mdate containing the mdate ion M4 minus now we obtained a yellow precipitate and this precipitate may have a rather variable composition but one author at least represents it as the ammonium taken three times four hydrogens phosphorus attached to six m207 groups this is called ammonium phosph mdate the other product of the reaction is water and this material precipitates is yellow and serves as an identification of the phosphate ion we then reacted red phosphorus with delute nitric acid we found that the equation can be balanced by the half reaction method with the phosphorus being oxidized to the phosphate Ion four waters are required eight hydrogens and five electrons needed for for balancing the nitrate ion in dilute concentration is reduced to nitric oxide three electrons are required for balancing so this equation must be multiplied by three and this by five for electrical balance and adding the two partials up we obtain uh three phosphorus atoms two molecules of water from the 12 Waters that entered in this partial and the 10 molecules of water produced in this partial leaving an n of two here and five nitrate ions producing three phosphate ions four hydrogen from the 24 hydrogens in the top partial and the 20 hydrogens in the bottom and that of four on the right and five molecules of nitric oxide we then added ammonium aldate and excess nitric acid and tested for the phosphate ions here in the same manner uh that we did previously and again obtained uh the yellow precipitate having this composition