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Chemistry: Aluminum


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  • Type: Video Tutorial
  • Length: 12:51
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 137 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Chemistry of Metals (8 lessons, $13.86)
Chemistry: Physical and Chemical Metals Processes (4 lessons, $7.92)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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The next stop on our tour of the main group elements is aluminum. Aluminum is the most abundant metal of the elements that exist in the lithosphere. It falls right behind oxygen and silicon. And like the alkaline metals and the alkaline earth metals, it does react with water. The metal reacts with water and oxygen. But we're going to see that there's a reason why it doesn't turn into a pile of dust - so, for instance, why aluminum cans are actually reasonably stable. Now aluminum exists in nature in a number of different important minerals, including bauxite, orthoclase, beryl, cryolite, and corundum. And you can see in the graphic that these all have formulas that include other elements, with the exception of bauxite, which only includes oxygen. Now bauxite turns out to be the principal source of aluminum from a metallurgical standpoint, and we've already talked about the Bayer process, in which bauxite is taken to aluminum oxide. And so we have bauxite, which is a mixture of aluminum hydroxide and various oxides of aluminum. And the first step in the Bayer process is that this mixture is digested in hot aqueous sodium hydroxide, and it forms the tetrahydroxaluminate anoin. And going along for the ride are the various soluble silicon hydroxides. The things like ferric hydroxide are not soluble in strong, hot base and so they're left as precipitates and those are filtered away.
And then, the next step is to lower the temperature and re-precipitate the aluminum hydroxides and the silicon hydroxides stay in solution, so it's possible to make these aluminum hydroxides reasonably pure. And then recognize that if we take aluminum hydroxide and dehydrate it, it becomes alumina, Al[2]O[3]. And this is the starting material for making aluminum metal.
Now, up to 1886, aluminum was largely a laboratory curiosity, and the reason is, it's very difficult to reduce aluminum oxide back to aluminum metal. And that should be really clear, based on the standard enthalpy of formation of aluminum. Remember, standard enthalpy of formation is the enthalpy change in going from the elements in their standard state to the compound of interest. And so if we get out 1,675.7 kilojoules per mole when we make aluminum oxide from the elements, if we're starting with aluminum oxide and we're trying to go back to aluminum metal, it's going to cost us this huge amount of energy. And so it's a very energy-intensive process to make aluminum metal from aluminum oxide. It's also, for reasons that I won't get into, just cumbersome - or was, at that time, before about 1886 - very cumbersome to make it. And one of the methods was to take aluminum chloride and reduce it in the presence of a potassium amalgam. And an amalgam is a solid solution of mercury with some other metal. And what it gives you is potassium chloride and then an aluminum amalgam. And the nice thing about amalgams is that in principle, if you heat them up, you can distill the mercury off. And so the mercury distilled off leaves aluminum metal.
Now, understand that aluminum was so valuable, up to 1886, that it is the metal that caps the top of the Washington Monument. So you'd have thought that something really cool like gold or silver would cap the top the Washington monument. But at the time, aluminum was the metal that was so valuable that it was the thing that was deemed suitable for capping something as important as the monument to the first president of the United States.
Now, all of this changed in 1886, when two guys who were in two different countries at the time and who happened to be only about 22 years old - Hall and Héraoult - decided that they would undertake the problem of trying to figure out a better way to reduce aluminum oxide to aluminum metal. And if you look at the graphic, you'll see that it's still an electrochemical reduction. It's really difficult, though, to melt aluminum oxide to make the molten salt that is the process that we use for making magnesium metal or sodium metal. Remember we take the molten salt and then we electrolyze it and we make the metal and then we make chlorine gas, for instance, if we're electrolyzing sodium chloride. Well, the problem is if you have aluminum, aluminum oxide just doesn't melt. It melts at about 2,050° Celsius. That's just way too high to be practical. And so what these guys figured out is that they could use something else as a solvent to dissolve the aluminum oxide. And then they could run their electrolytic cell at a much lower temperature. And in particular, what they discovered is that cryolite, which is a naturally occurring sodium aluminum fluoride, acts as both the electrolyte and the solvent for aluminum oxide. The anode in the Hall-Héraoult cell is graphite and the cathode is steel and the overall reaction is aluminum oxide plus carbon - so the graphite electrodes are actually consumed because the oxygen that is formed as a byproduct of the reaction reacts with the graphite electrodes - gives you aluminum metal, which being more dense, goes to the bottom of the cell. And so you can just periodically pull it off, and carbon dioxide is the other product. And the beauty is that the cryolite with the aluminum oxide dissolved in it actually melts at about 950° Celsius, as opposed to over 2,000, and so this is a much more convenient process for making aluminum metal.
Well, so aluminum metal has all kinds of applications. It's used in alloys, typically, with some magnesium and some copper. It's very lightweight and it's very strong and it's very low-density. So that means, you can make things like aluminum cans and they don't weigh a lot. Think about this when you're moving pop from where it's manufactured to where you sell it in the store, you don't want to be dragging along a lot of weight, which is really why we don't sell stuff in glass containers so much anymore. We sell things in aluminum or plastic. We want lightweight packaging. Aluminum metal is also a good conductor, so it's used in wires, and it's a lot cheaper than copper and it's almost as good a conductor as copper. And so we're using it in wires. You may know that it's an additive in Crystal Drano and what it does is it reacts with the base to form aluminum hydroxide and that generates a lot of heat and the heat probably does help with the process, but there's also been the suggestion that mostly what it does is fizzles and cracks and it makes you think that the Crystal Drano is doing something. What it does is it makes hydrogen and that hydrogen bubbles off and foams off. So it's not clear.
