Oxides and Oxoacids of Phosphorus

Transcript

0:00Introduction

Hello everyone! Let us begin with a small puzzle. Phosphorous acid, H three P O three, has three hydrogen atoms, so it should give three H plus ions in water, right? But it gives only two. And hypophosphorous acid, H three P O two, gives just one. So where did the other hydrogens go? Let us find out today. This topic comes up often in JEE, so let's begin.

0:27The oxides: P₄O₆ and P₄O₁₀

First, the oxides. We burn phosphorus, P four; the small lowered numbers after the letters are subscripts, counting atoms. In limited oxygen, O two, diluted with nitrogen, N two, we get P four O six. In excess oxygen, we get P four O ten. Okay? Both these oxides are cages built on the P four tetrahedron. So let me ask you this: how many edges does a tetrahedron have? It has six edges, yes, exactly! Each edge carries one bridging oxygen atom, between two phosphorus atoms. That is the cage. With me? In P four O six, each P bonds to three oxygens, so four times three gives twelve P O bonds; the dash means a single bond. P four O ten adds one oxygen on each P, as P double bond O, drawn with two lines. Clear? Now add water; the arrow means gives. P four O six plus six H two O gives four H three P O three. P four O ten plus six H two O gives four H three P O four. Right? And P four O ten is a powerful dehydrating agent. It turns HNO three into N two O five, and H two S O four into S O three. Got it?

1:49Oxoacids: count the P–OH groups

Now the oxoacids; the figure shows four of them, with the basicity written under each. In each one, P is tetrahedral and carries one P double bond O, at the top. Okay? Look at the second one, H three P O three. Two hydrogens sit on oxygen, as O H groups, and one sits directly on phosphorus. So which hydrogens can ionise, the ones on oxygen, or the one on phosphorus? The ones on oxygen, yes, exactly! Only hydrogens on oxygen, the O H ones, can ionise. Hydrogen bonded directly to phosphorus, in a P H bond, does not ionise at all. Clear? So here is the trap: do not just count the hydrogens. The rule is really simple: basicity equals the number of POH groups. With me? H three P O two, hypophosphorous acid, the first structure, has one POH group and two P H bonds, so it is monobasic. Right? And H three P O three, phosphorous acid, has two POH groups and one P H bond, so it is dibasic. Puzzle solved: the third hydrogen sits on phosphorus, and never ionises. Got it? Likewise, H three P O four, orthophosphoric acid, has three POH groups, so it is tribasic. And the last one, H four P two O seven, pyrophosphoric acid, has four, so it is tetrabasic. Okay?

3:15Oxidation states and more oxoacids

Next, oxidation states. With hydrogen plus one and oxygen minus two, P is plus one in H three P O two, plus three in H three P O three, and plus five in H three P O four. Clear? Now, H four P two O five, pyrophosphorous acid: P is plus three, with two P H bonds and a POP bridge, an oxygen joining two phosphorus atoms. It is dibasic, right? Now you try one. Hypophosphoric acid is H four P two O six. With hydrogen plus one and oxygen minus two, what is the oxidation state of phosphorus in it? It is plus four, very good! That is H four P two O six, hypophosphoric acid. The two phosphorus atoms are joined directly by a P P bond, and it is tetrabasic. Okay? And HPO three in brackets, with a small three outside for three units, is cyclotrimetaphosphoric acid. P is plus five, in a ring with three POP bridges, and it is tribasic. Got it?

4:15P–H bonds make the acid a reducing agent

Now, P H bonds make the acid a reducing agent. H three P O three reduces silver nitrate, A g N O three. With water, it gives two A g, silver metal, plus H three P O four, plus two HNO three. With me? Now H three P O two. With four A g N O three and two H two O, it gives four A g, plus H three P O four, plus four HNO three. Both acids end up as H three P O four, but this one gives four silver atoms, not two. So why twice the silver? Look at phosphorus. From plus three to plus five, it loses two electrons, and from plus one to plus five, it loses four. The arrow means goes to; e with a small minus is an electron. Each A g plus ion, its plus charge up top, takes one electron. Clear?

5:07On heating: P in +3 or +1 disproportionates

Next, heat. Acids with P in plus three or plus one disproportionate on heating: the same element is oxidised and reduced together. Four H three P O three gives three H three P O four, plus P H three, phosphine. Okay? Check the electrons. Three P atoms go up from plus three to plus five, losing six electrons. One P goes down to minus three in phosphine, gaining six. So it balances, right? And hypophosphorous acid does the same: two H three P O two gives H three P O four, plus P H three. Got it?

5:41Example 1: P–O–P, P=O and sigma bonds in P4O10

Now example one, an easy one. How many POP bonds, P double bond O bonds, and sigma bonds are present in P four O ten? Sigma is the Greek letter on screen. Pause the video, and try it first. Done? Okay, let's check. There are six bridging oxygens, one on each edge of the tetrahedron. So six POP links, and that means twelve single P O bonds. Okay? Then each P carries one terminal oxygen, so there are four P double bond O bonds. Now the sigma count. How many sigma bonds does one double bond contain? Just one sigma, plus one pi, the other Greek letter, yes, exactly! So there are sixteen sigma bonds, twelve plus four, and four pi bonds. The answer is six POP, four P double bond O, and sixteen sigma. Counting a double bond as two sigma gives twenty, so be careful. Clear?

6:40Example 2: true statements on H3PO3 and H3PO4

Now a harder one, in JEE pattern. Which are correct about the oxoacids of phosphorus? Option a: on heating, H three P O three disproportionates to H three P O four and P H three. Option b: H three P O three can act as a reducing agent but H three P O four cannot. Option c: H three P O three is monobasic. Option d: the H of the P H bond of H three P O three is not ionisable in water. Pause the video, and try it first. Done? Let's check. In H three P O three, P is plus three, so it disproportionates on heating, and option a is true. It has a P H bond, so it is a reducing agent, and H three P O four has none, so option b is true too. Okay? Now option c says that H three P O three is monobasic. Is that true, or false? That is false! It has two POH groups, so it is dibasic. Counting all three hydrogens is exactly the trap here. Right? And option d is true: the hydrogen bonded to phosphorus does not ionise, just like in our puzzle. So the correct options are a, b and d. Clear?

7:58Quick revision: oxides, basicity rule, P–H reducing acids

So, quick revision. P four O six forms in limited oxygen, P four O ten in excess, with oxygen bridges on the six edges. Basicity equals the number of POH groups; hydrogen on phosphorus never ionises. P H bonds mean a reducing acid, and P in plus three or plus one disproportionates on heating. Now try the practice problems on this page. All the best!