Phosphorus: Allotropes, Phosphine and Halides

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Here are two samples of phosphorus. The first glows in the dark, catches fire in air, and must be stored under water. The second does not glow, and is not even poisonous. Same atoms, so why are they so different? Let us find out, then meet phosphine and the phosphorus chlorides. This topic comes up often in JEE, so let's begin.

0:30White phosphorus: the P4 tetrahedron

So, white phosphorus. Look at the drawing on the left, P four, the small four below meaning four atoms in one discrete molecule, a tetrahedron. The other two are its oxides, not needed today. Each phosphorus bonds to the other three, giving six bonds, phosphorus to phosphorus, the P dash P on screen, and keeps one lone pair, so four lone pairs. Okay? The angle sign before PPP is the angle at each corner: sixty degrees, the small circle meaning degrees. That is really squeezed. So, what does it do to the bonds? Yes, exactly, it strains them! Strained bonds break easily, so white phosphorus is very reactive. It glows in the dark, called chemiluminescence, and catches fire in air. Clear? So it is stored under water. It is soluble in C S two, carbon disulphide, and poisonous. With me?

1:22Red and black phosphorus

Now, red phosphorus. We make it by heating white phosphorus at five hundred and seventy three kelvin, the capital K on screen, in an inert atmosphere. Okay? Here the P four units are joined into chains, so red phosphorus is polymeric. That relieves some strain, so it is less reactive, does not glow, and is not poisonous. So, which sample from our puzzle is this? Yes, it is the second one! Same atoms, but separate strained molecules in white phosphorus, and chains in red: that solves our puzzle. Black phosphorus is layered, the most stable and least reactive form, and it even conducts electricity. Got it?

2:00Making phosphine, PH3

Now, phosphine, P H three. White phosphorus is boiled with sodium hydroxide, under inert C O two. P four, plus three N a O H, plus three H two O, gives P H three, plus three N a H two P O two, sodium hypophosphite; the arrow means gives. Phosphorus starts at oxidation state zero in P four. Where does it go? One phosphorus goes down to minus three, in P H three, and three go up to plus one, in sodium hypophosphite; the small arrow means changes to. So phosphorus is reduced and oxidised at once, a disproportionation. Right? And a second method: calcium phosphide, C a three P two, plus six H two O, gives three C a, O H in brackets, taken twice, plus two P H three. With me?

2:49Phosphine: properties and reactions

Phosphine is colourless, it smells of rotting fish, and it is very poisonous. Now, does phosphine catch fire in air all by itself? If you said no, you are right, but only for pure phosphine, which does not catch fire by itself. It is the impurity, P two H four, diphosphane, that catches fire, and this is used in Holme's signals. Okay? It also precipitates phosphides: three C u S O four, copper sulphate, plus two P H three, gives C u three P two, plus three H two S O four. Remember it for example two. Got it?

3:22Phosphorus trichloride, PCl3

Now, the chlorides. P four plus six C l two gives four P C l three, phosphorus trichloride. It is pyramidal, and phosphorus is s p three hybridised, with the three written raised. Okay? Now, what acid forms when we add water? Many students write orthophosphoric acid, H three P O four. What do you say? No, no, wait, it gives H three P O three, phosphorous acid, plus three H C l. Orthophosphoric acid needs P C l five, as we will see. Right?

3:53Phosphorus pentachloride, PCl5

With more chlorine, P four plus ten C l two gives four P C l five, phosphorus pentachloride, trigonal bipyramidal in the gas. Okay? Its axial bonds are two hundred and forty picometres, the p m on screen, and its equatorial bonds two hundred and two. So why are the axial ones longer? Each axial bond pair is at ninety degrees to three equatorial pairs, but an equatorial pair to only two. More repulsion gives longer axial bonds, and that makes P C l five reactive. Clear? The solid is ionic: P C l four plus and P C l six minus, in square brackets with the charges raised. On heating, it gives P C l three and C l two; the double half arrows mean reversible. Right? Water hydrolyses it in two steps: P C l five plus H two O gives POC l three, plus two H C l, then POC l three plus three H two O gives H three P O four, orthophosphoric acid, plus three H C l. Got it?

4:55Example 1: volume of PH3 at STP from 18.2 g of Ca3P2

Now, example one. What volume of phosphine at STP is obtained from eighteen point two grams, the g on screen, of calcium phosphide and excess water? At STP, one mole of gas takes twenty two point four litres; L mol to the minus one means litres per mole. Pause the video and try it first. Done? Okay, let's check. The equation: calcium phosphide plus six H two O gives three calcium hydroxide, plus two P H three. Okay? The molar mass of calcium phosphide, capital M, is one hundred and eighty two, so eighteen point two over one hundred and eighty two gives zero point one zero zero moles, the mol on screen. So zero point one zero zero moles of phosphine too, right? No, no, look at the equation! One C a three P two gives two P H three, so we get double: zero point two zero zero moles. Clear? So the volume is zero point two zero zero times twenty two point four, which gives four point four eight litres. Got it?

5:58Example 2: mass of CuSO4 to absorb PH3 from 2.48 g of P4

Now example two, a JEE pattern question. Two point four eight grams of white phosphorus is boiled with sodium hydroxide solution in an inert atmosphere. The gas Q that comes off is completely absorbed in copper sulphate solution. What mass of copper sulphate, molar mass one hundred and fifty nine point six grams per mole, is used up? Pause the video and try it first. Done? Okay, let's check. White phosphorus with boiling sodium hydroxide gives phosphine, so Q is P H three. Now, P four has four phosphorus atoms, so four moles of phosphine per mole, right? Oh no, careful, remember the disproportionation: only one of the four atoms becomes P H three. So n, the moles of P four, is two point four eight over one hundred and twenty four, its molar mass, giving zero point zero two zero zero moles, and the same number of moles of phosphine. With me? Now the copper sulphate reaction: three C u S O four, plus two P H three, gives C u three P two, plus three H two S O four. Right? So three over two, times zero point zero two zero zero, gives zero point zero three zero zero moles of copper sulphate. Multiply by one hundred and fifty nine point six, and the mass is about four point seven nine grams; the wavy equals sign means approximately.

7:21Quick revision: allotropes, phosphine, PCl3 and PCl5

Clear? So, quick revision. White phosphorus, with its strained sixty degree angles, is very reactive; red has chains, and black is the most stable. Phosphine comes from white phosphorus with boiling sodium hydroxide, or calcium phosphide with water, and pure phosphine does not ignite by itself. With water, P C l three gives phosphorous acid, P C l five gives orthophosphoric acid, and P C l five has the longer axial bonds. Now try the practice problems on this page. All the best!