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Chemistry: Demo: Copper One-Pot Reactions

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  • Type: Video Tutorial
  • Length: 7:28
  • Media: Video/mp4
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  • Download: MP4 (iPod compatible)
  • Size: 80 MB
  • Posted: 07/14/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Transition Metals (9 lessons, $14.85)
Chemistry: Examining Transition Metals (2 lessons, $3.96)

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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Let's take a look at a series of reactions of Cu[2] nitrate with a bunch of other reagents. And the point that I'm going to show you is not that you should memorize what's going on here, but rather we can at least understand what's going on. We can recognize that a reaction has occurred either by a color change, or the formation of a precipitation, or the evolution of a gas. And we can write down reactions that can at least rationalize what we think is going on.
So again, what I have here is a solution of Cu[2] nitrate. It's a very pretty blue color. The nitrate, remember, is a very non-hydrolyzing anion of nitric acid. And so the copper is basically just got a bunch of waters in it's coordination sphere, and the nitrate is just floating around by itself. And what we're going to do is we've placed this Cu[2] nitrate into the test tubes. And what we're going to do is add reagents.
And the first reagent I'd like to take a look at--and let's see. Let's clear these out so we can actually see what's going on here--is sodium bromide. And sodium bromide is a source of sodium ions, but sodium ions don't do much with copper. And sodium cations certainly don't react with nitrate, but the bromide is going to react with the copper, so let's take a look at what happens. Or at least maybe the bromide will react with the copper, and maybe it won't. So what happened? Well it wasn't a dramatic color change, but a color change nonetheless. And what we observed is the formation of Cu[2] bromide in aqueous solution. The bromide is now coordinated to the copper to make Cu[2] bromide. So that's our first reaction.
Let's now look at the reaction of Cu[2] nitrate with oxalic acid. Oxalic acid is the acid that makes sour grass and rhubarb sour. And look at that. What we got is sort of a pale milky blue precipitate. And the identity of that precipitate is probably Cu[2] oxalate. And we could look up the solubility product of Cu[2] oxalate and see if in fact it's a relatively small number. And if it were, then that would support the identity of this milky blue precipitate as Cu[2] oxalate. So that's number two.
And let's look at the reaction of Cu[2] nitrate with aqueous ammonia. Ammonia is a Lewis Base, and it reacts with the copper and it forms, initially, a sort of milky white precipitate. Actually it's continuing to form a milky white precipitate. So copper apparently reacts with ammonia to form a milky white precipitate. I would have guessed that it would form some sort of copper ammonia complex. And the fact that it forms a precipitate doesn't fit with my expectation.
So let's add some more ammonia and see what happens. What happens, it goes to this deep blue color. And you'll have to take my word for it, but it's back in solution now. So that milky precipitate went away, and now it's become a deep blue solution again. Well what could be going on there? Let's add some more ammonia. Maybe we can even see with the camera that it has in fact gone into solution. I hesitate to put my thumb over this and give it a good shake, but maybe through the top you can see my finger here, that it has gone back in solution.
Originally what happens is, when you add the ammonia, remember ammonia is a weak base, so there's hydroxide in solution. And the solubility product for Cu[2] oxide is sufficiently small that what first happens when we add the ammonia is we precipitate out Cu[2] hydroxide. But as we add more and more ammonia, we can, by Le Châtelier's principle, make the copper ammonia complex, it's probably more than one ammonia coordinated to the copper, but that drags the equilibrium away from Cu[2] hydroxide. And so the precipitate redissolves. And we form the deep blue color of the copper ammonia complex.
Okay, let's keep going. We have here a solution of sodium carbonate. Sodium carbonate is a weak base. And we add the two together, and we got another precipitate. And I'm going to bet that this is, once again, Cu[2] hydroxide. The carbonate is a source of--oh, and there are some bubbles.
Hmmm, what could those bubbles be? Well it's a reasonable bet that, since we started with sodium carbonate, that the bubbles are carbon dioxide gas being evolved. And that makes some sense, because Cu[2] is a Lewis acid. And so the Cu[2] Lewis acid is going to form Brønsted acid. The copper aqual complex is probably a Brønsted acid, and that's going to react with the bicarbonate that's in solution. And it's going to form some carbon dioxide. You can see it's actually blowing some bubbles in here now where we have the slightly acidic copper nitrate solution reacting with the basic carbonate solution that we've thrown in, and it makes carbon dioxide. So in addition to Cu[2] hydroxides, we're clearly making some carbon dioxide gas.
And then the final reaction I want to take a look at is the reaction of potassium ferrous cyanide with Cu[2] nitrate. And what we get is sort of a yucky brown solid. In this reaction, the product is probably an analogue of Prussian blue. Prussian blue is one of the oldest, highly colored materials that was known. And it is the reaction of Fe[3] ions with Fe[2] hexacyano complexes. In this case we have the Cu[2] instead of the Fe[3], but it gives a very highly colored compound. And it turns out that the color of this compound is probably the result of some sort of charged transfer reaction, so the electron moving from the iron to the copper or vice versa. And that's what gives rise to this highly yucky-colored species.
So what have we done? Well we looked at a series of different reactions between Cu[2] in water, and we've made a variety of products. We can know that a reaction occurred by color changes, by the creation of a precipitate, by a gas evolution. And we can write reactions that explain, and look up data that explain things like why there should be a precipitate. Well it's because the solubility product is very small and we've exceeded it. Or why should this go from first being a precipitate to then having a deep blue color, but having it be redissolved? We can write both complex ion equilibria and solubility product equilibria that at least rationalize these results.
Transition Elements
Transition Metals
CIA Demonstration: Copper One-Pot Reactions Page [2 of 2]

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