Aluminium and Its Compounds

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Aluminium sits high in the reactivity series, so it should be very reactive, right? And yet aluminium pans sit in our kitchens for years and do not corrode. How is that possible? Keep that question in mind; we will answer it today. Questions on aluminium come up often in JEE, so let's begin.

0:28Bayer process: purifying bauxite

First, we need pure alumina, from bauxite, A l two O three dot two H two O, on screen. The small numbers below count the atoms, and the raised dot joins the water of hydration. Bauxite also carries iron oxide, F e two O three. Alumina is amphoteric, it reacts with acids and bases, but iron oxide is basic. So how can we separate them? Yes, exactly, with an alkali! Hot N a O H dissolves the alumina: A l two O three, plus two N a O H, plus three H two O, gives two N a, A l O H four. The arrow means gives, the square brackets hold the aluminium complex, and the round brackets hold the O H group, taken four times. Iron oxide stays undissolved. Okay? Now we pass C O two, which forms a weak acid in water, into the filtrate. It neutralises the alkali, so two N a, A l O H four, plus C O two, gives hydrated alumina, plus two N a HCO three. In the hydrated alumina, the letter x counts the water molecules, and the down arrow means it comes out as a solid. Right? Finally, we ignite it at one thousand four hundred and seventy kelvin, the capital K on screen, to get pure alumina. Got it?

1:47Hall–Héroult process: electrolysis

Next, the metal, by electrolysis in the Hall Héroult process. Pure alumina is dissolved in molten cryolite, N a three A l F six, with a little fluorspar, C a F two. Why not just melt pure alumina? Because pure alumina melts at two thousand three hundred and twenty three kelvin, very high! Cryolite and fluorspar lower this to about one thousand two hundred kelvin, as the arrow shows, and improve the conductivity. Clear? The graphite anodes burn away, so carbon appears in the overall equation: two A l two O three, plus three C, gives four A l, plus three C O two. Example two uses this equation. With me?

2:29Reactions of aluminium

Now, its reactions. With hydrochloric acid, two A l plus six H C l gives two A l C l three plus three H two. No surprise there, right? Now let me ask you this. Does aluminium also react with an alkali, like N a O H? Yes, it does, because aluminium is amphoteric! Two A l, plus two N a O H, plus six H two O, gives two N a, A l O H four, the same aluminium complex as in the Bayer process, plus three H two. Okay? And this answers our puzzle. In air, a thin, tightly adherent film of A l two O three forms and protects the metal, so aluminium pans do not corrode. Got it? Next, the thermite reaction: two A l plus F e two O three gives A l two O three plus two F e. Delta H, the enthalpy change, is very negative, and that heat is used to weld rails.

3:22Anhydrous aluminium chloride

Right? Now, anhydrous aluminium chloride, with no water at all. Its hydrate, A l C l three dot six H two O, is shown on screen. Why not just heat it to drive the water off? No, it does not work, and that is the trap! On heating, the water hydrolyses it: two A l C l three dot six H two O gives A l two O three, plus six H C l, plus nine H two O, nine waters. The H C l escapes, leaving alumina, not the chloride. Okay? So we use H C l gas; the small g in brackets means gas. Two A l plus six H C l gives two A l C l three plus three H two. With me?

4:05The dimer Al₂Cl₆

Now look at the figure. On the right is aluminium chloride in the vapour and in non-polar solvents: a dimer, A l two C l six. Only at high temperature does the vapour split into single A l C l three molecules. On the left, for comparison, is diborane, B two H six; we will not need its labels today. In a single A l C l three, aluminium has only six valence electrons. How does it get eight? From the other molecule, yes, exactly! One chlorine gives a lone pair to the other aluminium, and the other way round; those are the two arrows, the dative bonds. So we get two chlorine bridges, and each aluminium is tetrahedral. Clear? And the bond lengths: the bridging bonds are two hundred and twenty one picometres, written p m, and the terminal ones two hundred and six. The greater-than sign says it: the bridging bonds are longer. Right?

5:03Alums

Finally, alums. The general formula is M one, M three, S O four taken twice, dot twelve H two O, okay? The Roman one on top of the first M means a metal in the plus one state, and the Roman three means plus three. The best known is potash alum: K two S O four, dot A l two, S O four taken three times, dot twenty four waters. But the general formula said twelve! So which is right? Both, because this formula is simply doubled! Halve it, and you get K A l, S O four taken twice, dot twelve waters: potassium plus one, aluminium plus three. Right? Its big use is purifying water. Clay particles in muddy water are negatively charged, and the aluminium ion, with its high charge of three plus written on top, coagulates them, so the water clears. With me?

5:55Example 1: mass of Al to reduce 16 g Fe2O3 by thermite

Now, Example one, an easy start. What mass of aluminium is needed to reduce sixteen point zero grams of F e two O three by the thermite reaction? Pause the video and try it first. Done? Okay, let's check. Moles first; n means the amount in moles. Sixteen point zero over one hundred and fifty nine point seven, the molar mass, gives about zero point one zero zero moles; the wavy equals sign means approximately. So how many moles of aluminium? The equation needs two A l for each F e two O three, so we double it. Two times zero point one zero zero gives zero point two zero zero moles. Okay? Aluminium is twenty seven point zero grams per mole, so zero point two zero zero times twenty seven point zero gives five point four grams.

6:50Example 2: carbon anode burnt per kg of aluminium

Clear? Now, Example two, a little harder. Assume the only anode reaction is C plus two O two minus gives C O two plus four e minus; O two minus is the oxide ion, and e minus an electron. What mass of carbon anode is consumed per kilogram of aluminium produced? Pause the video and try it, using the overall equation. Done? Okay, let's check. The overall equation says two A l two O three plus three C gives four A l plus three C O two. So four moles of aluminium need three moles of carbon. Right? One kilogram is one thousand grams, and one thousand over twenty six point nine eight gives thirty seven point zero six moles of aluminium. Now, do we multiply by three over four, or by four over three? Three over four, yes, exactly! Four over three would be the trap. Three over four times thirty seven point zero six gives twenty seven point eight moles of carbon. With me? Carbon is twelve point zero grams per mole. Twenty seven point eight times twelve point zero is about three hundred and thirty four grams, about zero point three three kilograms. Got it? In practice, though, about zero point five kilograms burns, because some C O, carbon monoxide, also forms. That is why the question said this reaction only. Clear?

8:12Quick revision: Bayer, Hall-Heroult, AlCl3 dimer, alums

So, quick revision. Bayer: N a O H dissolves amphoteric alumina; Hall Héroult: electrolysis in molten cryolite. Heating hydrated aluminium chloride hydrolyses it, and the vapour is the dimer, A l two C l six. And in potash alum, the aluminium three plus ions coagulate clay and purify water. Now go and try the practice problems on this page. All the best!