Another application for aluminum, you may have seen on late night TV a magic plate that allows you to polish your silver without any work. You just touch the plate to the piece of silver in the sink with some magic activating formula in there. Well, that plate, that you pay so much money for, is just a piece of aluminum, and electrochemically, you polish the silver. And so the silver has sulfides on it, and by reacting with a strongly reducing metal like aluminum, you can reduce off those sulfides.
So, aluminum is also very malleable, so you can make big wide sheets of it, and of course aluminum foil is now standard in everybody's kitchen, but remember, in the late 1800's this would have been considered one of the most valuable metals - again, something suitable for capping the top of the Washington Monument.
One more thing - why aluminum cans don't rust. So, steel cans, tin cans - and tin cans are just steel cans that have some tin on them - they rust. Well, why doesn't an aluminum can rust? In other words, this guy is going to live in a dump for a long, long time if I don't recycle it. And the answer is that even though is aluminum is very strongly reducing - so it's very eager to get rid of electrons - what happens is that aluminum has an adherent aluminum oxide coating on it. So all aluminum has this adherent aluminum oxide coating on it. And because it's adherent, it protects the aluminum from further oxidation. This doesn't happen in iron. In iron, the rust, the ferric oxide - the Fe[2]O[3] - flakes off and it exposes more iron. So a piece of iron will rust into nothing, but aluminum cans will last a long time.
Now, let's talk about some of the compounds of aluminum. Aluminum oxide, Al[2]O[3], is used for catalyst supports. And we've talked about the catalytic converter before. And remember that there is the catalytic converter. The active component are palladium and platinum metals and things like that. But those metals are very expensive. And so what we do is, since we want to make thin films of them, because only the surface is reactive, what we do is we deposit them on catalyst supports and those catalyst supports have to be able to take the high temperature that exists inside a catalytic converter. And so what we do is, we make these alumina honeycombs, and then we deposit the palladium or platinum or whatever other metal we're going to use on these alumina honeycombs, and that gives us a high surface area on something that's heat resistant, something that's appropriate for the catalytic converter application.
Now, potassium aluminum sulfates are things like baking powder - that's the Lewis acid in baking powder that gives the leavening action of a baking powder. Aluminum hydroxide - Maalox for instance - is magnesium and aluminum hydroxide, so it's used as an antacid. Aluminum trichloride or aluminum chloride is an antiperspirant. And aluminum sulfate is used as a flocculating agent in making paper. So paper has, in addition to the cellulose fibers, lots of other things that make it good for picking up ink. And in particular, it has resins and clays and things like that - basically the distinction between toilet paper, which doesn't have any of these things. And so if you try to write on toilet paper, it just sort of makes a mess, versus a good sheet of paper with the presence of all these other sorts of additives above and beyond what's in the cellulose. Well, what you want to do is you want to make sure these things deposit onto the cellulose and the resins and the clays are typically colloids, so they're floating around in the water. And to make that colloid come together, or coagulate, what you do is you add flocculating agents. So aluminum sulfate has a very great use in the paper industry in coagulating out these colloids. Well, the problem is that the aluminum sulfate - and we talked about this earlier - is a Lewis acid, just like potassium aluminum sulfate is a Lewis acid. And when you dissolve it in water, they give rise to Brønsted acids. Well, when your paper is really acidic, what happens is it decomposes over time - it hydrolizes. The paper hydrolizes and it turns into a crumbly useless mass of nothing. And a lot of books in the Library of Congress, or in libraries all around the world, are crumbling because of the acids that we use in producing paper. We didn't do this a couple hundred years or more ago, because we were using cotton to make paper and we don't have to treat it with all of these acids to make good paper. So cotton papers aren't decomposing, but wood papers have a lot of acids, including the acids that we use as flocculating agents, and so they're causing all paper to decompose. And we have to worry about this because we're loosing things. We have libraries full of books that are just turning into dust.
And so aluminum is really a useful metal. The next metal that I'd like to look at, and only very briefly, is gallium. And gallium often occurs with aluminum. It's right below it in the periodic table. And the importance of gallium is that gallium is one-half of gallium arsenide. If you think about where silicon is in the periodic table, if you go to one element on either side, you get gallium arsenide. It turns out, gallium arsenide is also a semiconductor in exactly the same way silicon is a semiconductor. But gallium arsenide does something that a silicon can't do, and that's give off light. And so all of your light-emitting diodes are gallium arsenide - one-to-one gallium arsenide. And all the different colors are a result of doping some aluminum into gallium arsenide to make gallium aluminum arsenide. The bottom line is, the elements that are in the group 13 have a lot of applications, both industrial and medical. And it's good that there's so much aluminum in the lithosphere, because we need a lot of aluminum for doing everyday things, including wrapping leftovers.
Chemistry of Metals
Physical and Chemical Processes of Metals
Aluminum Page [1 of 3]

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